  The ultra-luminous X-ray (ULX) source ESO 243-49 HLX-1 currently provides the
strongest evidence for the existence of intermediate mass black holes. We
conduct an ongoing monitoring campaign with the Swift X-ray Telescope and found
that HLX-1 showed two fast rise and exponential decay with increases in the
count rate of a factor ~40 separated by 375+/-13 days. We obtained new
XMM-Newton and Chandra dedicated pointings that were triggered at the lowest
and highest luminosities, respectively. The unabsorbed luminosities ranged from
1.9x10^40 to 1.25x10^42 erg/s. 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 steep
power law state. 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+/-9%) in this state also suggests the
presence of a dominant disk. The spectral changes and long-term variability of
the source cannot be explained by variations of the beaming angle and are not
consistent with the source being in a super-Eddington accretion state. HLX-1 is
thus 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_sun can be derived, and the disk temperature in the thermal state
also suggests the presence of a black hole of a few 10^3 M_sun.
