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We report the results of Chandra and XMM-Newton observations of a newoutburst of an ultra-luminous X-ray source in M101. During a Chandra monitoringobservation of M101, M101 ULX-1 was found to be in outburst in 2004 December,the second outburst in 2004. The peak bolometric luminosity is about 3e40 erg/s(7e39 erg/s in 0.3-7 keV). The outburst spectra are very soft and can generallybe fitted with a blackbody model with temperatures of 40-80 eV, similar tosupersoft X-ray sources in the Milky Way and in the Magellanic Clouds. In oneChandra observation, the source spectrum appears to be harder with atemperature of 150 eV. Such a spectral state is rarely seen in M101 ULX-1 andno X-ray source in the Milky Way shows this kind of spectrum. However, such anunusual spectral state very likely belongs to a new class of X-ray sources,quasisoft X-ray sources, recently discovered in nearby galaxies. M101 ULX-1returned to supersoft state in a subsequent XMM-Newton observation. Based onthe two outbursts in 2004, the extremely high luminosity (L_bol=1e40-1e41erg/s), very soft X-ray spectra (kT=40-150 eV), transient behavior, and statetransition provide strong evidence that M101 ULX-1 harbors an intermediate-massblack hole.Comment: 4 pages, 2 figures, accepted for publication in ApJ

An Unusual Spectral State of an Ultraluminous Very Soft X-Ray Source during Outburst