Journal Article

Optical counterpart of HLX-1 during the 2010 outburst

Roberto Soria, Pasi J. Hakala, George K. T. Hau, Jeanette C. Gladstone and Albert K. H. Kong

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 420, issue 4, pages 3599-3608
Published in print March 2012 | ISSN: 0035-8711
Published online March 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.20281.x
Optical counterpart of HLX-1 during the 2010 outburst

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We studied the optical counterpart of the intermediate-mass black hole candidate HLX-1 in ESO 243−49. We used a set of Very Large Telescope imaging observations from 2010 November, integrated by Swift X-ray data from the same epoch. We measured standard Vega brightnesses U= 23.89 ± 0.18 mag, B= 25.19 ± 0.30 mag, V= 24.79 ± 0.34 mag and R= 24.71 ± 0.40 mag. Therefore, the source was ≈1 mag fainter in each band than in a set of Hubble Space Telescope images taken a couple of months earlier, when the X-ray flux was a factor of 2 higher. We conclude that during the 2010 September observations, the optical counterpart was dominated by emission from an irradiated disc (which responds to the varying X-ray luminosity), rather than by a star cluster around the black hole (which would not change). We modelled the Comptonized, irradiated X-ray spectrum of the disc, and found that the optical luminosity and colours in the 2010 November data are still consistent with emission from the irradiated disc, with a characteristic outer radius rout≈ 2800rin∼ 1013 cm and a reprocessing fraction ≈2 × 10−3. The optical colours are also consistent with a stellar population with age ≲6 Myr (at solar metallicity) and mass ≈104 M; this is only an upper limit to the mass, if there is also a significant contribution from an irradiated disc. We strongly rule out the presence of a young superstar cluster, which would be too bright. An old globular cluster might be associated with HLX-1, as long as its mass ≲2 × 106 M for an age of 10 Gyr, but it cannot significantly contribute to the observed very blue and variable optical/ultraviolet emission.

Keywords: accretion, accretion discs; black hole physics; X-rays: individual: HLX-1

Journal Article.  7568 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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