Investigating Slim Disk Solutions for HLX-1 in ESO 243-49
Godet, O., Plazolles, B., Kawaguchi, T., Lasota, J. -P., Barret, D., Farrell, S.~A., Braito, V., Servillat, M., Webb, N., & Gehrels, N.
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 (åisebox-0.5ex~2 × 10$^4$ M $_☉$) and the accretion flow is in the sub- Eddington regime. The disk radiation efficiency is \ensuremathη = 0.11 \ensuremath± 0.03. We also show that the source does follow the L$_X$ vprop 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 \i̊sebox-0.5ex~1 year recurrence timescale.