  The thermal dominant state in black hole binaries (BHBs) is well understood
but rarely seen in ultraluminous X-ray sources (ULXs). Using simultaneous
observations of M82 with Chandra and XMM-Newton, we report the first likely
identification of the thermal dominant state in a ULX based on the
disappearance of X-ray oscillations, low timing noise, and a spectrum dominated
by multicolor disk emission with luminosity varying to the 4th power of the
disk temperature. This indicates that ULXs are similar to Galactic BHBs. The
brightest X-ray spectrum can be fitted with a relativistic disk model with
either a highly super-Eddington (L_disk/L_Edd = 160) non-rotating black hole or
a close to Eddington (L_disk/L_Edd ~ 2) rapidly rotating black hole. The latter
interpretation is preferred, due to the absence of such highly super-Eddington
states in Galactic black holes and active galactic nuclei, and suggests that
the ULX in M82 contains a black hole of 200-800 solar masses with nearly
maximal spin. On long timescales, the source normally stays at a relatively low
flux level with a regular 62-day orbital modulation and occasionally exhibits
irregular flaring activity. The thermal dominant states are all found during
outbursts.
