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  • 1.
    Chashkina, Anna
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; Moscow State University, Russia; University of California, USA.
    Abolmasov, Pavel
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; University of California, USA.
    Super-Eddington accretion on to a magnetized neutron star2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, no 3, p. 2799-2813Article in journal (Refereed)
    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.

  • 2. Chashkina, Anna
    et al.
    Lipunova, Galina
    Abolmasov, Pavel
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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 stars2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A18Article in journal (Refereed)
    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.

  • 3. De Falco, V.
    et al.
    Kuiper, L.
    Bozzo, E.
    Ferrigno, C.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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-rays2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 603, article id A16Article in journal (Refereed)
    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.

  • 4. Doroshenko, Victor
    et al.
    Tsygankov, Sergey S.
    Mushtukov, Alexander A.
    Lutovinov, Alexander A.
    Santangelo, Andrea
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+532017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 466, no 2, p. 2143-2150Article in journal (Refereed)
    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.

  • 5. Kajava, J. J. E.
    et al.
    Sánchez-Fernández, C.
    Kuulkers, E.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    X-ray burst-induced spectral variability in 4U 1728-342017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, article id A89Article in journal (Refereed)
    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.

  • 6.
    Kosenkov, Ilia A.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (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
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; University of California, USA.
    High-precision optical polarimetry of the accreting black hole V404 Cyg during the 2015 June outburst2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 4, p. 4362-4373Article in journal (Refereed)
    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.

  • 7. Krivonos, Roman
    et al.
    Clavel, Maica
    Hong, JaeSub
    Mori, Kaya
    Ponti, Gabriele
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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 cloud2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 3, p. 2822-2835Article in journal (Refereed)
    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.

  • 8. Krivonos, Roman
    et al.
    Sazonov, Sergey
    Tsygankov, Sergey S.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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?2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, no 2, p. 2357-2364Article in journal (Refereed)
    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.

  • 9. Li, Z.
    et al.
    De Falco, V.
    Falanga, M.
    Bozzo, E.
    Kuiper, L.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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 outburst2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, article id A114Article in journal (Refereed)
    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.

  • 10. Li, Zhaosheng
    et al.
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Space Research Institute of the Russian Academy of Sciences, Russia; University of Turku, Finland.
    Salmi, Tuomo
    Falanga, Maurizio
    Nättilä, Joonas
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Xu, Renxin
    Evidence for the Photoionization Absorption Edge in a Photospheric Radius Expansion X-Ray Burst from GRS 1747-312 in Terzan 62018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, no 1, article id 53Article in journal (Refereed)
    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.

  • 11. Lutovinov, Alexander A.
    et al.
    Tsygankov, Sergey S.
    Krivonos, Roman A.
    Molkov, Sergey V.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    PROPELLER EFFECT IN THE TRANSIENT X-RAY PULSAR SMC X-22017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 834, no 2, article id 209Article in journal (Refereed)
    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.

  • 12. Mereminskiy, Ilya A.
    et al.
    Semena, Andrey N.
    Bykov, Sergey D.
    Filippova, Ekaterina V.
    Lutovinov, Alexander A.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Russian Academy of Sciences, Russia; University of Turku, Finland.
    Studying temporal variability of GRS 1739-278 during the 2014 outburst2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, no 1, p. 1392-1405Article in journal (Refereed)
    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.

  • 13. Mushtukov, Alexander A.
    et al.
    Nagirner, Dmitrij I.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Compton scattering S matrix and cross section in strong magnetic field2016In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 93, no 10, article id 105003Article in journal (Refereed)
    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.

  • 14. Mönkkönen, Juhani
    et al.
    Tsygankov, Sergey S.
    Mushtukov, Alexander A.
    Doroshenko, Victor
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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 analysis2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A106Article in journal (Refereed)
    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.

