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Dickinson, Hugh J.
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Publications (10 of 44) Show all publications
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Uchiyama, Y. (2015). Probing the gamma-ray emission from HESS J1834-087 using HESS and Fermi LAT observations. Astronomy and Astrophysics, 574, Article ID A27.
Open this publication in new window or tab >>Probing the gamma-ray emission from HESS J1834-087 using HESS and Fermi LAT observations
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2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 574, article id A27Article in journal (Refereed) Published
Abstract [en]

Aims. Previous observations with the High Energy Stereoscopic System (H.E.S.S.) have revealed an extended very-high-energy (VHE; E > 100 GeV) gamma-ray source, HESS J1834-087, coincident with the supernova remnant (SNR) W41. The origin of the gamma-ray emission was investigated in more detail with the H.E.S.S. array and the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. Methods. The gamma-ray data provided by 61 h of observations with H.E.S.S., and four years with the Fermi LAT were analyzed, covering over five decades in energy from 1.8 GeV up to 30 TeV. The morphology and spectrum of the TeV and GeV sources were studied and multiwavelength data were used to investigate the origin of the gamma-ray emission toward W41. Results. The TeV source can be modeled with a sum of two components: one point-like and one significantly extended (sigma(TeV) = 0.17 degrees +/- 0.01 degrees), both centered on SNR W41 and exhibiting spectra described by a power law with index Gamma(TeV) similar or equal to 2.6. The GeV source detected with Fermi LAT is extended (sigma(GeV) = 0.15 degrees +/- 0.03 degrees) and morphologically matches the VHE emission. Its spectrum can be described by a power-law model with an index Gamma(GeV) = 2.15 +/- 0.12 and smoothly joins the spectrum of the whole TeV source. A break appears in the gamma-ray spectra around 100 GeV. No pulsations were found in the GeV range. Conclusions. Two main scenarios are proposed to explain the observed emission: a pulsar wind nebula (PWN) or the interaction of SNR W41 with an associated molecular cloud. X-ray observations suggest the presence of a point-like source (a pulsar candidate) near the center of the remnant and nonthermal X-ray diffuse emission that could arise from the possibly associated PWN. The PWN scenario is supported by the compatible positions of the TeV and GeV sources with the putative pulsar. However, the spectral energy distribution from radio to gamma-rays is reproduced by a one-zone leptonic model only if an excess of low-energy electrons is injected following a Maxwellian distribution by a pulsar with a high spin-down power (> 10(37) erg s(-1)). This additional low-energy component is not needed if we consider that the point-like TeV source is unrelated to the extended GeV and TeV sources. The interacting SNR scenario is supported by the spatial coincidence between the gamma-ray sources, the detection of OH (1720 MHz) maser lines, and the hadronic modeling.

Keywords
acceleration of particles, ISM: supernova remnants, ISM: clouds, cosmic rays
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-115931 (URN)10.1051/0004-6361/201322694 (DOI)000349467000027 ()2-s2.0-84921455684 (Scopus ID)
Note

AuthorCount:228;

Available from: 2015-04-14 Created: 2015-04-08 Last updated: 2022-10-14Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Horan, D. (2015). The high-energy gamma-ray emission of AP Librae. Astronomy and Astrophysics, 573, Article ID A31.
Open this publication in new window or tab >>The high-energy gamma-ray emission of AP Librae
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2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 573, article id A31Article in journal (Refereed) Published
Abstract [en]

The gamma-ray spectrum of the low-frequency-peaked BL Lac (LBL) object AP Librae is studied, following the discovery of very-high-energy (VHE; E > 100 GeV) gamma-ray emission up to the TeV range by the H.E.S.S. experiment. Thismakes AP Librae one of the few VHE emitters of the LBL type. The measured spectrum yields a flux of (8.8 +/- 1.5(stat) +/- 1.8(sys)) x 10(-12) cm(-2) s(-1) above 130 GeV and a spectral index of Gamma = 2.65 +/- 0.19(stat) +/- 0.20(sys). This study also makes use of Fermi-LAT observations in the high energy (HE, E > 100 MeV) range, providing the longest continuous light curve (5 years) ever published on this source. The source underwent a flaring event between MJD 56 306-56 376 in the HE range, with a flux increase of a factor of 3.5 in the 14 day bin light curve and no significant variation in spectral shape with respect to the low-flux state. While the H.E.S.S. and (low state) Fermi-LAT fluxes are in good agreement where they overlap, a spectral curvature between the steep VHE spectrum and the Fermi-LAT spectrum is observed. The maximum of the gamma-ray emission in the spectral energy distribution is located below the GeV energy range.

