Endre søk
Begrens søket
12 1 - 50 of 63
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Treff pr side
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
Merk
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1. Abolmasov, Pavel
    et al.
    Nättilä, Joonas
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Columbia University, USA; Flatiron Institute, USA.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Kilohertz quasi-periodic oscillations from neutron star spreading layers2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 638, artikkel-id A142Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When the accretion disc around a weakly magnetised neutron star (NS) meets the stellar surface, it should brake down to match the rotation of the NS, forming a boundary layer. As the mechanisms potentially responsible for this braking are apparently inefficient, it is reasonable to consider this layer as a spreading layer (SL) with negligible radial extent and structure. We perform hydrodynamical 2D spectral simulations of an SL, considering the disc as a source of matter and angular momentum. Interaction of new, rapidly rotating matter with the pre-existing, relatively slow material co-rotating with the star leads to instabilities capable of transferring angular momentum and creating variability on dynamical timescales. For small accretion rates, we find that the SL is unstable for heating instability that disrupts the initial latitudinal symmetry and produces large deviations between the two hemispheres. This instability also results in breaking of the axial symmetry as coherent flow structures are formed and escape from the SL intermittently. At enhanced accretion rates, the SL is prone to shearing instability and acts as a source of oblique waves that propagate towards the poles, leading to patterns that again break the axial symmetry. We compute artificial light curves of an SL viewed at different inclination angles. Most of the simulated light curves show oscillations at frequencies close to 1 kHz. We interpret these oscillations as inertial modes excited by shear instabilities near the boundary of the SL. Their frequencies, dependence on flux, and amplitude variations can explain the high-frequency pair quasi-periodic oscillations observed in many low-mass X-ray binaries.

  • 2. Abolmasov, Pavel
    et al.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Gamma-ray opacity of the anisotropic stratified broad-line regions in blazars2017Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 464, nr 1, s. 152-169Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The GeV-range spectra of blazars are shaped not only by non-thermal emission processes internal to the relativistic jet but also by external pair-production absorption on the thermal emission of the accretion disc and the broad-line region (BLR). For the first time, we compute here the pair-production opacities in the GeV range produced by a realistic BLR accounting for the radial stratification and radiation anisotropy. Using photoionization modelling with the CLOUDY code, we calculate a series of BLR models of different sizes, geometries, cloud densities, column densities and metallicities. The strongest emission features in the model BLR are Ly alpha and He II Ly alpha. Contribution of recombination continua is smaller, especially for hydrogen, because Ly continuum is efficiently trapped inside the large optical depth BLR clouds and converted to Lyman emission lines and higher order recombination continua. The largest effects on the gamma-ray opacity are produced by the BLR geometry and localization of the gamma-ray source. We show that when the gamma-ray source moves further from the central source, all the absorption details move to higher energies and the overall level of absorption drops because of decreasing incidence angles between the gamma-rays and BLR photons. The observed positions of the spectral breaks can be used to measure the geometry and the location of the gamma-ray emitting region relative to the BLR. Strong dependence on geometry means that the soft photons dominating the pair-production opacity may be actually produced by a different population of BLR clouds than the bulk of the observed broad line emission.

  • 3. Abolmasov, Pavel
    et al.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Mechanical model of a boundary layer for the parallel tracks of kilohertz quasi-periodic oscillations in accreting neutron stars2021Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 647, artikkel-id A45Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Kilohertz-scale quasi-periodic oscillations (kHz QPOs) are a distinct feature of the variability of neutron star low-mass X-ray binaries. Among all the variability modes, they are especially interesting as a probe for the innermost parts of the accretion flow, including the accretion boundary layer (BL) on the surface of the neutron star. All the existing models of kHz QPOs explain only part of their rich phenomenology. Here, we show that some of their properties can be explained by a very simple model of the BL that is spun up by accreting rapidly rotating matter from the disk and spun down by the interaction with the neutron star. In particular, if the characteristic time scales for the mass and the angular momentum transfer from the BL to the star are of the same order of magnitude, our model naturally reproduces the so-called parallel tracks effect, where the QPO frequency is correlated with luminosity at time scales of hours but becomes uncorrelated at time scales of days. The closeness of the two time scales responsible for mass and angular momentum exchange between the BL and the star is an expected outcome of the radial structure of the BL.

  • 4.
    Chashkina, Anna
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Moscow State University, Russia; University of California, USA.
    Abolmasov, Pavel
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; University of California, USA.
    Super-Eddington accretion on to a magnetized neutron star2017Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, nr 3, s. 2799-2813Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small Eddington limit. The accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. We propose a model of a nearly standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. Our calculations show that the size of the magnetosphere may be represented as the classical Alfven radius times a dimensionless factor. which depends only on the disc thickness. In the case of radiation-pressure-dominated disc, the size of the magnetosphere does not depend on the mass accretion rate. In general, increasing the disc thickness leads to a larger magnetosphere size in units of the Alfven radius. For large enough mass accretion rates and magnetic moments, it is important to take into account not only the pressure of the magnetic field and the radiation pressure inside the disc, but also the pressure of the radiation produced close to the surface of the neutron star in accretion column. The magnetospheric size may increase by up to factor of 2 as a result of the effects related to the disc thickness and the irradiation from the central source. Accounting for these effects reduces the estimate of the neutron star magnetic moment by a factor of several orders.

  • 5. Chashkina, Anna
    et al.
    Lipunova, Galina
    Abolmasov, Pavel
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Super-Eddington accretion discs with advection and outflows around magnetized neutron stars2019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, artikkel-id A18Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a model for a super-Eddington accretion disc around a magnetized neutron star taking into account advection of heat and the mass loss by the wind. The model is semi-analytical and predicts radial profiles of all the basic physical characteristics of the accretion disc. The magnetospheric radius is found as an eigenvalue of the problem. When the inner disc is in radiation-pressure-dominated regime but does not reach its local Eddington limit, advection is mild, and the radius of the magnetosphere depends weakly on the accretion rate. Once it approaches the local Eddington limit the disc becomes advection-dominated, and the scaling for the magnetospheric radius with the mass accretion rate is similar to the classical Alfven relation. Allowing for the mass loss in a wind leads to an increase in the magnetospheric radius. Our model can be applied to a wide variety of magnetized neutron stars accreting close to or above their Eddington limits: ultra-luminous X-ray pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of our model we discuss the observational properties of NGC5907 X-1, the brightest ultra-luminous pulsar currently known, and NGC300 ULX1, which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst.

  • 6. De Falco, V.
    et al.
    Kuiper, L.
    Bozzo, E.
    Ferrigno, C.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Stella, L.
    Falanga, M.
    The transitional millisecond pulsar IGR J18245-2452 during its 2013 outburst at X-rays and soft gamma-rays2017Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 603, artikkel-id A16Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    IGR J18245-2452/PSR J1824-2452I is one of the rare transitional accreting millisecond X-ray pulsars, showing direct evidence of switches between states of rotation-powered radio pulsations and accretion-powered X-ray pulsations, dubbed transitional pulsars. IGR J18245-2452 with a spin frequency of similar to 254.3 Hz is the only transitional pulsar so far to have shown a full accretion episode, reaching an X-ray luminosity of similar to 10(37) erg s(-1) permitting its discovery with INTEGRAL in 2013. In this paper, we report on a detailed analysis of the data collected with the IBIS/ISGRI and the two JEM-X monitors on-board INTEGRAL at the time of the 2013 outburst. We make use of some complementary data obtained with the instruments on-board XMM-Newton and Swift in order to perform the averaged broad-band spectral analysis of the source in the energy range 0.4-250 keV. We have found that this spectrum is the hardest among the accreting millisecond X-ray pulsars. We improved the ephemeris, now valid across its full outburst, and report the detection of pulsed emission up to similar to 60 keV in both the ISGRI(10.9 similar to) and Fermi/GBM (5.9 sigma) bandpass. The alignment of the ISGRI and Fermi GBM 20-60 keV pulse profiles are consistent at a similar to 25 mu s level. We compared the pulse profiles obtained at soft X-rays with XMM-Newton with the soft gamma-ray ones, and derived the pulsed fractions of the fundamental and first harmonic, as well as the time lag of the fundamental harmonic, up to 150 mu s, as a function of energy. We report on a thermonuclear X-ray burst detected with INTEGRAL, and using the properties of the previously type-I X-ray burst, we show that all these events are powered primarily by helium ignited at a depth of y(ign) similar to 2.7 x 10(8) g cm(-2). For such a helium burst the estimated recurrence time of Delta t(rec) similar to 5.6 d is in agreement with the observations.

