  We report on the identification of a third, new ultraluminous X-ray source
(ULX) in the starburst galaxy M82. Previously, the source was observed at
fluxes consistent with the high state of Galactic black hole binaries (BHBs).
We observe fluxes up to (6.5 +/- 0.3)E39 ergs/s in the ultraluminous regime.
When the source is not in the the low/hard state, spectral fitting using a
multicolor disk model shows that the disk luminosity varies as the disk inner
temperature raised to the power 4.8 +/- 0.9, consistent with the behavior of
Galactic BHBs in the thermal dominant state. Fitting the spectrum with a
multicolor disk model with general relativistic corrections suggests that the
source harbors a rapidly spinning black hole with a mass less than 100 solar
masses. A soft excess was found in the source spectrum that could be blackbody
emission from a photosphere created by a massive outflow. The source also
showed soft dips during a flare.
