Combined Analysis of Hubble and VLT Photometry of the Intermediate Mass
  Black Hole ESO 243-49 HLX-1
  In this paper we present a combined analysis of data obtained with the Hubble
Space Telescope (HST), Very Large Telescope (VLT), and Swift X-ray telescope
(XRT) of the intermediate mass black hole ESO 243-49 HLX-1 that were taken 2
months apart between September and November 2010. Previous separate analyses of
these data found that they were consistent with an irradiated accretion disc
with contribution from either a very young or very old stellar population, and
also indicated that the optical flux of the HLX-1 counterpart could be
variable. Such variability could only be attributed to a varying accretion
disc, so simultaneous analysis of all data sets should break the degeneracies
in the model fits. We thus simultaneously fit the broad-band spectral energy
distribution (SED) from near-infrared through to X-ray wavelengths of the two
epochs of data with a model consisting of an irradiated accretion disc and a
stellar population. We show that this combined analysis rules out an old
stellar population, finding that the SED is dominated by emission from an
accretion disc with moderate reprocessing in the outer disc around an
intermediate mass black hole imbedded in a young (20 Myr) stellar cluster with
a mass of 1E5 Msun. We also place an upper limit on the mass of an additional
hidden old stellar population of 1E6 Msun. However, optical r-band observations
of HLX-1 obtained with the Gemini-South telescope covering part of the decay
from a later X-ray outburst are consistent with constant optical flux,
indicating that the observed variability between the HST and VLT observations
could be spurious caused by differences in the background subtraction applied
to the two optical data sets. In this scenario the contribution of the stellar
population, and thus the stellar mass of the cluster, may be higher (abridged).