  • 7. Doroshenko, Victor
    et al.
    Tsygankov, Sergey S.
    Mushtukov, Alexander A.
    Lutovinov, Alexander A.
    Santangelo, Andrea
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+532017Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 466, nr 2, s. 2143-2150Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report on the analysis of NuSTAR observations of the Be-transient X-ray pulsar V 0332+ 53 during the giant outburst in 2015 and another minor outburst in 2016. We confirm the cyclotronline energy-luminosity correlation previously reported in the source and the line energy decrease during the giant outburst. Based on 2016 observations, we find that a year later the line energy has increased again essentially reaching the pre-outburst values. We discuss this behaviour and conclude that it is likely caused by a change of the emission region geometry rather than previously suggested accretion-induced decay of the neutron stars magnetic field. At lower luminosities, we find for the first time a hint of departure from the anticorrelation of line energy with flux, which we interpret as a transition from super-to sub-critical accretion associated with the disappearance of the accretion column. Finally, we confirm and briefly discuss the orbital modulation observed in the outburst light curve of the source.

  • 8. Kajava, J. J. E.
    et al.
    Sánchez-Fernández, C.
    Kuulkers, E.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    X-ray burst-induced spectral variability in 4U 1728-342017Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, artikkel-id A89Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Aims. INTEGRAL has been monitoring the Galactic center region for more than a decade. Over this time it has detected hundreds of type-I X-ray bursts from the neutron star low-mass X-ray binary 4U 1728-34, also known as the slow burster. Our aim is to study the connection between the persistent X-ray spectra and the X-ray burst spectra in a broad spectral range.

    Methods. We performed spectral modeling of the persistent emission and the X-ray burst emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI instruments.

    Results. We constructed a hardness intensity diagram to track spectral state variations. In the soft state, the energy spectra are characterized by two thermal components likely coming from the accretion disc and the boundary/spreading layer, together with a weak hard X-ray tail that we detect in 4U 1728-34 for the first time in the similar to 40 to 80 keV range. In the hard state, the source is detected up to similar to 200 keV and the spectrum can be described by a thermal Comptonization model plus an additional component: either a powerlaw tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect emission up to 80 keV during the X-ray bursts. We find that during the bursts the emission above 40 keV decreases by a factor of approximately three with respect to the persistent emission level.

    Conclusions. Our results suggest that the enhanced X-ray burst emission changes the spectral properties of the accretion disc in the hard state. The likely cause is an X-ray burst induced cooling of the electrons in the inner hot flow near the neutron star.

  • 9.
    Kosenkov, Ilia A.
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; St. Petersburg State University, Russia.
    Berdyugin, Andrei V.
    Piirola, Vilppu
    Tsygankov, Sergey S.
    Pallé, Enric
    Miles-Páez, Paulo A.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; University of California, USA.
    High-precision optical polarimetry of the accreting black hole V404 Cyg during the 2015 June outburst2017Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, nr 4, s. 4362-4373Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Our simultaneous three-colour (BVR) polarimetric observations of the low-mass black hole X-ray binary V404 Cyg show a small but statistically significant change of polarization degree (Delta(p) similar to 1 per cent) between the outburst in 2015 June and the quiescence. The polarization of V404 Cyg in the quiescent state agrees within the errors with that of the visually close (1.4 arc-sec) companion (pR = 7.3 +/- 0.1 per cent), indicating that it is predominantly of interstellar origin. The polarization pattern of the surrounding field stars supports this conclusion. From the observed variable polarization during the outburst, we show that the polarization degree of the intrinsic component peaks in the V band, p(V) = 1.1 +/- 0.1 per cent, at the polarization position angle of theta(V) =-7 degrees+/- 2 degrees, which is consistent in all three passbands. We detect significant variations in the position angle of the intrinsic polarization in the R band from -30. to similar to 0 degrees during the outburst peak. The observed wavelength dependence of the intrinsic polarization does not support non-thermal synchrotron emission from a jet as a plausible mechanism, but it is in better agreement with the combined effect of electron (Thomson) scattering and absorption in a flattened plasma envelope or outflow surrounding the illuminating source. Alternatively, the polarization signal can be produced by scattering of the disc radiation in a mildly relativistic polar outflow. The position angle of the intrinsic polarization, nearly parallel to the jet direction (i. e. perpendicular to the accretion disc plane), is in agreement with these interpretations.

  • 10. Kosenkov, Ilia A.
    et al.
    Veledina, Alexandra
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Berdyugin, Andrei
    Kravtsov, Vadim
    Piirola, Vilppu
    Berdyugina, Svetlana
    Sakanoi, Takeshi
    Kagitani, Masato
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Disc and wind in black hole X-ray binary MAXI J1820+070 observed through polarized light during its 2018 outburst2020Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 496, nr 1, s. L96-L100Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We describe the first complete polarimetric data set of the entire outburst of a low-mass black hole X-ray binary system and discuss the constraints for geometry and radiative mechanisms it imposes. During the decaying hard state, when the optical flux is dominated by the non-thermal component, the observed polarization is consistent with the interstellar values in all filters. During the soft state, the intrinsic polarization of the source is small, similar to 0.15 per cent in B and V filters, and is likely produced in the irradiated disc. A much higher polarization, reaching similar to 0.5 per cent in V and R filters, at a position angle of similar to 25 degrees observed in the rising hard state coincides in time with the detection of winds in the system. This angle coincides with the position angle of the jet. The detected optical polarization is best explained by scattering of the non-thermal (hot flow or jet base) radiation in an equatorial wind.

  • 11. Kosenkov, Ilia A.
    et al.
    Veledina, Alexandra
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Colors and patterns of black hole X-ray binary GX 339-42020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 638, artikkel-id A127Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Black hole X-ray binaries show signs of nonthermal emission in the optical to near-infrared range. We analyzed optical to near-infrared SMARTS data on GX 339-4 over the 2002–2011 period. Using soft state data, we estimated the interstellar extinction toward the source and characteristic color temperatures of the accretion disk. We show that various spectral states of regular outbursts occupy similar regions on color-magnitude diagrams, and that transitions between the states proceed along the same tracks despite substantial differences in the morphology of the observed light curves. We determine the typical duration of hard-to-soft and soft-to-hard state transitions and the hard state at the decaying stage of the outburst to be one, two, and four weeks, respectively. We find that the failed outbursts cannot be easily distinguished from the regular outbursts at their early stages, but if the source reaches 16 mag in V band, it transits to the soft state. By subtracting the contribution of the accretion disk, we obtain spectra of the nonthermal component, which have constant, nearly flat shapes during the transitions between the hard and soft states. In contrast to the slowly evolving nonthermal component seen at optical and near-infrared wavelengths, the mid-infrared spectrum is strongly variable on short timescales and sometimes shows a prominent excess with a cutoff below 1014 Hz. We show that the radio to optical spectrum can be modeled using three components corresponding to the jet, hot flow, and irradiated accretion disk.

  • 12. Kravtsov, Vadim
    et al.
    Berdyugin, Andrei V.
    Piirola, Vilppu
    Kosenkov, Ilia A.
    Tsygankov, Sergey S.
    Chernyakova, Maria
    Malyshev, Denys
    Sakanoi, Takeshi
    Kagitani, Masato
    Berdyugina, Svetlana V.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Orbital variability of the optical linear polarization of the γ-ray binary LS I+61 degrees 303 and new constraints on the orbital parameters2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 643, artikkel-id A170Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We studied the variability of the linear polarization and brightness of the gamma -ray binary LS I +61 degrees 303. High-precision BVR photopolarimetric observations were carried out with the Dipol-2 polarimeter on the 2.2 m remotely controlled UH88 telescope at Mauna Kea Observatory and the 60 cm Tohoku telescope at Haleakala bservatory (Hawaii) over 140 nights in 2016-2019. We also determined the degree and angle of the interstellar polarization toward LS I +61 degrees 303 using two out of four nearby field stars that have Gaia's parallaxes. After subtracting the interstellar polarization, we determined the position angle of the intrinsic polarization theta similar or equal to 11 degrees, which can either be associated with the projection of the Be star's decretion disk axis on the plane of sky, or can differ from it by 90 degrees. Using the Lomb-Scargle method, we performed timing analyses and period searches of our polarimetric and photometric data. We found statistically significant periodic variability of the normalized Stokes parameters q and u in all passbands. The most significant period of variability, P-Pol=13.244 +/- 0.012 d, is equal to one half of the orbital period P-orb=26.496 d. The fits of the polarization variability curves with Fourier series show a dominant contribution from the second harmonic which is typical for binary systems with circular orbits and nearly symmetric distribution of light scattering material with respect to the orbital plane. The continuous change of polarization with the orbital phase implies co-planarity of the orbit of the compact object and the Be star's decretion disk. Using a model of Thomson scattering by a cloud that orbits the Be star, we obtained constraints on the orbital parameters, including a small eccentricity e<0.2 and periastron phase of <phi>(p)approximate to 0.6, which coincides with the peaks in the radio, X-ray, and TeV emission. These constraints are independent of the assumption about the orientation of the decretion disk plane on the sky. We also extensively discuss the apparent inconsistency with the previous measurements of the orbital parameters from radial velocities. By folding the photometry data acquired during a three-year time span with the orbital period, we found a linear phase shift of the moments of the brightness maximum, confirming the possible existence of superorbital variability.

