Optical counterpart of HLX-1 during the 2010 outburst

Abstract

We studied the optical counterpart of the intermediate-mass black hole candidate HLX-1 in ESO243-49. 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 ± 0.18mag, B= 25.19 ± 0.30mag, V= 24.79 ± 0.34mag and R= 24.71 ± 0.40mag. Therefore, the source was ≈1mag 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 r out≈ 2800r in~ 10 13cm and a reprocessing fraction ≈2 × 10 -3. The optical colours are also consistent with a stellar population with age ≲6Myr (at solar metallicity) and mass ≈10 4M ⊙; 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 × 10 6M ⊙ for an age of 10Gyr, but it cannot significantly contribute to the observed very blue and variable optical/ultraviolet emission. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.