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

摘要

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 ～ 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 kcV 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.