Evidence of an intermediate-mass black hole:: Chandra and XMM-Newton observations of the ultraluminous supersoft X-ray source in M101 during its 2004 outburst

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 similar to 10(37) ergs s(-1)) 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 3 x 10(40) ergs s(-1), with a bolometric luminosity of about 10(41) ergs s(-1). The outburst spectra were very soft and can be generally fitted with a blackbody model with temperatures of 50 - 100 eV. In two of the observations, absorption edges at 0.33, 0.56, 0.66, 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.