Keywords
galaxies: active, BL Lacertae objects: individual: AP Librae, gamma rays: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-115466 (URN)10.1051/0004-6361/201321436 (DOI)000346901300001 ()2-s2.0-84936998181 (Scopus ID)
Note

AuthorCount:223;

Available from: 2015-03-26 Created: 2015-03-24 Last updated: 2022-10-17Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Zechlin, H.-S. -. (2014). Flux upper limits for 47 AGN observed with HESS in 2004-2011. Astronomy and Astrophysics, 564, A9
Open this publication in new window or tab >>Flux upper limits for 47 AGN observed with HESS in 2004-2011
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 564, p. A9-Article in journal (Refereed) Published
Abstract [en]

Context. About 40% of the observation time of the High Energy Stereoscopic System (H.E.S.S.) is dedicated to studying active galactic nuclei (AGN), with the aim of increasing the sample of known extragalactic very-high-energy (VHE, E > 100 GeV) sources and constraining the physical processes at play in potential emitters. Aims. H.E.S.S. observations of AGN, spanning a period from April 2004 to December 2011, are investigated to constrain their gamma-ray fluxes. Only the 47 sources without significant excess detected at the position of the targets are presented. Methods. Upper limits on VHE fluxes of the targets were computed and a search for variability was performed on the nightly time scale. Results. For 41 objects, the flux upper limits we derived are the most constraining reported to date. These constraints at VHE are compared with the flux level expected from extrapolations of Fermi-LAT measurements in the two-year catalog of AGN. The H.E.S.S. upper limits are at least a factor of two lower than the extrapolated Fermi-LAT fluxes for 11 objects Taking into account the attenuation by the extragalactic background light reduces the tension for all but two of them, suggesting intrinsic curvature in the high-energy spectra of these two AGN. Conclusions. Compilation efforts led by current VHE instruments are of critical importance for target-selection strategies before the advent of the Cherenkov Telescope Array (CTA).

Keywords
gamma rays: galaxies, galaxies: active
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-103960 (URN)10.1051/0004-6361/201322897 (DOI)000334671000009 ()2-s2.0-84897468203 (Scopus ID)
Note

AuthorCount:219;

Available from: 2014-06-09 Created: 2014-05-27 Last updated: 2022-10-11Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Zechlin, H.-S. -. (2014). HESS J1640-465-an exceptionally luminous TeV gamma-ray supernova remnant. Monthly notices of the Royal Astronomical Society, 439(3), 2828-2836
Open this publication in new window or tab >>HESS J1640-465-an exceptionally luminous TeV gamma-ray supernova remnant
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2014 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 439, no 3, p. 2828-2836Article in journal (Refereed) Published
Abstract [en]

The results of follow-up observations of the TeV gamma-ray source HESS J1640-465 from 2004 to 2011 with the High Energy Stereoscopic System (HESS) are reported in this work. The spectrum is well described by an exponential cut-off power law with photon index Gamma = 2.11 +/- 0.09(stat) +/- 0.10(sys), and a cut-off energy of E-2 = 6.0(-1.2)(+2.0) TeV. The TeV emission is significantly extended and overlaps with the northwestern part of the shell of the SNR G338.3-0.0. The new HESS results, a re-analysis of archival XMM-Newton data and multiwavelength observations suggest that a significant part of the gamma-ray emission from HESS J1640-465 originates in the supernova remnant shell. In a hadronic scenario, as suggested by the smooth connection of the GeV and TeV spectra, the product of total proton energy and mean target density could be as high as W(p)n(H) similar to 4 x 10(52)(d/10kpc)(2) erg cm(-3).

