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 ( \sim 20 Myr ) stellar cluster with a mass of \sim 10 ^ { 5 } M _ { \odot } .
We also place an upper limit on the mass of an additional hidden old stellar population of \sim 10 ^ { 6 } M _ { \odot } .
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.Nonetheless , variability of < 50 \% can not be ruled out by the Gemini data and thus they are still consistent within the errors with an exponential decay similar to that observed in X-rays .