Discovery of an optical counterpart to the hyperluminous X-ray source in ESO 243-49

The existence of black holes of masses ∼10(2)-10(5)M⊙ has important implications for the formation and evolution of star clusters and supermassive black holes. One of the strongest candidates to date is the hyperluminous X-ray source (HLX1), possibly located in the S0-a galaxy ESO 243-49, but the lack of an identifiable optical counterpart had hampered its interpretation. Using the Magellan telescope, we have discovered an unresolved optical source with R = 23.80±0.25 mag and V = 24.5±0.3 mag within HLX1's positional error circle. This implies an average X-ray/optical flux ratio ∼500. Taking the same distance as ESO 243-49, we obtain an intrinsic brightness Mr = -11.0±0.3 mag, comparable to that of a massive globular cluster. Alternatively, the optical source is consistent with a main-sequence M star in the Galactic halo (for example an M4.4 star at ≈2.5 kpc). We also examined the properties of ESO 243–49 by combining Swift/Ultraviolet/Optical Telescope (UVOT) observations with stellar population modelling. We found that the overall emission is dominated by a ~5 -Gyr -old stellar population, but the UV emission at ≈2000. Å is mostly due to ongoing star formation at a rate of ~0.03 M⊙ yr -1. The UV emission is more intense (at least a 9σ enhancement above the mean) north-east of the nucleus, in the same quadrant as HLX1. With the combined optical and X-ray measurements, we put constraints on the nature of HLX1. We rule out a foreground star and a background AGN. Two alternative scenarios are still viable. HLX1 could be an accreting intermediate mass black hole in a star cluster, which may itself be the stripped nucleus of a dwarf galaxy that passed through ESO 243-49, an event which might have caused the current episode of star formation. Or, it could be a neutron star in the Galactic halo, accreting from an M4-M5 donor star.