Keywords
radiation mechanisms: non-thermal, ISM: individual objects: G338.3-0.0, ISM: supernova remnants
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-104568 (URN)10.1093/mnras/stu139 (DOI)000334114900046 ()2-s2.0-84896446798 (Scopus ID)
Note

AuthorCount:220;

Available from: 2014-06-12 Created: 2014-06-11 Last updated: 2022-10-11Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Zechlin, H.-S. -. (2014). HESS J1818-154, a new composite supernova remnant discovered in TeV gamma rays and X-rays. Astronomy and Astrophysics, 562, 562
Open this publication in new window or tab >>HESS J1818-154, a new composite supernova remnant discovered in TeV gamma rays and X-rays
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 562, p. 562-Article in journal (Refereed) Published
Abstract [en]

Composite supernova remnants (SNRs) constitute a small subclass of the remnants of massive stellar explosions where non-thermal radiation is observed from both the expanding shell-like shock front and from a pulsar wind nebula (PWN) located inside of the SNR. These systems represent a unique evolutionary phase of SNRs where observations in the radio, X-ray, and gamma-ray regimes allow the study of the co-evolution of both these energetic phenomena. In this article, we report results from observations of the shell-type SNR G15.4+0.1 performed with the High Energy Stereoscopic System (H. E. S. S.) and XMM-Newton. A compact TeV gamma-ray source, HESS J1818-154, located in the center and contained within the shell of G15.4+0.1 is detected by H. E. S. S. and featurs a spectrum best represented by a power-law model with a spectral index of -2.3 +/- 0.3(stat) +/- 0.2(sys) and an integral flux of F(>0.42 TeV) = (0.9 +/- 0.3(stat) +/- 0.2(sys)) x 10(-12) cm(-2) s(-1). Furthermore, a recent observation with XMM-Newton reveals extended X-ray emission strongly peaked in the center of G15.4+0.1. The X-ray source shows indications of an energy-dependent morphology featuring a compact core at energies above 4 keV and more extended emission that fills the entire region within the SNR at lower energies. Together, the X-ray and VHE gamma-ray emission provide strong evidence of a PWN located inside the shell of G15.4+0.1 and this SNR can therefore be classified as a composite based on these observations. The radio, X-ray, and gamma-ray emission from the PWN is compatible with a one-zone leptonic model that requires a low average magnetic field inside the emission region. An unambiguous counterpart to the putative pulsar, which is thought to power the PWN, has been detected neither in radio nor in X-ray observations of G15.4+0.1.

Keywords
X-rays: individuals: G15.4+0.1, gamma rays: general, methods: observational, supernovae: individual: HESS J1818-154, X-rays: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-102980 (URN)10.1051/0004-6361/201322914 (DOI)000332161800133 ()2-s2.0-84893848704 (Scopus ID)
Note

AuthorCount:219;

Available from: 2014-04-28 Created: 2014-04-25 Last updated: 2022-10-11Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Zechlin, H.-S. -. (2014). HESS observations of the Crab during its March 2013 GeV gamma-ray flare. Astronomy and Astrophysics, 562
Open this publication in new window or tab >>HESS observations of the Crab during its March 2013 GeV gamma-ray flare
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 562Article in journal (Refereed) Published
Abstract [en]

Context. On March 4, 2013 the Fermi-EAT and AGILE reported a flare from the direction of the Crab nebula in which the high-energy (HE; E > 100 MeV) flux was six times above its quiescent level. Simultaneous observations in other energy bands give us hints about the emission processes during the flare episode and the physics of pulsar wind nebulae in general. Aims. We search for variability in the emission of the Crab nebula at very-high energies (VHF,; E > 100 GeV), using contemporaneous data taken with the H.E.S.S. array of Cherenkov telescopes. Methods. Observational data taken with the H.E.S.S. instrument on five consecutive days during the flare were analysed for the flux and spectral shape of the emission from the Crab nebula. Night-wise light curves are presented with energy thresholds of 1 TeV and 5 TeV. Results. The observations conducted with H.E.S.S. on March 6 to March 10, 2013 show no significant changes in the flux. They limit the variation in the integral flux above 1 TeV to less than 63% and the integral flux above 5 TeV to less than 78% at a 95% confidence level.

