Journal Article

Comparing spectral models for ultraluminous X-ray sources with NGC 4517 ULX1

D. J. Walton, J. C. Gladstone, T. P. Roberts, A. C. Fabian, M. D. Caballero-Garcia, C. Done and M. J. Middleton

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 414, issue 2, pages 1011-1022
Published in print June 2011 | ISSN: 0035-8711
Published online June 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18397.x
Comparing spectral models for ultraluminous X-ray sources with NGC 4517 ULX1

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We present the previously unanalysed high-quality XMM–Newton spectrum of an ultraluminous X-ray source (ULX) candidate in NGC 4517. As with other high-quality ULX spectra, a downturn in the spectrum is observed at ∼6 keV. Both of the recent disc reflection and Comptonization interpretations of this feature are applied, in order to present a direct comparison, and are found to provide statistically equivalent representations of the current data. We find that the reflection model requires the accretion disc to have a highly supersolar iron abundance, while the Comptonization model requires low-temperature Comptonizing electrons, and for the corona to be optically thick. These physical requirements are discussed in detail, and physically motivated scenarios are highlighted in which each model can be considered a viable explanation for the observed emission. By extending our consideration of these two interpretations to high energies, we demonstrate that observations of ULXs at energies ≳10 keV should be extremely useful when attempting to distinguish between them. With current instrumentation, it is only viable to perform these observations for M82 X-1, but future high angular resolution hard X-ray imaging spectrometers, such as the Hard X-ray Imaging System due to fly on Astro-H, should go a long way to resolving this issue.

Keywords: black hole physics; X-rays: binaries

Journal Article.  10766 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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