  We have studied a highly variable ultraluminous X-ray source (ULX) in the
Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations
between 2002 and 2006. In 2006 April, the source peaked at a luminosity ~ 3 x
10^{40} erg/s in the 0.3-10 keV band (similar to the maximum luminosity found
by ASCA in 1995), and declined on an e-folding timescale ~ 3 days. The X-ray
spectrum is always dominated by a broad power-law-like component. When the
source is seen at X-ray luminosities ~ 10^{40} erg/s, an additional soft
thermal component (which we interpret as emission from the accretion disk)
contributes ~ 1/4 of the X-ray flux; when the luminosity is higher, ~ 3 x
10^{40} erg/s, the thermal component is not detected and must contribute < 10%
of the flux. At the beginning of the decline, ionized absorption is detected
around 0.5-2 keV; it is a possible signature of a massive outflow. The
power-law is always hard, with a photon index Gamma ~ 1.7 (and even flatter at
times), as is generally the case with bright ULXs. We speculate that this
source and perhaps most other bright ULXs are in a high/hard state: as the
accretion rate increases well above the Eddington limit, more and more power is
extracted from the inner region of the inflow through non-radiative channels,
and is used to power a Comptonizing corona, jet or wind. The observed thermal
component comes from the standard outer disk; the transition radius between
outer standard disk and Comptonizing inner region moves further out and to
lower disk temperatures as the accretion rate increases. This produces the
observed appearance of a large, cool disk. Based on X-ray luminosity and
spectral arguments, we suggest that this accreting black hole has a likely mass
~ 50-150 Msun (even without accounting for possible beaming).