  • 13. Krivonos, Roman
    et al.
    Clavel, Maica
    Hong, JaeSub
    Mori, Kaya
    Ponti, Gabriele
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Rahoui, Farid
    Tomsick, John
    Tsygankov, Sergey
    NuSTAR and XMM-Newton observations of the Arches cluster in 2015: fading hard X-ray emission from the molecular cloud2017Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, nr 3, s. 2822-2835Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present results of long Nuclear Spectroscopic Telescope Array (NuSTAR; 200 ks) and XMM-Newton (100 ks) observations of the Arches stellar cluster, a source of bright thermal (kT similar to 2 keV) X-rays with prominent Fe XXV K alpha 6.7 keV line emission and a nearby molecular cloud, characterized by an extended non-thermal hard X-ray continuum and fluorescent Fe K alpha 6.4 keV line of a neutral or low-ionization state material around the cluster. Our analysis demonstrates that the non-thermal emission of the Arches cloud underwent a dramatic change, with its homogeneous morphology, traced by fluorescent Fe K alpha line emission, vanishing after 2012, revealing three bright clumps. The declining trend of the cloud emission, if linearly fitted, is consistent with half-life decay time of similar to 8 yr. Such strong variations have been observed in several other molecular clouds in the Galactic Centre, including the giant molecular cloud Sgr B2, and point towards a similar propagation of illuminating fronts, presumably induced by the past flaring activity of Sgr A(star). We also detect a significant drop of the equivalent width of the fluorescent Fe Ka line, which could mean either that the new clumps have a different position along the line of sight or that the contribution of cosmic ray has become more dominant.

  • 14. Krivonos, Roman
    et al.
    Sazonov, Sergey
    Tsygankov, Sergey S.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Space Research Institute of the Russian Academy of Sciences, Russia; University of Turku, Finland.
    NuSTAR observations of the ultraluminous X-ray source M33 X-8: a black hole in a very high state?2018Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, nr 2, s. 2357-2364Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The closest known ultraluminous X-ray source (ULX), M33 X-8, has been recently observed with NuSTAR during its Extragalactic Legacy program, which includes a hard X-ray survey of the M33 galaxy. We present results of two long observations of M33 taken in 2017 March and July, with M33 X-8 in the field of view. The source demonstrates a nearly constant flux during the observations, and its 3-20 keV spectrum can be well described by two distinct components: a standard accretion disc with a temperature of similar to 1 keV at the inner radius and a power law with a photon index Gamma approximate to 3, which is significantly detected up to 20 keV. There is also an indication of a high-energy cut-off in the spectrum, corresponding to a temperature of the Comptonizing medium of greater than or similar to 10 keV. The broad-band spectral properties of M33 X-8 resemble black hole X-ray binaries in their very high states, suggesting that M33 X-8 is a black hole accreting at a nearly Eddington rate, in contrast to super-Eddington accretion believed to take place in more luminous ULXs.

  • 15. Kuiper, L.
    et al.
    Tsygankov, S. S.
    Falanga, M.
    Mereminskiy, I. A.
    Galloway, D. K.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Li, Z.
    High-energy characteristics of the accretion-powered millisecond pulsar IGR J17591-2342 during its 2018 outburst: XMM-Newton, NICER, NuSTAR, and INTEGRAL view of the 0.3-300 keV X-ray band2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 641, artikkel-id A37Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    IGR J17591−2342 is an accreting millisecond X-ray pulsar, discovered with INTEGRAL, which went into outburst around July 21, 2018. To better understand the physics acting in these systems during the outburst episode, we performed detailed temporal-, timing-, and spectral analyses across the 0.3–300 keV band using data from NICER, XMM-Newton, NuSTAR, and INTEGRAL. The hard X-ray 20–60 keV outburst profile covering ∼85 days is composed of four flares. Over the course of the maximum of the last flare, we discovered a type-I thermonuclear burst in INTEGRAL JEM-X data, posing constraints on the source distance. We derived a distance of 7.6 ± 0.7 kpc, adopting Eddington-limited photospheric radius expansion and assuming anisotropic emission. In the timing analysis, using all NICER 1–10 keV monitoring data, we observed a rather complex set of behaviours starting with a spin-up period (MJD 58345–58364), followed by a frequency drop (MJD 58364–58370), an episode of constant frequency (MJD 58370–58383), concluded by irregular behaviour till the end of the outburst. The 1–50 keV phase distributions of the pulsed emission, detected up to ∼120 keV using INTEGRAL ISGRI data, was decomposed in three Fourier harmonics showing that the pulsed fraction of the fundamental increases from ∼10% to ∼17% going from ∼1.5 to ∼4 keV, while the harder photons arrive earlier than the soft photons for energies ≲10 keV. The total emission spectrum of IGR J17591−2342 across the 0.3–150 keV band could adequately be fitted in terms of an absorbed COMPPS model yielding as best fit parameters a column density of NH = (2.09 ± 0.05) × 1022 cm−2, a blackbody seed photon temperature kTbb, seed of 0.64 ± 0.02 keV, electron temperature kTe = 38.8 ± 1.2 keV and Thomson optical depth τT = 1.59 ± 0.04. The fit normalisation results in an emission area radius of 11.3 ± 0.5 km adopting a distance of 7.6 kpc. Finally, the results are discussed within the framework of accretion physics- and X-ray thermonuclear burst theory.

  • 16. Li, Z.
    et al.
    De Falco, V.
    Falanga, M.
    Bozzo, E.
    Kuiper, L.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Cumming, A.
    Galloway, D. K.
    Zhang, S.
    Mixed H/He bursts in SAX J1748.9-2021 during the spectral change of its 2015 outburst2018Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, artikkel-id A114Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    SAX J1748.9-2021 is a transiently accreting X-ray millisecond pulsar. It is also known as an X-ray burster source discovered by Beppo-SAX. We analyzed the persistent emission and type-I X-ray burst properties during its 2015 outburst. The source changed from hard to soft state within half day. We modeled the broadband spectra of the persistent emission in the (1-250) keV energy band for both spectral states using the quasi-simultaneous INTEGRAL and Swift data. The broadband spectra are well fitted by an absorbed thermal Componization model, COMPPS, in a slab geometry. The best-fits for the two states indicate significantly different plasma temperature of 18 and 5 keV and the Thomson optical depths of three and four, respectively. In total, 56 type-I X-ray bursts were observed during the 2015 outburst, of which 26 detected by INTEGRAL in the hard state, 25 by XMM-Newton in the soft state, and five by Swift in both states. As the object transited from the hard to the soft state, the recurrence time for X-ray bursts decreased from approximate to 2 to approximate to 1 h. The relation between the recurrence time, Delta t(rec )and the local mass accretion rate per unit area onto the compact object, in, is fitted by a power-law model, and yielded as best fit at Delta t(rec )similar to <(m) over dot >(-1)(.0)(2 +/-)(0.)(03) using all X-ray bursts. In both cases, the observed recurrence times are consistent with the mixed hydrogen and helium bursts. We also discuss the effects of type-I X-ray bursts prior to the hard to soft transition.

  • 17. Li, Z. S.
    et al.
    Kuiper, L.
    Falanga, M.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Tsygankov, S. S.
    Galloway, D. K.
    Bozzo, E.
    Pan, Y. Y.
    Huang, Y.
    Zhang, S. N.
    Zhang, S.
    Broadband X-ray spectra and timing of the accreting millisecond pulsar Swift J1756.9-2508 during its 2018 and 2019 outbursts2021Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 649, artikkel-id A76Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The accreting millisecond X-ray pulsar Swift J1756.9-2508 launched into an outburst in April 2018 and June 2019 - 8.7 years after the previous period of activity. We investigated the temporal, timing, and spectral properties of these two outbursts using data from NICER, XMM-Newton, NuSTAR, INTEGRAL, Swift, and Insight-HXMT. The two outbursts exhibited similar broadband spectra and X-ray pulse profiles. For the first time, we report the detection of the pulsed emission up to similar to 100 keV that was observed by Insight-HXMT during the 2018 outburst. We also found the pulsation up to similar to 60 keV that was observed by NICER and NuSTAR during the 2019 outburst. We performed a coherent timing analysis combining the data from the two outbursts. The binary system is well described by a constant orbital period over a time span of similar to 12 years. The time-averaged broadband spectra are well fitted by the absorbed thermal Comptonization model COMPPS in a slab geometry with an electron temperature, kT(e)=40-50 keV, Thomson optical depth tau similar to 1.3, blackbody seed photon temperature kT(bb, seed)similar to 0.7-0.8 keV, and hydrogen column density of N-H similar to 4.2x10(22) cm(-2). We searched the available data for type-I (thermonuclear) X-ray bursts, but found none, which is unsurprising given the estimated low peak accretion rate (approximate to 0.05 of the Eddington rate) and generally low expected burst rates for hydrogen-poor fuel. Based on the history of four outbursts to date, we estimate the long-term average accretion rate at roughly 5x10(-12) M-circle dot yr(-1) for an assumed distance of 8 kpc. The expected mass transfer rate driven by gravitational radiation in the binary implies the source may be no closer than 4 kpc. Swift J1756.9-2508 is the third low mass X-ray binary exhibiting double outbursts, which are separated by much shorter intervals than what we typically see and are likely to result from interruption of the accretion flow from the disk onto the neutron star. Such behavior may have important implications for the disk instability model.

