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P. Kaaret, A.H. Prestwich, A. Zezas, S.S. Murray, D.-W. Kim, R.E. Kilgard, E.M. Schlegel (Harvard-Smithsonian CfA), M.J. Ward (Leicester)
We have analyzed early Chandra High Resolution Camera observations of the starburst galaxy M82, concentrating on the most luminous x-ray source. We find an accurate position and report detection of an x-ray oscillation with a period in the range 573--613~s. The x-ray position shows that the luminous source is not at the dynamical center of M82 or coincident with any suggested radio AGN candidate. The detection of ~600~s oscillations excludes the possibility that the source is a supernova or remnant, suggests that the source is a compact object, and can be used to place an upper bound of 1.3 \times 106 \, M\odot on the mass of the compact object. The x-ray luminosity suggests a compact object mass in excess of 600 \, M\odot. Thus, the luminous source in M82 likely represents a new class of compact object with a mass intermediate between those of stellar mass black hole candidates and supermassive black holes found in the centers of galaxies.