Investigating Slim Disk Solutions for HLX-1 in ESO 243-49

The hyperluminous X-ray source HLX-1 in the galaxy ESO 243-49, currently the best intermediate-mass black hole (BH) candidate, displays spectral transitions similar to those observed in Galactic BH binaries, but with a luminosity 100-1000 times higher. We investigated the X-ray properties of this unique source by fitting multi-epoch data collected by Swift, XMM-Newton, and Chandra with a disk model computing spectra for a wide range of sub- and super-Eddington accretion rates assuming a non-spinning BH and a face-on disk (i = 0°). Under these assumptions we find that the BH in HLX-1 is in the intermediate-mass range (~2 × 104 M ☉) and the accretion flow is in the sub-Eddington regime. The disk radiation efficiency is η = 0.11 ± 0.03. We also show that the source does follow the LX ∝ T 4 relation for our mass estimate. At the outburst peaks, the source radiates near the Eddington limit. The accretion rate then stays constant around 4 × 10–4 M ☉ yr–1 for several days and then decreases exponentially. Such "plateaus" in the accretion rate could be evidence that enhanced mass-transfer rate is the driving outburst mechanism in HLX-1. We also report on the new outburst observed in 2011 August by the Swift X-Ray Telescope. The time of this new outburst further strengthens the ~1 year recurrence timescale.