  • 18. Li, Zhaosheng
    et al.
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Space Research Institute of the Russian Academy of Sciences, Russia; University of Turku, Finland.
    Salmi, Tuomo
    Falanga, Maurizio
    Nättilä, Joonas
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Xu, Renxin
    Evidence for the Photoionization Absorption Edge in a Photospheric Radius Expansion X-Ray Burst from GRS 1747-312 in Terzan 62018Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, nr 1, artikkel-id 53Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Thermonuclear X-ray bursts on the surface of neutron stars (NSs) can enrich the photosphere with metals, which may imprint photoionization edges on the burst spectra. We report here the discovery of absorption edges in the spectra of the type I X-ray burst from the NS low-mass X-ray binary GRS 1747-312 in Terzan 6 during observations by the Rossi X-ray Timing Explorer. We find that the edge energy evolves from 9.45 +/- 0.51 to similar to 6 keV and then back to 9.44 +/- 0.40 keV during the photospheric radius expansion phase and remains at 8.06 +/- 0.66 keV in the cooling tail. The photoionization absorption edges of hydrogen-like Ni, Fe, or an Fe/Ni mixture and the bound-bound transitions of metals may be responsible for the observed spectral features. The ratio of the measured absorption edge energy in the cooling tail to the laboratory value of the hydrogen-like Ni(Fe) edge energy allows us to estimate the gravitational redshift factor 1 + z = 1.34 +/- 0.11(1 + z = 1.15 +/- 0.09). The evolution of the spectral parameters during the cooling tail are well described by metal-rich atmosphere models. The combined constraints on the NS mass and radius from the direct cooling method and the tidal deformability strongly suggest very high atmospheric abundance of the iron group elements and limit the distance to the source to 11 +/- 1 kpc.

  • 19. Loktev, Vladislav
    et al.
    Salmi, Tuomo
    Nättilä, Joonas
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Oblate Schwarzschild approximation for polarized radiation from rapidly rotating neutron stars2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 643, artikkel-id A84Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have developed a complete theory for the calculation of the observed Stokes parameters for radiation emitted from the surface of a rapidly rotating neutron star (NS) using the oblate Schwarzschild approximation. We accounted for the rotation of the polarization plane due to relativistic effects along the path from the stellar surface to the observer. The results were shown to agree with those obtained by performing full numerical general relativistic ray-tracing with the ARCMANCER code. We showed that the obtained polarization angle profiles may differ substantially from those derived for a spherical star. We demonstrated that assuming incorrect shape for the star can lead to biased constraints for NS parameters when fitting the polarization data. Using a simplified model, we also made a rough estimate of how accurately the geometrical parameters of an accreting NS can be determined using the X-ray polarization measurements of upcoming polarimeters like the Imaging X-ray Polarimeter Explorer or the enhanced X-ray Timing and Polarimetry mission.

  • 20. Loktev, Vladislav
    et al.
    Veledina, Alexandra
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Analytical techniques for polarimetric imaging of accretion flows in the Schwarzschild metric2022Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 660, artikkel-id A25Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Emission from an accretion disk around compact objects, such as neutron stars and black holes, is expected to be significantly polarized. The polarization can be used to put constraints on the geometrical and physical parameters of the compact sources – their radii, masses, and spins – as well as to determine the orbital parameters. The radiation escaping from the innermost parts of the disk is strongly affected by the gravitational field of the compact object and the relativistic velocities of the matter. The straightforward calculation of the observed polarization signatures involves a computationally expensive ray-tracing technique. At the same time, having fast computational routines for direct data fitting is becoming increasingly important in light of the currently observed images of the accretion flow around the supermassive black hole in M 87 by the Event Horizon Telescope and infrared polarization signatures coming from Sgr A*, as well as the upcoming X-ray polarization measurements by the Imaging X-ray Polarimetry Explorer and enhanced X-ray Timing and Polarimetry mission. In this work, we obtain an exact analytical expression for the rotation angle of the polarization plane in the Schwarzschild metric accounting for the effects of light bending and relativistic aberration. We show that the calculation of the observed flux, polarization degree, and polarization angle as a function of energy can be performed analytically with a high level of accuracy using an approximate light-bending formula, eliminating the need for the precomputed tabular models in fitting routines.

  • 21. Lutovinov, Alexander A.
    et al.
    Tsygankov, Sergey S.
    Krivonos, Roman A.
    Molkov, Sergey V.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    PROPELLER EFFECT IN THE TRANSIENT X-RAY PULSAR SMC X-22017Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 834, nr 2, artikkel-id 209Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report the results of the monitoring campaign of the transient X-ray pulsar SMC X-2 performed with the Swift/XRT telescope over the period of 2015 September-2016 January during the Type II outburst. During this event, the bolometric luminosity of the source ranged from similar or equal to 10(39) down to several x 10(34) erg s(-1). Moreover, we discovered its dramatic drop by a factor of more than 100 below the limiting value of L-lim similar or equal to 4 x 10(36) erg s(-1), which can be interpreted as a transition to the propeller regime. These measurements make SMC X-2 the sixth pulsating X-ray source where such a transition is observed and allow us to estimate the magnetic field of the neutron star in the system B similar or equal to 3 x 10(12) G, which is in agreement with independent results of the spectral analysis.

  • 22. Mereminskiy, Ilya A.
    et al.
    Semena, Andrey N.
    Bykov, Sergey D.
    Filippova, Ekaterina V.
    Lutovinov, Alexander A.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Russian Academy of Sciences, Russia; University of Turku, Finland.
    Studying temporal variability of GRS 1739-278 during the 2014 outburst2019Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, nr 1, s. 1392-1405Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report a discovery of low-frequency quasi-periodic oscillation at 0.3-0.7 Hz in the power spectra of the accreting black hole GRS 1739-278 in the hard-intermediate state during its 2014 outburst based on the NuSTAR and Swift/XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase lags are generally positive (hard) in the 0.1-3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be R-in < 7.3R(g). In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with M-BH a parts per thousand 100 M-aS (TM) is required.

  • 23. Monkkonen, Juhani
    et al.
    Doroshenko, Victor
    Tsygankov, Sergey S.
    Nabizadeh, Armin
    Abolmasov, Pavel
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Space ResearchUniversity of Turku, Finland; Institute of the Russian Academy of Sciences, Russia.
    Discovery of a retrogradely rotating neutron star in the X-ray pulsar GX 301-22020Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 494, nr 2, s. 2178-2182Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report on the analysis of the spin evolution of a slow X-ray pulsar GX 301-2 along the orbit using long-term monitoring by Fermi/Gamma-ray Burst Monitor. Based on the observationally confirmed accretion scenario and an analytical model for the accretion of angular momentum, we demonstrate that in this system, the neutron star spins retrogradely, that is, in a direction opposite to the orbital motion. This first-of-a-kind discovery of such a system proves the principal possibility of retrograde rotation in accreting systems with suitable accretion torque, and might have profound consequences for our understanding of the spin evolution of X-ray pulsars, estimates of their initial spin periods, and the ultimate result of their evolution.

  • 24. Mushtukov, Alexander A.
    et al.
    Nagirner, Dmitrij I.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Compton scattering S matrix and cross section in strong magnetic field2016Inngår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 93, nr 10, artikkel-id 105003Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Compton scattering of polarized radiation in a strong magnetic field is considered. The recipe for calculation of the scattering matrix elements, the differential and total cross sections based on quantum electrodynamic second-order perturbation theory is presented for the case of arbitrary initial and final Landau level, electron momentum along the field and photon momentum. Photon polarization and electron spin state are taken into account. The correct dependence of natural Landau level width on the electron spin state is taken into account in a general case of arbitrary initial photon momentum for the first time. A number of steps in the calculations were simplified analytically making the presented recipe easy to use. The redistribution functions over the photon energy, momentum and polarization states are presented and discussed. The paper generalizes already known results and offers a basis for the accurate calculation of radiation transfer in a strong B field, for example, in strongly magnetized neutron stars.

