  We report the results of Chandra and XMM-Newton observations of a new
outburst of an ultraluminous supersoft X-ray source in M101. CXOU
J140332.3+542103 was observed in a low luminosity state (L_x~1e37 erg/s)
between 2004 January and May. The low-state X-ray spectra were relatively hard;
the combined low state spectrum can be fitted with a combination of a power-law
with photon index of 1.4 and a blackbody of 63 eV. During 2004 July, the source
underwent a strong outburst and the peak 0.3-7 keV luminosity reached 3e40
erg/s, with a bolometric luminosity of about 1e41 erg/s. The outburst spectra
were very soft and can generally be fitted with a blackbody model with
temperatures of 50-100 eV. In two of the observations, absorption edges at 0.33
keV, 0.56 keV, 0.66 keV, and 0.88 keV were found. An XMM-Newton observation was
also performed during the decay of the outburst, and a power-law tail was seen
in addition to the supersoft spectrum. We consider different accretion models;
one involving an intermediate-mass black hole can explain the observations.