Keywords
gamma rays: ISM, ISM: individual objects: Crab nebula, radiation mechanisms: non-thermal, relativistic processes
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-102982 (URN)10.1051/0004-6361/201323013 (DOI)000332161800140 ()2-s2.0-84893853075 (Scopus ID)
Note

AuthorCount:220;

Available from: 2014-04-28 Created: 2014-04-25 Last updated: 2022-10-11Bibliographically approved
Conrad, J., Dickinson, H. J. & Farnier, C. (2014). LONG-TERM TeV AND X-RAY OBSERVATIONS OF THE GAMMA- RAY BINARY HESS J0632+057. Astrophysical Journal, 780(2), 168
Open this publication in new window or tab >>LONG-TERM TeV AND X-RAY OBSERVATIONS OF THE GAMMA- RAY BINARY HESS J0632+057
2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 780, no 2, p. 168-Article in journal (Refereed) Published
Abstract [en]

HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observations with ground-based observatories in both the northern and southern hemispheres. Here we report on long-term observations of HESS J0632+057 conducted with the Very Energetic Radiation Imaging Telescope Array System and High Energy Stereoscopic System Cherenkov telescopes and the X-ray satellite Swift, spanning a time range from 2004 to 2012 and covering most of the system's orbit. The very-high-energy (VHE) emission is found to be variable and is correlated with that at X-ray energies. An orbital period of 315(-4)(+6) days is derived from the X-ray data set, which is compatible with previous results, P = (321 +/- 5) days. The VHE light curve shows a distinct maximum at orbital phases close to 0.3, or about 100 days after periastron passage, which coincides with the periodic enhancement of the X-ray emission. Furthermore, the analysis of the TeV data shows for the first time a statistically significant (> 6.5 sigma) detection at orbital phases 0.6-0.9. The obtained gamma-ray and X-ray light curves and the correlation of the source emission at these two energy bands are discussed in the context of the recent ephemeris obtained for the system. Our results are compared to those reported for other gamma-ray binaries.

Keywords
acceleration of particles, binaries: general, gamma rays: general(HESS J0632+057, VER J0633+057)
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-100860 (URN)10.1088/0004-637X/780/2/168 (DOI)000329097100055 ()2-s2.0-84891124368 (Scopus ID)
Note

AuthorCount:299;

Available from: 2014-02-20 Created: 2014-02-17 Last updated: 2022-10-12Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Zechlin, H.-S. (2014). Search for TeV Gamma-ray Emission from GRB 100621A, an extremely bright GRB in X-rays, with HESS. Astronomy and Astrophysics, 565, A16
Open this publication in new window or tab >>Search for TeV Gamma-ray Emission from GRB 100621A, an extremely bright GRB in X-rays, with HESS
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 565, p. A16-Article in journal (Refereed) Published
Abstract [en]

The long gamma-ray burst (GRB) 100621A, at the time the brightest X-ray transient ever detected by Swift-XRT in the 0.3-10 keV range, has been observed with the H.E.S.S. imaging air Cherenkov telescope array, sensitive to gamma radiation in the very-high-energy (VHE, >100 GeV) regime. Due to its relatively small redshift of z similar to 0.5, the favourable position in the southern sky and the relatively short follow-up time (<700 s after the satellite trigger) of the H.E.S.S. observations, this GRB could be within the sensitivity reach of the HESS. instrument. The analysis of the HESS. data shows no indication of emission and yields an integral flux upper limit above similar to 380 GeV of 4.2 x 10(-12) cm(-2) s(-1) s (95% confidence level), assuming a simple Band function extension model. A comparison to a spectral-temporal model, normalised to the prompt flux at sub-MeV energies, constraints the existence of a temporally extended and strong additional hard power law, as has been observed in the other bright X-ray GRB 130427A. A comparison between the HESS. upper limit and the contemporaneous energy output in X-rays constrains the ratio between the X-ray and VHE gamma-ray fluxes to be greater than 0.4. This value is an important quantity for modelling the afterglow and can constrain leptonic emission scenarios, where leptons are responsible for the X-ray emission and might produce VHE gamma rays.

