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.Comment: Accepted by ApJ Letter, 2 figure
