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

TL;DR

This study re-examines X-ray data of HLX1, showing its properties could indicate either an intermediate-mass black hole or a foreground neutron star, challenging previous interpretations.

Contribution

It provides a revised analysis demonstrating that HLX1's X-ray properties are consistent with both an intermediate-mass black hole and a foreground neutron star, questioning earlier conclusions.

Findings

X-ray spectral and timing properties fit both scenarios

No luminosity change associated with spectral state transition

Thermal component becomes hotter during outburst, compatible with either model

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 of about a few times 10^{32} erg/s ~ 10^{-6} L_{Edd}, located at a distance of about 1.5 to 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.