  We report the results of Chandra and XMM-Newton observations of an
ultra-luminous supersoft X-ray source in M101. M101 ULX-1 underwent 2 outbursts
in 2004 during which the peak bolometric luminosities reached 1e41 erg/s. The
outburst spectra were very soft and can generally be fitted with a blackbody
model with temperatures of 50-160 eV. In two of the observations, absorption
edges at 0.33 keV, 0.56 keV, 0.66 keV, and 0.88 keV were found. A cool
accretion disk was also found in the 2004 December outburst. During the low
luminosity state, a power-law tail was seen up to 7 keV. It is clear the source
changed from a low/hard state to a high/soft state. In addition, it showed at
least 5 outbursts between 1996 and 2004. This is the first ultra-luminous X-ray
source for which recurrent outbursts with state transitions similar to Galactic
X-ray binaries have been observed. From the Hubble Space Telescope data, we
found an optical counterpart to the source. During the 2004 outbursts, we also
performed radio and ground-based optical observations. All the results strongly
suggest that the accreting object is a > 2800 solar mass black hole.
