We compare the outer radius of the accretion disc in the intermediate-mass black hole candidate HLX-1 as estimated from the UV/optical continuum , with the values estimated from its outburst decline timescales .
We fit the Swift 2010 outburst decline lightcurve with an exponential decay , a knee and a linear decay .
We find that the disk has an outer radius 10 ^ { 12 } cm \la R _ { out } \la 10 ^ { 13 } 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 \approx a few 10 ^ { 14 } 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 disk radius .
We obtain that e \ga 0.95 , that the radius of the donor star is \la 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 .