Abstract
We report the discovery of a recurrent ultraluminous X-ray source (ULX) in the starburst galaxy M82. From a series of Chandra observations, we found an ULX (Lx 1e40 erg/s) that was active for about 100 days in late 1999. The source turned off in 2000 January for about 50 days and has been active since then, indicating at least a factor of 50 in brightness variation. During its active state, the source luminosity varies between 7e39 erg/s and 1e40 erg/s and shows unusual spectral changes. The X-ray spectra are best fit with a power-law with photon index of 1.2-1.7. By comparing with IR images taken with HST/NICMOS, the ULX is found to be located within a young star cluster. Radio imaging indicates that it is associated with an extended radio source. We suggest that the ULX is likely a >100 solar mass intermediate-mass black hole in the low/hard state. In addition to the transient, we also found a very luminous (Lx 2e39 erg/s) X-ray emitting supernova remnant and a highly absorbed hard ULX within 3 arcsec of the transient. Finally, we will discuss the diverse properties of ULXs in M82.