We report the results of Chandra and XMM-Newton observations of a new outburst of an ultra-luminous X-ray source in M101 .
During a Chandra monitoring observation 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 3 \times 10 ^ { 40 } \hbox { ergs } \thinspace \hbox { s } ^ { -1 } ( 7 \times 10 ^ { 39 } \hbox { ergs } \thinspace \hbox { s } ^ { -1 } in 0.3–7 keV ) .
The outburst spectra are very soft and can generally be fitted with a blackbody model with temperatures of 40–80 eV , similar to supersoft X-ray sources in the Milky Way and in the Magellanic Clouds .
In one Chandra observation , the source spectrum appears to be harder with a temperature of 150 eV .
Such a spectral state is rarely seen in M101 ULX-1 and no X-ray source in the Milky Way shows this kind of spectrum .
However , such an unusual spectral state very likely belongs to a new class of X-ray sources , quasisoft X-ray sources , recently discovered in nearby galaxies .
M101 ULX-1 returned to supersoft state in a subsequent XMM-Newton observation .
Based on the two outbursts in 2004 , the extremely high luminosity ( L _ { bol } = 10 ^ { 40 } -10 ^ { 41 } ergs s ^ { -1 } ) , very soft X-ray spectra ( kT = 40 - 150 eV ) , transient behavior , and state transition provide strong evidence that M101 ULX-1 harbors an intermediate-mass black hole .