Eccentricity of HLX-1

Abstract

We compare the outer radius of the accretion disc in the intermediate-mass black hole candidate HLX-1 as estimated from the ultraviolet/optical continuum, with the values estimated from its outburst decline time-scales. We fit the Swift 2010 outburst decline light curve with an exponential decay, a knee and a linear decay. We find that the disc has an outer radius of 1012 ≤ Rout ≤ 1013 cm, only an order of magnitude larger than typical accretion discs in the high/soft state of Galactic black holes. By contrast, the semimajor axis is ≈ a few ×1014 cm. This discrepancy can be explained with a highly eccentric orbit. We estimate the tidal truncation radius and circularization radius around the black hole at periastron, and impose that they are similar or smaller than the outer disc radius. We obtain that e ≥ 0.95, that the radius of the donor star is ≤ a few solar radii and that the donor star is not at risk of tidal disruption. If the companion star fills its Roche lobe and impulsively transfers mass only around periastron, secular evolution of the orbit is expected to increase eccentricity and semimajor axis even further. We speculate that such extremely eccentric systems may have the same origin as the S stars in the Galactic Centre.