AN UNUSUAL SPECTRAL STATE OF AN ULTRALUMINOUS VERY SOFT X-RAY SOURCE DURING OUTBURST

摘要

We report the results of Chandra and XMM-Newton observations of a new outburst of an ultraluminous 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 x 10~(40)) ergs s~(-1) (7 x 10~(39) ergs 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, quasi-soft 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.