  • 15. Nabizadeh, Armin
    et al.
    Tsygankov, Sergey S.
    Karasev, Dmitrij I.
    Mönkkönen, Juhani
    Lutovinov, Alexander A.
    Nagirner, Dmitrij I.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; Russian Academy of Sciences, Russia.
    Properties of the transient X-ray pulsar Swift J1816.7-1613 and its optical companion2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, article id A198Article in journal (Refereed)
    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.

  • 16. Nattila, J.
    et al.
    Steiner, A. W.
    Kajava, J. J. E.
    Suleimanov, V. F.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Equation of state constraints for the cold dense matter inside neutron stars using the cooling tail method2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 591, article id A25Article in journal (Refereed)
    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.

  • 17.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Rosseland and Flux Mean Opacities for Compton Scattering2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 835, no 2, article id 119Article in journal (Refereed)
    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.

  • 18.
    Poutanen, Juri
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Veledina, Alexandra
    Stockholm University, Nordic Institute for Theoretical Physics (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 binaries2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id A79Article in journal (Refereed)
    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.

  • 19. Salmi, Tuomo
    et al.
    Suleimanov, Valery F.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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 pulsars2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, article id A39Article in journal (Refereed)
    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.

  • 20. Suleimanov, V. F.
    et al.
    Klochkov, D.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Werner, K.
    Probing the possibility of hotspots on the central neutron star in HESSJ1731-3472017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, article id A43Article in journal (Refereed)
    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.

  • 21. Suleimanov, V. F.
    et al.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (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 stars2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id A114Article in journal (Refereed)
    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.

  • 22. Suleimanov, Valery F.
    et al.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Nättilä, Joonas
    Stockholm University, Nordic Institute for Theoretical Physics (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 radii2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 466, no 1, p. 906-913Article in journal (Refereed)
    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.

  • 23. Tsygankov, S. S.
    et al.
    Doroshenko, V.
    Lutovinov, A. A.
    Mushtukov, A. A.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    SMC X-3: the closest ultraluminous X-ray source powered by a neutron star with non-dipole magnetic field2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, article id A39Article in journal (Refereed)
    Abstract [en]

    Aims. The magnetic field of accreting neutron stars determines their overall behavior including the maximum possible luminosity. Some models require an above-average magnetic field strength (greater than or similar to 10(13) G) in order to explain super-Eddington mass accretion rate in the recently discovered class of pulsating ultraluminous X-ray sources (ULX). The peak luminosity of SMCX-3 during its major outburst in 2016-2017 reached similar to 2.5x10(39) erg s(-1) comparable to that in ULXs thus making this source the nearest ULX-pulsar. Determination of the magnetic field of SMCX-3 is the main goal of this paper.

    Methods. SMCX-3 belongs to the class of transient X-ray pulsars with Be optical companions, and exhibited a giant outburst in July 2016-March 2017. The source has been observed over the entire outburst with the Swift/XRT and Fermi/GBM telescopes, as well as the NuSTAR observatory. Collected data allowed us to estimate the magnetic field strength of the neutron star in SMCX-3 using several independent methods.

    Results. Spin evolution of the source during and between the outbursts, and the luminosity of the transition to the so-called propeller regime in the range of (0.3-7) x 10(35) erg s(-1) imply a relatively weak dipole field of (1-5) x 10(12) G. On the other hand, there is also evidence for a much stronger field in the immediate vicinity of the neutron star surface. In particular, transition from super-to sub-critical accretion regime associated with the cease of the accretion column and very high peak luminosity favor a field that is an order of magnitude stronger. This discrepancy makes SMCX-3 a good candidate for possessing significant non-dipolar components of the field, and an intermediate source between classical X-ray pulsars and accreting magnetars which may constitute an appreciable fraction of ULX population.

  • 24. Tsygankov, S. S.
    et al.
    Lutovinov, A. A.
    Doroshenko, V.
    Mushtukov, A. A.
    Suleimanov, V.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and V 0332+532016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 593, article id A16Article in journal (Refereed)
    Abstract [en]

    Aims. We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well-known X-ray pulsars 4U 0115+63 and V 0332+53.

