FIRST EVIDENCE FOR SPECTRAL STATE TRANSITIONS IN THE ESO 243-49 HYPERLUMINOUS X-RAY SOURCE HLX-1

The brightest ultra-luminous X-ray source, ESO 243-49 HLX-1, with a 0.2–10 keV X-ray luminosity of up to 1042 erg s−1, provides the strongest evidence to date for the existence of intermediate mass black holes (BHs). Although small-scale X-ray spectral variability has already been demonstrated, we have initiated a monitoring campaign with the X-ray Telescope (XRT) onboard the Swift satellite to search for luminosity-related spectral changes and to compare its behavior with the better-studied stellar mass BHs. In this Letter, 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.