The ultra-luminous X-ray ( ULX ) source ESO 243–49 HLX-1 , which reaches a maximum luminosity of 10 ^ { 42 } erg s ^ { -1 } ( 0.2–10 keV ) , currently provides the strongest evidence for the existence of intermediate mass black holes .
To study the spectral variability of the source , we conduct an ongoing monitoring campaign with the Swift X-ray Telescope , which now spans more than two years .
We found that HLX-1 showed two fast rise and exponential decay ( FRED ) type outbursts in the Swift XRT light-curve with increases in the count rate of a factor \sim 40 separated by 375 \pm 13 days .
We obtained new XMM-Newton and Chandra dedicated pointings that were triggered at the lowest and highest luminosities , respectively .
From spectral fitting , the unabsorbed luminosities ranged from 1.9 \times 10 ^ { 40 } to 1.25 \times 10 ^ { 42 } erg s ^ { -1 } .
We confirm here the detection of spectral state transitions from HLX-1 reminiscent of Galactic black hole binaries : at high luminosities , the X-ray spectrum showed a thermal state dominated by a disk component with temperatures of 0.26 keV at most , and at low luminosities the spectrum is dominated by a hard power law with a photon index in the range 1.4–2.1 , consistent with a hard state .
The source was also observed in a state consistent with the steep power law state , with a photon index of \sim 3.5 .
In the thermal state , the luminosity of the disk component appears to scale with the fourth power of the inner disk temperature which supports the presence of an optically thick , geometrically thin accretion disk .
The low fractional variability ( rms of 9 \pm 9 % ) in this state also suggests the presence of a dominant disk .
The spectral changes and long-term variability of the source can not be explained by variations of the beaming angle and are not consistent with the source being in a super-Eddington accretion state as is proposed for most ULX sources with lower luminosities .
All this indicates that HLX-1 is an unusual ULX as it is similar to Galactic black hole binaries , which have non-beamed and sub-Eddington emission , but with luminosities 3 orders of magnitude higher .
In this picture , a lower limit on the mass of the black hole of > 9000 M _ { \odot } can be derived , and the relatively low disk temperature in the thermal state also suggests the presence of an intermediate mass black hole of a few 10 ^ { 3 } M _ { \odot } .