  • 25. Mushtukov, Alexander A.
    et al.
    Portegies Zwart, Simon
    Tsygankov, Sergey S.
    Nagirner, Dmitrij
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Space Research Institute of the Russian Academy of Sciences, Russia; University of Turku, Finland.
    Pulsating ULXs: large pulsed fraction excludes strong beaming2021Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 501, nr 2, s. 2424-2429Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The recent discovery of pulsating ultraluminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars can exceed the Eddington luminosity by a factor of 100s. The relation between the actual and apparent luminosity is a key ingredient in theoretical models of ULXs, but it is still under debate. A typical feature of the discovered pulsating ULXs is a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high PF. We argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ULX pulsars can show PF above 10 per cent. Discrepancy between this conclusion and current observations indicates that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.

  • 26. Mushtukov, Alexander A.
    et al.
    Tsygankov, Sergey S.
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; University of California, USA.
    Ultraluminous X-ray sources as neutrino pulsars2018Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 476, nr 3, s. 2867-2873Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The classical limit on the accretion luminosity of a neutron star is given by the Eddington luminosity. The advanced models of accretion on to magnetized neutron stars account for the appearance of magnetically confined accretion columns and allow the accretion luminosity to be higher than the Eddington value by a factor of tens. However, the recent discovery of pulsations from ultraluminous X-ray source (ULX) in NGC 5907 demonstrates that the accretion luminosity can exceed the Eddington value up to by a factor of 500. We propose a model explaining observational properties of ULX-1 in NGC 5907 without any ad hoc assumptions. We show that the accretion column at extreme luminosity becomes advective. Enormous energy release within a small geometrical volume and advection result in very high temperatures at the bottom of accretion column, which demand to account for the energy losses due to neutrino emission which can be even more effective than the radiation energy losses. We show that the total luminosity at the mass accretion rates above 1021 g s(-1) is dominated by the neutrino emission similarly to the case of core-collapse supernovae. We argue that the accretion rate measurements based on detected photon luminosity in case of bright ULXs powered by neutron stars can be largely underestimated due to intense neutrino emission. The recently discovered pulsating ULX-1 in galaxy NGC 5907 with photon luminosity of similar to 10(41) erg s(-1) is expected to be even brighter in neutrinos and is thus the first known Neutrino Pulsar.

  • 27. Mönkkönen, Juhani
    et al.
    Tsygankov, Sergey S.
    Mushtukov, Alexander A.
    Doroshenko, Victor
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis2019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, artikkel-id A106Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The X-ray pulsar GRO J1744-28 is a unique source that shows both pulsations and type-II X-ray bursts, allowing studies of the interaction of the accretion disk with the magnetosphere at huge mass-accretion rates exceeding 10(19) g s(-1) during its super-Eddington outbursts. The magnetic field strength in the source, B approximate to 5 x 10(11) G, is known from the cyclotron absorption feature discovered in the energy spectrum around 4.5 keV. Here, we have explored the flux variability of the source in context of interaction of its magnetosphere with the radiation-pressure dominated accretion disk. Specifically, we present the results of the analysis of noise power density spectra (PDS) using the observations of the source in 1996-1997 by the Rossi X-ray Timing Explorer (RXTE). Accreting compact objects commonly exhibit a broken power-law PDS shape with a break corresponding to the Keplerian orbital frequency of matter at the innermost disk radius. The observed frequency of the break can thus be used to estimate the size of the magnetosphere. We find, however, that the observed PDS of GRO J1744-28 differs dramatically from the canonical shape. The observed break frequency appears to be significantly higher than expected based on the magnetic field estimated from the cyclotron line energy. We argue that these observational facts can be attributed to the existence of the radiation-pressure dominated region in the accretion disk at luminosities above similar to 2 x10(37) erg s(-1). We discuss a qualitative model for the PDS formation in such disks, and show that its predictions are consistent with our observational findings. The presence of the radiation-pressure dominated region can also explain the observed weak luminosity dependence of the inner radius, and we argue that the small inner radius can be explained by a quadrupole component dominating the magnetic field of the neutron star.

  • 28. Nabizadeh, Armin
    et al.
    Monkkonen, Juhani
    Tsygankov, Sergey S.
    Doroshenko, Victor
    Molkov, Sergey V.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    NuSTAR observations of wind-fed X-ray pulsar GX301-2 during unusual spin-up event2019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 629, artikkel-id A101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report on NuSTAR observations of the well-known wind-accreting X-ray pulsar GX301-2 during a strong spin-up episode that took place in January-March 2019. A measurement of high luminosity of the source in the most recent observation allowed us to detect a positive correlation of the cyclotron line energy with luminosity. Beyond that, only minor differences in spectral and temporal properties of the source during the spin-up, presumably associated with the formation of a transient accretion disk, and the normal wind-fed state could be detected. Finally, we discuss conditions for the formation of the disk and possible reasons for lack of any appreciable variations in most of the observed source properties induced by the change of the accretion mechanism, and conclude that the bulk of the observed X-ray emission is still likely powered by direct accretion from the wind.

  • 29. Nabizadeh, Armin
    et al.
    Tsygankov, Sergey S.
    Ji, Long
    Doroshenko, Victor
    Molkov, Sergey
    Tuo, Youli
    Zhang, Shuang-Nan
    Lu, Fan-Jun
    Zhang, Shu
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Spectral evolution of X-ray pulsar 4U 1901+03 during the 2019 outburst based on Insight-HXMT and NuSTAR observations2021Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 652, artikkel-id A89Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report on a detailed spectral analysis of emission from X-ray pulsar 4U 1901+03 using data obtained by the Insight-HXMT and NuSTAR observatories during the 2019 outburst of the source. Thanks to the extensive coverage of the outburst by Insight-HXMT, we were able to investigate the spectral evolution of the source as a function of flux, and compare these results to the previous reports, focusing on the properties of a putative absorption feature at around 10 keV. In particular, we demonstrate that the broadband X-ray continuum of 4U 1901+03 can be well described with a two-component continuum model without an absorption line at 10 keV, which casts doubt on its interpretation as a cyclotron line. The high quality of the data also allowed us to perform both phase-averaged and phase-resolved spectral analyses as a function of luminosity. Finally, we performed a detailed investigation of another absorption feature in the spectrum of the source around 30 keV recently reported in the NuSTAR data. We show that this feature appears to be significantly detected both in phase-averaged and phase-resolved spectra irrespective of the continuum model.

  • 30. Nabizadeh, Armin
    et al.
    Tsygankov, Sergey S.
    Karasev, Dmitrij I.
    Mönkkönen, Juhani
    Lutovinov, Alexander A.
    Nagirner, Dmitrij I.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Properties of the transient X-ray pulsar Swift J1816.7-1613 and its optical companion2019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, artikkel-id A198Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present results of investigation of the poorly studied X-ray pulsar Swift J1816.7-1613 during its transition from the type I outburst to the quiescent state. Our studies are based on the data obtained from X-ray observatories Swift, NuSTAR, and Chandra alongside with the latest IR data from UKIDSS/GPS and Spitzer/GLIMPSE surveys. The aim of the work is to determine the parameters of the system, namely the strength of the neutron star magnetic field and the distance to the source, which are required for the interpretation of the source behaviour in the framework of physically motivated models. No cyclotron absorption line was detected in the broad-band energy spectrum. However, the timing analysis hints at the typical for the X-ray pulsars magnetic field from a few x10(11) to a few x10(12) G. We also estimated the type of the IR-companion as a B0-2e star located at a distance of 7-13 kpc.

  • 31. Nabizadeh, Armin
    et al.
    Tsygankov, Sergey S.
    Molkov, Sergey V.
    Karasev, Dmitri I.
    Ji, Long
    Lutovinov, Alexander A.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Broad-band analysis of X-ray pulsar 2S 1845-0242022Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 657, artikkel-id A58Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present the results of a detailed investigation of the poorly studied X-ray pulsar 2S 1845−024 based on data obtained at the NuSTAR observatory during the type I outburst in 2017. Neither pulse phase-averaged nor phase-resolved spectra of the source show evidence for a cyclotron absorption feature. We also used data obtained from other X-ray observatories (Swift, XMM-Newton and Chandra) to study the spectral properties as a function of orbital phase. The analysis reveals a high hydrogen column density for the source reaching ∼1024 cm−2 around periastron. Using high-quality Chandra data we were able to obtain an accurate localization of 2S 1845−024 at RA = 18h48m16.s8 and Dec = −2°25′25.″1 (J2000), which allowed us to use infrared (IR) data to roughly classify the optical counterpart of the source as an OB supergiant at a distance of ≳15 kpc.

