X-ray study of HLX1: intermediate-mass black hole or foreground neutron star?

Abstract

We re-assess the XMM-Newton and Swift observations of HLX1 to examine the evidence for its identification as an intermediate-mass black hole. We show that the X-ray spectral and timing properties are equally consistent with an intermediate-mass black hole in a high state or with a foreground neutron star with a luminosity ∼ a few ×10³² erg s^-1∼ 10^-6L_Edd, located at a distance of ≈1.5-3 kpc. Contrary to previously published results, we find that the X-ray spectral change between the two XMM-Newton observations of 2004 and 2008 (going from power-law dominated to thermal dominated) is not associated with a change in the X-ray luminosity. The thermal component becomes more dominant (and hotter) during the 2009 outburst seen by Swift but in a way that is consistent with either scenario.