Explaining the outbursts of the intermediate-mass BH HLX-1 as a wind-driven instability

We model the optical/UV emission of the intermediate-mass black hole HLX-1, using three sets of HST observations (from 2010, 2012, 2013), together with XMM-Newton and Swift data. We quantify the relative contributions of a bluer component, function of X-ray irradiation, and a redder component, constant and likely coming from an old stellar population. By combining optical and X-ray data, in particular around its state transitions, we estimate a BH mass 2 x 10^4 Msun, a spin parameter a/M 0.9 for a moderately face-on view, and a peak outburst luminosity 0.3 times the Eddington luminosity. We discuss the discrepancy between the characteristic size inferred from the short X-ray timescale (R a few 10^{11} cm) and the characteristic size of the irradiated optical emitter (R sqrt[cos theta] 2 x 10^{13} cm). One possibility is that the optical emitter is a circumbinary disk; however, we disfavour this scenario because it would require a very small donor star. A more plausible scenario is that the disk is large but only the inner annuli are involved in the X-ray outburst cycle. We propose that the recurrent outbursts are caused by an accretion-rate oscillation driven by wind instability in the inner disk. We argue that the system has a long-term-average accretion rate of a few percent of Eddington, just below the upper limit of the low/hard state; a wind-driven oscillation can trigger transitions to the high/soft state, with a recurrence period 1 year (much longer than the binary period, which we estimate as 10 days). The oscillation that dominated the system in the last decade is now damped such that the accretion rate more rarely reaches the level required to trigger a state transition. Finally, we highlight similarities between the role of disk winds in HLX-1 and in the Galactic BH V404 Cyg.