  • 32. Nattila, J.
    et al.
    Steiner, A. W.
    Kajava, J. J. E.
    Suleimanov, V. F.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Equation of state constraints for the cold dense matter inside neutron stars using the cooling tail method2016Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 591, artikkel-id A25Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The cooling phase of thermonuclear (type-I) X-ray bursts can be used to constrain neutron star (NS) compactness by comparing the observed cooling tracks of bursts to accurate theoretical atmosphere model calculations. By applying the so-called cooling tail method, where the information from the whole cooling track is used, we constrain the mass, radius, and distance for three different NSs in low-mass X-ray binaries 4U 1702-429, 4U 1724-307, and SAX J1810.8-260. Care is taken to use only the hard state bursts where it is thought that the NS surface alone is emitting. We then use a Markov chain Monte Carlo algorithm within a Bayesian framework to obtain a parameterized equation of state (EoS) of cold dense matter from our initial mass and radius constraints. This allows us to set limits on various nuclear parameters and to constrain an empirical pressure-density relationship for the dense matter. Our predicted EoS results in NS a radius between 10.5-12.8 km (95% confidence limits) for a mass of 1.4 M-circle dot, depending slightly on the assumed composition. Because of systematic errors and uncertainty in the composition, these results should be interpreted as lower limits for the radius.

  • 33. Papitto, A.
    et al.
    Falanga, M.
    Hermsen, W.
    Mereghetti, S.
    Kuiper, L.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Bozzo, E.
    Ambrosino, F.
    Coti Zelati, F.
    De Falco, V.
    de Martino, D.
    Di Salvo, T.
    Esposito, P.
    Ferrigno, C.
    Forot, M.
    Gotz, D.
    Gouiffes, C.
    Iaria, R.
    Laurent, P.
    Li, J.
    Li, Z.
    Mineo, T.
    Moran, P.
    Neronov, A.
    Paizis, A.
    Rea, N.
    Riggio, A.
    Sanna, A.
    Savchenko, V.
    Slowikowska, A.
    Shearer, A.
    Tiengo, A.
    Torres, D. F.
    The INTEGRAL view of the pulsating hard X-ray sky: from accreting and transitional millisecond pulsars to rotation-powered pulsars and magnetars2020Inngår i: New astronomy reviews (Print), ISSN 1387-6473, E-ISSN 1872-9630, Vol. 91, artikkel-id 101544Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In the last 25 years a new generation of X-ray satellites imparted a significant leap forward in our knowledge of X-ray pulsars. The discovery of accreting and transitional millisecond pulsars proved that disk accretion can spin up a neutron star to a very high rotation speed. The detection of MeV-GeV pulsed emission from a few hundreds of rotation-powered pulsars probed particle acceleration in the outer magnetosphere, or even beyond. Also, a population of two dozens of magnetars has emerged. INTEGRAL played a central role to achieve these results by providing instruments with high temporal resolution up to the hard X-ray/soft, gamma-ray band and a large field of view imager with good angular resolution to spot hard X-ray transients. In this article we review the main contributions by INTEGRAL to our understanding of the pulsating hard X-ray sky, such as the discovery and characterization of several accreting and transitional millisecond pulsars, the generation of the first catalog of hard X-ray/soft gamma-ray rotation-powered pulsars, the detection of polarization in the hard X-ray emission from the Crab pulsar, and the discovery of persistent hard X-ray emission from several magnetars.

  • 34.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Accurate analytic formula for light bending in Schwarzschild metric2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 640, artikkel-id A24Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We propose new analytic formulae describing light bending in the Schwarzschild metric. For an emission radii above the photon orbit at the 1.5 Schwarzschild radius, the formulae have an accuracy of better than 0.2% for the bending angle and 3% for the lensing factor for any trajectories that turn around a compact object by less than about 160 degrees. In principle, they can be applied to any emission point above the horizon of the black hole. The proposed approximation can be useful for problems involving emission from neutron stars and accretion discs around compact objects when fast accurate calculations of light bending are required. It can also be used to test the codes that compute light bending using exact expressions via elliptical integrals.

  • 35.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Relativistic rotating vector model for X-ray millisecond pulsars2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 641, artikkel-id A166Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The X-ray radiation produced on the surface of accreting magnetised neutron stars is expected to be strongly polarised. A swing of the polarisation vector with the pulsar phase gives a direct measure of the source inclination and magnetic obliquity. In the case of rapidly rotating millisecond pulsars, the relativistic motion of the emission region causes additional rotation of the polarisation plane. Here, we develop a relativistic rotating vector model, where we derive analytical expression for the polarisation angle as a function of the pulsar phase accounting for relativistic aberration and gravitational light bending in the Schwarzschild metric. We show that in the case of fast pulsars the rotation of the polarisation plane can reach tens of degrees, strongly influencing the observed shape of the polarisation angle’s phase dependence. The rotation angle grows nearly linearly with the spin rate but it is less sensitive to the neutron star radius. Overall, this angle is large even for large spots. Our results have implications with regard to the modelling of X-ray polarisation from accreting millisecond pulsars that are to be observed with the upcoming Imaging X-ray Polarimeter Explorer and the enhanced X-ray Timing and Polarimetry mission. The X-ray polarisation may improve constraints on the neutron star mass and radius coming from the pulse profile modelling.

  • 36.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Rosseland and Flux Mean Opacities for Compton Scattering2017Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 835, nr 2, artikkel-id 119Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Rosseland mean opacity plays an important role in theories of stellar evolution and X-ray burst models. In the high-temperature regime, when most of the gas is completely ionized, the opacity is dominated by Compton scattering. Our aim here is to critically evaluate previous works on this subject and to compute the exact Rosseland mean opacity for Compton scattering over a broad range of temperature and electron degeneracy parameter. We use relativistic kinetic equations for Compton scattering and compute the photon mean free path as a function of photon energy by solving the corresponding integral equation in the diffusion limit. As a byproduct we also demonstrate the way to compute photon redistribution functions in the case of degenerate electrons. We then compute the Rosseland mean opacity as a function of temperature and electron degeneracy and present useful approximate expressions. We compare our results to previous calculations and find a significant difference in the low-temperature regime and strong degeneracy. We then proceed to compute the flux mean opacity in both free-streaming and diffusion approximations, and show that the latter is nearly identical to the Rosseland mean opacity. We also provide a simple way to account for the true absorption in evaluating the Rosseland and flux mean opacities.

  • 37.
    Poutanen, Juri
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute (IKI) of the Russian Academy of Sciences, Russia.
    Veledina, Alexandra
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute (IKI) of the Russian Academy of Sciences, Russia.
    Berdyugin, Andrei
    Berdyugina, Svetlana
    Jermak, Helen
    Jonker, Peter G.
    Kajava, Jari J. E.
    Kosenkov, Ilia A.
    Kravtsov, Vadim
    Piirola, Vilppu
    Shrestha, Manisha
    Perez Torres, Manuel A.
    Tsygankov, Sergey S.
    Black hole spin-orbit misalignment in the x-ray binary MAXI J1820+0702022Inngår i: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 375, nr 6583, s. 874-876Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The observational signatures of black holes in x-ray binary systems depend on their masses, spins, accretion rate, and the misalignment angle between the black hole spin and the orbital angular momentum. We present optical polarimetric observations of the black hole x-ray binary MAXI J1820+070, from which we constrain the position angle of the binary orbital. Combining this with previous determinations of the relativistic jet orientation. which traces the black hole spin, and the inclination of the orbit, we determine a lower limit of 40 degrees on the spin-orbit misalignment angle. The misalignment must originate from either the binary evolution or black hole formation stages. If other x-ray binaries have similarly large misalignments, these would bias measurements of black hole masses and spins from x-ray observations.

  • 38.
    Poutanen, Juri
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Veledina, Alexandra
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Zdziarski, Andrzej A.
    Doughnut strikes sandwich: the geometry of hot medium in accreting black hole X-ray binaries2018Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, artikkel-id A79Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the effects of the mutual interaction of hot plasma and cold medium in black hole binaries in their hard spectral state. We consider a number of different geometries. In contrast to previous theoretical studies, we use a modern energy-conserving code for reflection and reprocessing from cold media. We show that a static corona above an accretion disc extending to the innermost stable circular orbit produces spectra not compatible with those observed. They are either too soft or require a much higher disc ionization than that observed. This conclusion confirms a number of previous findings, but disproves a recent study claiming an agreement of that model with observations. We show that the cold disc has to be truncated in order to agree with the observed spectral hardness. However, a cold disc truncated at a large radius and replaced by a hot flow produces spectra which are too hard if the only source of seed photons for Comptonization is the accretion disc. Our favourable geometry is a truncated disc coexisting with a hot plasma either overlapping with the disc or containing some cold matter within it, also including seed photons arising from cyclo-synchrotron emission of hybrid electrons, i.e. containing both thermal and non-thermal parts.

