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In this paper we present a combined analysis of data obtained with the HubbleSpace 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 2months apart between September and November 2010. Previous separate analyses ofthese data found that they were consistent with an irradiated accretion discwith contribution from either a very young or very old stellar population, andalso indicated that the optical flux of the HLX-1 counterpart could bevariable. Such variability could only be attributed to a varying accretiondisc, so simultaneous analysis of all data sets should break the degeneraciesin the model fits. We thus simultaneously fit the broad-band spectral energydistribution (SED) from near-infrared through to X-ray wavelengths of the twoepochs of data with a model consisting of an irradiated accretion disc and astellar population. We show that this combined analysis rules out an oldstellar population, finding that the SED is dominated by emission from anaccretion disc with moderate reprocessing in the outer disc around anintermediate mass black hole imbedded in a young (20 Myr) stellar cluster witha mass of 1E5 Msun. We also place an upper limit on the mass of an additionalhidden old stellar population of 1E6 Msun. However, optical r-band observationsof HLX-1 obtained with the Gemini-South telescope covering part of the decayfrom a later X-ray outburst are consistent with constant optical flux,indicating that the observed variability between the HST and VLT observationscould be spurious caused by differences in the background subtraction appliedto the two optical data sets. In this scenario the contribution of the stellarpopulation, and thus the stellar mass of the cluster, may be higher (abridged).

Combined Analysis of Hubble and VLT Photometry of the Intermediate Mass Black Hole ESO 243-49 HLX-1