    Methods. Both sources form binary systems with Be optical companions and undergo so-called giant outbursts every 3 4 years. The current observational campaigns were performed with the Swift/XRT telescope in the soft X-ray band (0.5 10 keV) during the declining phases of the outbursts exhibited by both sources in 2015.

    Results. The transitions to the propeller regime were detected at the threshold luminosities of (1 : 4 +/- 0 : 4) x 10(36) erg s(-1) and (2 : 0 +/- 0 : 4) x 10(36) erg s(-1) for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into the propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star.

  • 25. Tsygankov, Sergey S.
    et al.
    Doroshenko, Victor
    Mushtukov, Alexander A.
    Lutovinov, Alexander A.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    On the magnetic field of the first Galactic ultraluminous X-ray pulsar Swift J0243.6+61242018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 479, no 1, p. L134-L138Article in journal (Refereed)
    Abstract [en]

    We report on the monitoring of the final stage of the outburst from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124, which reached similar to 40 Eddington luminosities. The main aim of the monitoring program with the Swift/XRT telescope was to measure the magnetic field of the neutron star using the luminosity of transition to the 'propeller' state. The visibility constraints, unfortunately, did not permit us to observe the source down to the fluxes low enough to detect such a transition. The tight upper limit on the propeller luminosity L-prop < 6.8x10(35) erg s(-1) implies the dipole component of the magnetic field B< 10(13) G. On the other hand, the observed evolution of the pulse profile and of the pulsed fraction with flux points to a change of the emission region geometry at the critical luminosity L-crit similar to 3 x 10(38) erg s(-1) both in the rising and declining parts of the outburst. We associate the observed change with the onset of the accretion column, which allows us to get an independent estimate of the magnetic field strength close to the neutron stars surface of B > 10(13) G. Given the existing uncertainty in the effective magnetosphere size, we conclude that both estimates are marginally compatible with each other.

  • 26. Tsygankov, Sergey S.
    et al.
    Lutovinov, Alexander A.
    Krivonos, Roman A.
    Molkov, Sergey V.
    Jenke, Peter J.
    Finger, Mark H.
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    NuSTAR discovery of a cyclotron absorption line in the transient X-ray pulsar 2S 1553-5422016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 457, no 1, p. 258-266Article in journal (Refereed)
    Abstract [en]

    We report the results of a spectral and timing analysis of the poorly studied transient X-ray pulsar 2S 1553-542 using data collected with the NuSTAR and Chandra observatories and the Fermi/GBM instrument during an outburst in 2015. The properties of the source at high energies (>30 keV) are studied for the first time and the sky position has been essentially improved. The source broad-band spectrum has a quite complicated shape and can be reasonably described by a composite model with two continuum components - a blackbody emission with the temperature about 1 keV at low energies and a power law with an exponential cut-off at high energies. Additionally, an absorption feature at similar to 23.5 keV is discovered both in phase-averaged and phase-resolved spectra and interpreted as the cyclotron resonance scattering feature corresponding to the magnetic field strength of the neutron star B similar to 3 x 10(12) G. Based on the Fermi/ GBM data, the orbital parameters of the system were substantially improved, which allowed us to determine the spin period of the neutron star P = 9.27880(3) s and a local spin-up. (P) over dot similar or equal to -7.5 x 10(-10) s s(-1) due to the mass accretion during the NuSTAR observations. Assuming accretion from the disc and using standard torque models, we estimated the distance to the system as d = 20 +/- 4 kpc.

  • 27. Tsygankov, Sergey S.
    et al.
    Wijnands, Rudy
    Lutovinov, Alexander A.
    Degenaar, Nathalie
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; University of California, USA.
    The X-ray properties of Be/X-ray pulsars in quiescence2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, no 1, p. 126-141Article in journal (Refereed)
    Abstract [en]

