First evidence for spectral state transitions in the ESO243-49 hyper luminous X-ray source HLX-1

TL;DR

This study presents the first evidence of spectral state transitions in the hyper-luminous X-ray source HLX-1, indicating behavior similar to stellar-mass black holes and supporting the presence of an intermediate mass black hole.

Contribution

The paper reports the detection of spectral state transitions in HLX-1, a hyper-luminous X-ray source, through a dedicated monitoring campaign, providing new insights into its nature.

Findings

Observed a factor of ~8 decrease in X-ray count rate with spectral hardening.

Detected a re-brightening by a factor of ~21 with spectral softening.

First evidence of state transitions in HLX-1, akin to stellar-mass black holes.

Abstract

The brightest Ultra-Luminous X-ray source (ULX), ESO 243-49 HLX-1, with a 0.2 - 10 keV X-ray luminosity of up to 10^42 erg s^-1, provides the strongest evidence to date for the existence of intermediate mass black holes. Although small scale X-ray spectral variability has already been demonstrated, we have initiated a monitoring campaign with the X-ray Telescope onboard the Swift satellite to search for luminosity-related spectral changes and to compare its behavior with the better studied stellar mass black holes. In this paper, we report a drop in the XRT count rate by a factor of ~8 which occurred simultaneously with a hardening of the X-ray spectrum. A second observation found that the source had re-brightened by a factor of ~21 which occurred simultaneously with a softening of the X-ray spectrum. This may be the first evidence for a transition between the low/hard and high/soft states.