  • 39. Salganik, Alexander
    et al.
    Tsygankov, Sergey S.
    Djupvik, Anlaug A.
    Karasev, Dmitri I.
    Lutovinov, Alexander A.
    Buckley, David A. H.
    Gromadzki, Mariusz
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Space Research Institute of the Russian Academy of Sciences, Russia; University of Turku, Finland.
    On the nature of the X-ray pulsar XTE J1859+083 and its broad-band properties2022Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 509, nr 4, s. 5955-5963Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work is devoted to the study of the broad-band 0.8–79 keV spectral and timing properties of the poorly studied X-ray pulsar XTE J1859+083 during its 2015 outburst based on the data from the NuSTAR and Swift observatories. We show that the source pulse profile has complex shape that depends on the energy band. Pulse fraction of XTE J1859+083 has constant value around 35 per cent in the broad energy band, this behaviour is atypical for X-ray pulsars. At the same time its energy spectrum is typical of this class of objects and has a power-law shape with an exponential cutoff at high energies. No cyclotron absorption line was discovered in the source spectrum. On the basis of indirect method and the absence of a cyclotron line, an estimation was made for the magnetic field strength as less than 5 × 1011 G or belonging to the interval from 5 × 1012 to 2.0×1013 G. Data from the NOT and SALT telescopes as well as optical and IR sky surveys allowed us also to study the nature of its optical companion. We have proposed and studied new possible candidates for the optical companion of XTE J1859+083 and the most likely candidate was identified. The results of the optical and IR photometry and spectroscopy of these possible companions showed that the system is a Be X-ray binary, showing Brγ, He I, and strong H α spectral lines. 

  • 40. Salmi, T.
    et al.
    Nättilä, Joonas
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Institute of the Russian Academy of Sciences, Russia.
    Bayesian parameter constraints for neutron star masses and radii using X-ray timing observations of accretion-powered millisecond pulsars2018Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 618, artikkel-id A161Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a Bayesian method to constrain the masses and radii of neutron stars (NSs) using the information encoded in the X-ray pulse profiles of accreting millisecond pulsars. We model the shape of the pulses using oblate Schwarzschild approximation, which takes into account the deformed shape of the star together with the special and general relativistic corrections to the photon trajectories and angles. The spectrum of the radiation is obtained from an empirical model of Comptonization in a hot slab in which a fraction of seed black-body photons is scattered into a power-law component. By using an affine-invariant Markov chain Monte Carlo ensemble sampling method, we obtain posterior probability distributions for the different model parameters, especially for the mass and the radius. To test the robustness of our method, we first analysed self-generated synthetic data with known model parameters Similar analysis was then applied for the observations of SAX J1808.4-3658 by the Rossi X-ray Timing Explorer (RXTE). The results show that our method can reproduce the model parameters of the synthetic data, and that accurate constraints for the radius can be obtained using the RXTE pulse profile observations if the mass is a priori known. For a mass in the range 1.5-1.8 M-circle dot, the radius of the NS in SAX J1808.4-3658 is constrained between 9 and 13 km. If the mass is accurately known, the radius can be determined with an accuracy of 5% (68% credibility). For example, for the mass of 1.7 M-circle dot the equatorial radius is R-eq = 11.9(-0.4)(+0.5) km. Finally, we show that further improvements can be obtained when the X-ray polarization data from the Imaging X-ray Polarimeter Explorer will become available.

  • 41. Salmi, Tuomo
    et al.
    Loktev, Vladislav
    Korsman, Karri
    Baldini, Luca
    Tsygankov, Sergey S.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Neutron star parameter constraints for accretion-powered millisecond pulsars from the simulated IXPE data2021Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 646, artikkel-id A23Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have simulated the X-ray polarization data that can be obtained with the Imaging X-ray Polarimetry Explorer, when observing accretion-powered millisecond pulsars. We estimated the necessary exposure times for SAX J1808.4−3658 in order to obtain different accuracy in the measured time-dependent Stokes profiles integrated over all energy channels. We found that the measured relative errors strongly depend on the relative configuration of the observer and the emitting hotspot. The improvement in the minimum relative error in Stokes Q and U parameters as a function of observing time t scales as 1/√t, and it spans the range from 30–90% with a 200 ks exposure time to 20–60% with a 500 ks exposure time (in the case of data binned in 19 phase bins). The simulated data were also used to predict how accurate measurements of the geometrical parameters of the neutron star can be made when modelling only Q and U parameters, but not the flux. We found that the observer inclination and the hotspot co-latitude could be determined with better than 10° accuracy for most of the cases we considered. In addition, we show that the position of a secondary hotspot can also be constrained when the spot is not obscured by an accretion disc. These measurements can be used to further constrain the neutron star mass and radius when combined with modelling of the X-ray pulse profile.

  • 42. Salmi, Tuomo
    et al.
    Suleimanov, Valery F.
    Nättilä, Joonas
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Columbia University, USA; Flatiron Institute, USA.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Magnetospheric return-current-heated atmospheres of rotation-powered millisecond pulsars2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 641, artikkel-id A15Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We computed accurate atmosphere models of rotation-powered millisecond pulsars in which the polar caps of a neutron star (NS) are externally heated by magnetospheric return currents. The external ram pressure, energy losses, and stopping depth of the penetrating charged particles were computed self-consistently with the atmosphere model, instead of assuming a simplified deep-heated atmosphere in radiative equilibrium. We used exact Compton scattering formalism to model the properties of the emergent X-ray radiation. The deep-heating approximation was found to be valid only if most of the heat originates from ultra-relativistic bombarding particles with Lorentz factors of γ ≳ 100. In the opposite regime, the atmosphere attains a distinct two-layer structure with an overheated optically thin skin on top of an optically thick cool plasma. The overheated skin strongly modifies the emergent radiation: It produces a Compton-upscattered high-energy tail in the spectrum and alters the radiation beaming pattern from limb darkening to limb brightening for emitted hard X-rays. This kind of drastic change in the emission properties can have a significant impact on the inferred NS pulse profile parameters as performed, for example, by Neutron star Interior Composition ExploreR. Finally, the connection between the energy distribution of the return current particles and the atmosphere emission properties offers a new tool to probe the exact physics of pulsar magnetospheres.

  • 43. Salmi, Tuomo
    et al.
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars2019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, artikkel-id A39Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The aim of this work is to study the possible effects and biases on the radius constraints for rotation-powered millisecond pulsars when using Thomson approximation to describe electron scattering in the atmosphere models, instead of using exact formulation for Compton scattering. We compare the differences between the two models in the energy spectrum and angular distribution of the emitted radiation. We also analyse a self-generated, synthetic, phase-resolved energy spectrum, based on Compton atmosphere and the most X-ray luminous, rotation-powered millisecond pulsars observed by the Neutron star Interior Composition ExploreR (NICER). We derive constraints for the neutron star parameters using both the Compton and Thomson models. The results show that the method works by reproducing the correct parameters with the Compton model. However, biases are found in both the size and the temperature of the emitting hotspot, when using the Thomson model. The constraints on the radius are still not significantly changed, and therefore the Thomson model seems to be adequate if we are interested only in the radius measurements using NICER.

  • 44. Semena, Andrey N.
    et al.
    Lutovinov, Alexander A.
    Mereminskiy, Ilya A.
    Tsygankov, Sergey S.
    Shtykovsky, Andrey E.
    Molkov, Sergey V.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Space Research Institute of the Russian Academy of Sciences, Russia; University of Turku, Finland.
    Observational constraints on the magnetic field of the bright transient Be/X-ray pulsar SXP 4.782019Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 490, nr 3, s. 3355-3364Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report results of the spectral and timing analysis of the Be/X-ray pulsar SXP 4.78 using the data obtained during its recent outburst with NuSTAR, Swift, Chandra, and NICER observatories. Using an overall evolution of the system luminosity, spectral analysis, and variability power spectrum we obtain constraints on the neutron star magnetic field strength. We found a rapid evolution of the variability power spectrum during the rise of the outburst, and absence of the significant changes during the flux decay. Several low frequency quasiperiodic oscillation features are found to emerge on the different stages of the outburst, but no clear clues on their origin were found in the energy spectrum and overall flux behaviour. We use several indirect methods to estimate the magnetic field strength on the neutron star surface and found that most of them suggest magnetic field B less than or similar to 2 x 10(12) G. The strictest upper limit comes from the absence of the cyclotron absorption features in the energy spectra and suggests relatively weak magnetic field B < 6 x 10(11) G.)