    Observations of accreting neutron stars (NSs) with strong magnetic fields can be used not only for studying the accretion flow interaction with the NS magnetospheres, but also for understanding the physical processes inside NSs and for estimating their fundamental parameters. Of particular interest are (i) the interaction of a rotating NS (magnetosphere) with the infalling matter at different accretion rates, and (ii) the theory of deep crustal heating and the influence of a strong magnetic field on this process. Here, we present results of the first systematic investigation of 16 X-ray pulsars with Be optical companions during their quiescent states, based on data from the Chandra, XMM-Newton and Swift observatories. The whole sample of sources can be roughly divided into two distinct groups: (i) relatively bright objects with a luminosity around similar to 10(34) erg s(-1) and (hard) power-law spectra, and (ii) fainter ones showing thermal spectra. X-ray pulsations were detected from five objects in group (i) with quite a large pulse fraction of 50-70 per cent. The obtained results are discussed within the framework of the models describing the interaction of the infalling matter with the NS magnetic field and those describing heating and cooling in accreting NSs.

  • 28.
    Veledina, Alexandra
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Berdyugin, Andrei
    Kosenkov, Ilia A.
    Kajava, Jari J. E.
    Tsygankov, Sergey S.
    Piirola, Vilppu
    Berdyugin, Svetlana
    Sakanoi, Takeshi
    Kagitani, Masato
    Kravtsov, Vadim
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland; Space Research Institute of the Russian Academy of Sciences, Russia.
    Evolving optical polarisation of the black hole X-ray binary MAXI J1820+0702019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A75Article in journal (Refereed)
    Abstract [en]

    Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, similar to 0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between similar to 0.3% and 0.7%, and the polarisation position angle is between similar to 10 degrees-30 degrees. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.

  • 29. Watts, Anna L.
    et al.
    Yu, WenFei
    Poutanen, Juri
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, .
    Zhang, Shu
    Bhattacharyya, Sudip
    Bogdanov, Slavko
    Ji, Long
    Patruno, Alessandro
    Riley, Thomas E.
    Bakala, Pavel
    Baykal, Altan
    Bernardini, Federico
    Bombaci, Ignazio
    Brown, Edward
    Cavecchi, Yuri
    Chakrabarty, Deepto
    Chenevez, Jerome
    Degenaar, Nathalie
    Del Santo, Melania
    Di Salvo, Tiziana
    Doroshenko, Victor
    Falanga, Maurizio
    Ferdman, Robert D.
    Feroci, Marco
    Gambino, Angelo F.
    Ge, MingYu
    Greif, Svenja K.
    Guillot, Sebastien
    Gungor, Can
    Hartmann, Dieter H.
    Hebeler, Kai
    Heger, Alexander
    Homan, Jeroen
    Iaria, Rosario
    in 't Zand, Jean
    Kargaltsev, Oleg
    Kurkela, Aleksi
    Lai, XiaoYu
    Li, Ang
    Li, XiangDong
    Li, ZhaoSheng
    Linares, Manuel
    Lu, FangJun
    Mahmoodifar, Simin
    Mendez, Mariano
    Miller, M. Coleman
    Morsink, Sharon
    Nättilä, Joonas
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Turku, Finland.
    Possenti, Andrea
    Prescod-Weinstein, Chanda
    Qu, JinLu
    Riggio, Alessandro
    Salmi, Tuomo
    Sanna, Andrea
    Santangelo, Andrea
    Schatz, Hendrik
    Schwenk, Achim
    Song, LiMing
    Sramkova, Eva
    Stappers, Benjamin
    Stiele, Holger
    Strohmayer, Tod
    Tews, Ingo
    Tolos, Laura
    Torok, Gabriel
    Tsang, David
    Urbanec, Martin
    Vacchi, Andrea
    Xu, RenXin
    Xu, YuPeng
    Zane, Silvia
    Zhang, GuoBao
    Zhang, ShuangNan
    Zhang, WenDa
    Zheng, ShiJie
    Zhou, Xia
    Dense matter with eXTP2019In: Science China Physics, Mechanics & Astronomy, ISSN 1674-7348, Vol. 62, no 2, article id 029503Article, review/survey (Refereed)
    Abstract [en]

    In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to be launched in the mid 2020s.

1 - 29 of 29
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