Optical counterpart of HLX-1 during the 2010 outburst

TL;DR

This study analyzes the optical emission of HLX-1 during its 2010 outburst, concluding it was dominated by an irradiated accretion disk rather than a star cluster, with implications for the nature of its environment.

Contribution

It provides the first detailed optical analysis of HLX-1 during an outburst, modeling the irradiated disk and constraining the properties of potential surrounding stellar populations.

Findings

Optical emission was dominated by an irradiated disk during the outburst.

The disk's outer radius is approximately 2800 times the inner radius.

A young super-star cluster is ruled out as the source of optical emission.

Abstract

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 disk (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 disk, and found that the optical luminosity and colours in the 2010 November data are still consistent with emission from the irradiated disk, with a characteristic outer radius r_{out} ~ 2800 r_{in} ~ 10^{13} cm and a reprocessing fraction ~ 2 x 10^{-3}. The optical colours are also consistent with a stellar population with age <~ 6 Myr (at solar metallicity) and mass ~ 10^4 M_{sun}; this is only an upper limit to the mass, if there is also a significant contribution from an irradiated disk. We strongly rule out the presence of a young super-star-cluster, which would be too bright. An old globular cluster might be associated with HLX-1, as long as its mass <~ 2 x 10^6 M_{sun} for an age of 10 Gyr, but it cannot significantly contribute to the observed very blue and variable optical/UV emission.