  • 45. Soffitta, Paolo
    et al.
    Bucciantini, Niccolo'
    Churazov, Eugene
    Costa, Enrico
    Dovciak, Michal
    Feng, Hua
    Heyl, Jeremy
    Ingram, Adam
    Jahoda, Keith
    Kaaret, Philip
    Kallman, Timothy
    Karas, Vladimir
    Khabibullin, Ildar
    Krawczynski, Henric
    Malzac, Julien
    Marin, Frédéric
    Marshall, Herman
    Matt, Giorgio
    Muleri, Fabio
    Mundell, Carole
    Pearce, Mark
    Petrucci, Pierre-Olivier
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Romani, Roger
    Santangelo, Andrea
    Tagliaferri, Gianpiero
    Taverna, Roberto
    Turolla, Roberto
    Vink, Jacco
    Zane, Silvia
    A polarized view of the hot and violent universe2021Inngår i: Experimental astronomy, ISSN 0922-6435, E-ISSN 1572-9508, Vol. 51, nr 3, s. 1109-1141Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    X-ray polarimetry has long been considered the 'holy grail' of X-ray astronomy. Fortunately, after a silence of more than 40 years, the field is now rejuvenating. In fact, an X-ray polarimeter onboard a Cube-sat nano-satellite has been recently successfully operated. IXPE, the Imaging X-ray Polarimetry Explorer, will be launched in 2021 while eXTP, containing a larger version of IXPE, is expected to be launched in 2027. Although at present it is difficult to predict the discoveries that, given their exploratory nature, IXPE and eXTP will obtain, the path for a follow-up mission can already be envisaged. In this paper we describe the scientific goals of such a follow-up mission, and present a medium-size mission profile that can accomplish this task.

  • 46. Suleimanov, V. F.
    et al.
    Klochkov, D.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Werner, K.
    Probing the possibility of hotspots on the central neutron star in HESSJ1731-3472017Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, artikkel-id A43Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The X-ray spectra of the neutron stars located in the centers of supernova remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models. These fits yield plausible neutron star sizes for the known or estimated distances to these supernova remnants. The evidence in favor of the presence of a pure carbon envelope at the neutron star surface is rather indirect and is based on the assumption that the emission is generated uniformly by the entire stellar surface. Although this assumption is supported by the absence of pulsations, the observational upper limit on the pulsed fraction is not very stringent. In an attempt to quantify this evidence, we investigate the possibility that the observed spectrum of the neutron star in HESS J1731-347 is a combination of the spectra produced in a hydrogen atmosphere of the hotspots and of the cooler remaining part of the neutron star surface. The lack of pulsations in this case has to be explained either by a sufficiently small angle between the neutron star spin axis and the line of sight, or by a sufficiently small angular distance between the hotspots and the neutron star rotation poles. As the observed flux from a non-uniformly emitting neutron star depends on the angular distribution of the radiation emerging from the atmosphere, we have computed two new grids of pure carbon and pure hydrogen atmosphere model spectra accounting for Compton scattering. Using new hydrogen models, we have evaluated the probability of a geometry that leads to a pulsed fraction below the observed upper limit to be about 8.2%. Such a geometry thus seems to be rather improbable but cannot be excluded at this stage.

  • 47. Suleimanov, V. F.
    et al.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Space Research Institute of the Russian Academy of Sciences, Russia; University of Turku, Finland.
    Werner, K.
    Accretion heated atmospheres of X-ray bursting neutron stars2018Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, artikkel-id A114Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20-70% of the maximum luminosity. The amplitude of the deviations and the critical luminosity correlate with the persistent luminosity, which leads us to suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We computed the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the velocity of accreted particles, their temperature and the penetration angle. We show that the heated atmosphere develops two different regions. The upper one is the hot (20-100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra correspondingly have two components: a blackbody with the temperature close to that of the isothermal region and a hard Comptonized component (a power law with an exponential decay). Their relative contribution depends on the ratio of the energy dissipation rate of the accreted particles to the intrinsic flux from the neutron star surface. These spectra deviate strongly from those of undisturbed, passively cooling neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820-30.

  • 48. Suleimanov, Valery F.
    et al.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Nättilä, Joonas
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland.
    Kajava, Jari J. E.
    Revnivtsev, Mikhail G.
    Werner, Klaus
    The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii2017Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 466, nr 1, s. 906-913Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Determining neutron star (NS) radii and masses can help to understand the properties of matter at supra-nuclear densities. Thermal emission during thermonuclear X-ray bursts from NSs in low-mass X-ray binaries provides a unique opportunity to study NS parameters, because of the high fluxes, large luminosity variations and the related changes in the spectral properties. The standard cooling tail method uses hot NS atmosphere models to convert the observed spectral evolution during cooling stages of X-ray bursts to the Eddington flux F-Edd and the stellar angular size Omega. These are then translated to the constraints on the NS massMand radius R. Here we present the improved, direct cooling tail method that generalizes the standard approach. First, we adjust the cooling tail method to account for the bolometric correction to the flux. Then, we fit the observed dependence of the blackbody normalization on flux with a theoretical model directly on theM-R plane by interpolating theoretical dependences to a given gravity, hence ensuring only weakly informative priors for M and R instead of FEdd and Omega. The direct cooling method is demonstrated using a photospheric radius expansion burst from SAX J1810.8-2609, which has happened when the system was in the hard state. Comparing to the standard cooling tail method, the confidence regions are shifted by 1 sigma towards larger radii, giving R = 11.5-13.0 km at M = 1.3-1.8M(circle dot) for this NS.

  • 49. Suleimanov, Valery F.
    et al.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Werner, Klaus
    Observational appearance of rapidly rotating neutron stars: X-ray bursts, cooling tail method, and radius determination2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 639, artikkel-id A33Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Neutron stars (NSs) in low-mass X-ray binaries rotate at frequencies high enough to significantly deviate from sphericity (nu (*) similar to 200-600 Hz). First, we investigate the effects of rapid rotation on the observational appearance of a NS. We propose analytical formulae relating gravitational mass and equatorial radius of the rapidly rotating NS to the mass M and radius R of a non-rotating NS of the same baryonic mass using accurate fully relativistic computations. We assume that the NS surface emission is described by the Planck function with two different emission patterns: the isotropic intensity and that corresponding to the electron-scattering dominated atmosphere. For these two cases we compute spectra from an oblate rotating NS observed at different inclination angles using the modified oblate Schwarzschild approximation, where light bending is computed in Schwarzschild metric, but frame dragging and quadrupole moment of a NS are approximately accounted for in the photon redshift calculations. In particular, we determine the solid angle at which a rotating NS is seen by a distant observer, the observed colour temperature and the blackbody normalization. Then, we investigate how rapid rotation affects the results of NS radius determination using the cooling tail method applied to the X-ray burst spectral evolution. We approximate the local spectra from the NS surface by a diluted blackbody with the luminosity-dependent dilution factor using previously computed NS atmosphere models. We then generalize the cooling tail method to the case of a rapidly rotating NS to obtain the most probable values of M and R of the corresponding non-rotating NS with the same baryonic mass. We show that the NS radius could be overestimated by 3-3.5 km for face-on stars of R approximate to 11 km rotating at nu (*)= 700 Hz if the version of the cooling tail method for a non-rotating NS is used. We apply the method to an X-ray burst observed from the NS rotating at nu (*)approximate to 532 Hz in SAX J1810.8-2609. The resulting radius of the non-rotating NS (assuming M=1.5 M-circle dot) becomes 11.8 +/- 0.5 km if it is viewed at inclination i=60 degrees and R=11.2 +/- 0.5 km for a face-on view, which are smaller by 0.6 and 1.2 km than the radius obtained using standard cooling tail method ignoring rotation. The corresponding equatorial radii of these rapidly rotating NSs are 12.3 +/- 0.6 km (for i=60 degrees) and 11.6 +/- 0.6 km (for i=0 degrees).

  • 50. Sánchez-Fernández, C.
    et al.
    Kajava, J. J. E.
    Poutanen, Juri
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Kuulkers, E.
    Suleimanov, V. F.
    Burst-induced coronal cooling in GS 1826-24 The clock wagging its tail2020Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 634, artikkel-id A58Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Type I X-ray bursts in GS 1826-24, and in several other systems, may induce cooling of the hot inner accretion flow that surrounds the bursting neutron star. Given that GS 1826-24 remained persistently in the hard state over the period 2003-2008 and presented regular bursting properties, we stacked the spectra of the X-ray bursts detected by INTEGRAL (JEM-X and ISGRI) and XMM-Newton (RGS) during that period to study the effect of the burst photons on the properties of the Comptonizing medium. The extended energy range provided by these instruments allows the simultaneous observation of the burst and persistent emission spectra. We detect an overall change in the shape of the persistent emission spectrum in response to the burst photon shower. For the first time, we observe simultaneously a drop in the hard X-ray emission, together with a soft X-ray excess with respect to the burst blackbody emission. The hard X-ray drop can be explained by burst-induced coronal cooling, while the bulk of the soft X-ray excess can be described by fitting the burst emission with an atmosphere model, instead of a simple blackbody model. Traditionally, the persistent emission was assumed to be invariant during X-ray bursts, and more recently to change only in normalization but not in spectral shape; the observed change in the persistent emission level during X-ray bursts may thus trigger the revision of existing neutron star mass-radius constraints, as the derived values rely on the assumption that the persistent emission does not change during X-ray bursts. The traditional burst fitting technique leads to up to a 10% overestimation of the bolometric burst flux in GS 1826-24, which significantly hampers the comparisons of the KEPLER and MESA model against this textbook burster.

12 1 - 50 of 63
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf