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 multiepoch 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 degrees). Under these assumptions we find that the BH in HLX-1 is in the intermediate-mass range (similar to 2 x 10(4) M-circle dot) and the accretion flow is in the sub-Eddington regime. The disk radiation efficiency is eta = 0.11 +/- 0.03. We also show that the source does follow the L-X alpha 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 x 10(-4) M-circle dot 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 similar to 1 year recurrence timescale.