X-RAY VARIABILITY AND HARDNESS OF ESO 243-49 HLX-1: CLEAR EVIDENCE FOR SPECTRAL STATE TRANSITIONS

Abstract

The ultra-luminous X-ray (ULX) source ESO 243-49 HLX-1 currently provides the\nstrongest evidence for the existence of intermediate mass black holes. We\nconduct an ongoing monitoring campaign with the Swift X-ray Telescope and found\nthat HLX-1 showed two fast rise and exponential decay with increases in the\ncount rate of a factor ~40 separated by 375+/-13 days. We obtained new\nXMM-Newton and Chandra dedicated pointings that were triggered at the lowest\nand highest luminosities, respectively. The unabsorbed luminosities ranged from\n1.9x10^40 to 1.25x10^42 erg/s. We confirm here the detection of spectral state\ntransitions from HLX-1 reminiscent of Galactic black hole binaries: at high\nluminosities, the X-ray spectrum showed a thermal state dominated by a disk\ncomponent with temperatures of 0.26 keV at most, and at low luminosities the\nspectrum is dominated by a hard power law with a photon index in the range\n1.4-2.1, consistent with a hard state. The source was also observed in a steep\npower law state. In the thermal state, the luminosity of the disk component\nappears to scale with the fourth power of the inner disk temperature which\nsupports the presence of an optically thick, geometrically thin accretion disk.\nThe low fractional variability (rms of 9+/-9%) in this state also suggests the\npresence of a dominant disk. The spectral changes and long-term variability of\nthe source cannot be explained by variations of the beaming angle and are not\nconsistent with the source being in a super-Eddington accretion state. HLX-1 is\nthus an unusual ULX as it is similar to Galactic black hole binaries, which\nhave non-beamed and sub-Eddington emission, but with luminosities 3 orders of\nmagnitude higher. In this picture, a lower limit on the mass of the black hole\nof >9000 M_sun can be derived, and the disk temperature in the thermal state\nalso suggests the presence of a black hole of a few 10^3 M_sun.\n