Journal Article

X-ray study of HLX1: intermediate-mass black hole or foreground neutron star?

Roberto Soria, Luca Zampieri, Silvia Zane and Kinwah Wu

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 3, pages 1886-1894
Published in print January 2011 | ISSN: 0035-8711
Published online January 2011 | e-ISSN: 1365-2966 | DOI:
X-ray study of HLX1: intermediate-mass black hole or foreground neutron star?

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We re-assess the XMM–Newton and Swift observations of HLX1 to examine the evidence for its identification as an intermediate-mass black hole. We show that the X-ray spectral and timing properties are equally consistent with an intermediate-mass black hole in a high state or with a foreground neutron star with a luminosity ∼ a few ×1032 erg s−1∼ 10−6LEdd, located at a distance of ≈1.5–3 kpc. Contrary to previously published results, we find that the X-ray spectral change between the two XMM–Newton observations of 2004 and 2008 (going from power-law dominated to thermal dominated) is not associated with a change in the X-ray luminosity. The thermal component becomes more dominant (and hotter) during the 2009 outburst seen by Swift but in a way that is consistent with either scenario.

Keywords: accretion, accretion discs; black hole physics; stars: neutron; X-rays: binaries; X-rays: individual: HLX1

Journal Article.  5559 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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