Keywords
gamma rays: general, gamma-ray burst: individual: GRB 100621A, gamma rays: stars, X-rays: stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-105874 (URN)10.1051/0004-6361/201322984 (DOI)000336730900016 ()2-s2.0-84899126766 (Scopus ID)
Note

AuthorCount:215;

Available from: 2014-07-08 Created: 2014-07-07 Last updated: 2022-10-13Bibliographically approved
Abramowski, A., Aharonian, F., Benkhali, F. A., Akhperjanian, A. G., Anguener, E., Anton, G., . . . Zechlin, H.-S. -. (2014). TeV gamma-ray observations of the young synchrotron-dominated SNRs G1.9+0.3 and G330.2+1.0 with HESS. Monthly notices of the Royal Astronomical Society, 441(1), 790-799
Open this publication in new window or tab >>TeV gamma-ray observations of the young synchrotron-dominated SNRs G1.9+0.3 and G330.2+1.0 with HESS
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2014 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 441, no 1, p. 790-799Article in journal (Refereed) Published
Abstract [en]

The non-thermal nature of the X-ray emission from the shell-type supernova remnants (SNRs) G1.9+0.3 and G330.2+1.0 is an indication of intense particle acceleration in the shock fronts of both objects. This suggests that the SNRs are prime candidates for very-high-energy (VHE; E > 0.1 TeV) gamma-ray observations. G1.9+0.3, recently established as the youngest known SNR in the Galaxy, also offers a unique opportunity to study the earliest stages of SNR evolution in the VHE domain. The purpose of this work is to probe the level of VHE gamma-ray emission from both SNRs and use this to constrain their physical properties. Observations were conducted with the H. E. S. S. (High Energy Stereoscopic System) Cherenkov Telescope Array over a more than six-year period spanning 2004-2010. The obtained data have effective livetimes of 67 h for G1.9+0.3 and 16 h for G330.2+1.0. The data are analysed in the context of the multiwavelength observations currently available and in the framework of both leptonic and hadronic particle acceleration scenarios. No significant gamma-ray signal from G1.9+0.3 or G330.2+1.0 was detected. Upper limits (99 per cent confidence level) to the TeV flux from G1.9+0.3 and G330.2+1.0 for the assumed spectral index Gamma = 2.5 were set at 5.6 x 10(-1)3 cm(-2) s(-1) above 0.26 TeV and 3.2 x 10(-12) cm(-2) s(-1) above 0.38 TeV, respectively. In a one-zone leptonic scenario, these upper limits imply lower limits on the interior magnetic field to B-G1.9 greater than or similar to 12 mu G for G1.9+0.3 and to B-G330 greater than or similar to 8 mu G for G330.2+1.0. In a hadronic scenario, the low ambient densities and the large distances to the SNRs result in very low predicted fluxes, for which the H.E.S.S. upper limits are not constraining.

Keywords
radiation mechanisms: non-thermal, ISM: individual objects: SNR G1.9+0.3, ISM: individual objects: SNR G330.2+1.0, ISM: magnetic fields, ISM: supernova remnants, gamma-rays: ISM
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-105180 (URN)10.1093/mnras/stu459 (DOI)000336249300061 ()2-s2.0-84900007602 (Scopus ID)
Note

AuthorCount:219;

Available from: 2014-07-04 Created: 2014-06-24 Last updated: 2022-10-13Bibliographically approved
Middleton, M. J., Miller-Jones, J. C. A., Markoff, S., Fender, R., Henze, M., Hurley-Walker, N., . . . Grainge, K. (2013). Bright radio emission from an ultraluminous stellar-mass microquasar in M 31. Nature, 493(7431), 187-190
Open this publication in new window or tab >>Bright radio emission from an ultraluminous stellar-mass microquasar in M 31
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2013 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 493, no 7431, p. 187-190Article in journal (Refereed) Published
Abstract [en]

A subset of ultraluminous X-ray sources (those with luminosities of less than 10(40) erg s(-1); ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of similar to 5-20M(circle dot), probably by means of an accretion disk(2,3). The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole(4,5), with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way(6), and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 10(39) erg s(-1). The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-87696 (URN)10.1038/nature11697 (DOI)000313259600032 ()2-s2.0-84872166397 (Scopus ID)
Funder
EU, European Research Council
Note

AuthorCount:37;

Available from: 2013-02-15 Created: 2013-02-14 Last updated: 2022-10-04Bibliographically approved
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