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Multiwavelength Observations and State Transitions of an Ultra-luminous Supersoft X-ray Source: Evidence for an Intermediate-Mass Black Hole

A. K. H. Kong·M. P. Rupen·L. O. Sjouwerman·R. Di Stefano
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Abstract

We report the results of Chandra and XMM-Newton observations of an ultra-luminous supersoft X-ray source in M101. M101 ULX-1 underwent 2 outbursts in 2004 during which the peak bolometric luminosities reached 1e41 erg/s. The outburst spectra were very soft and can generally be fitted with a blackbody model with temperatures of 50-160 eV. In two of the observations, absorption edges at 0.33 keV, 0.56 keV, 0.66 keV, and 0.88 keV were found. A cool accretion disk was also found in the 2004 December outburst. During the low luminosity state, a power-law tail was seen up to 7 keV. It is clear the source changed from a low/hard state to a high/soft state. In addition, it showed at least 5 outbursts between 1996 and 2004. This is the first ultra-luminous X-ray source for which recurrent outbursts with state transitions similar to Galactic X-ray binaries have been observed. From the Hubble Space Telescope data, we found an optical counterpart to the source. During the 2004 outbursts, we also performed radio and ground-based optical observations. All the results strongly suggest that the accreting object is a > 2800 solar mass black hole.

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A. K. H. Kong, M. P. Rupen, L. O. Sjouwerman, R. Di Stefano. 2005-03-22. Multiwavelength Observations and State Transitions of an Ultra-luminous Supersoft X-ray Source: Evidence for an Intermediate-Mass Black Hole. https://arxiv.org/abs/astro-ph/0503465

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