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

TL;DR

This study reports the discovery of an optical counterpart to the hyperluminous X-ray source HLX1 in ESO 243-49, supporting the hypothesis that it is an intermediate-mass black hole or a neutron star in the Galactic halo.

Contribution

The paper presents the first optical detection of HLX1, providing new constraints on its nature and ruling out some previous hypotheses, while proposing two viable scenarios.

Findings

Optical counterpart has R=23.80 and V=24.5 mag, consistent with a globular cluster or a Galactic M star.

The host galaxy ESO243-49 has a ~5 Gyr old stellar population with ongoing star formation.

HLX1 is unlikely to be a foreground star or background AGN, supporting its association with the galaxy.

Abstract

The existence of black holes of masses ~ 10^2-10^5 Msun 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 ESO243-49, 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 ESO243-49, we obtain an intrinsic brightness M_R = (-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 ESO243-49 by combining Swift/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 Ang is mostly due to ongoing star-formation at a rate of ~ 0.03 Msun/yr. The UV emission is more intense (at least a 9-sigma 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 ESO243-49, 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.