1999ApJ...517L..85P

We report on the detection of hard (2-10 keV) X-ray variability in the starburst galaxy M82 over the course of nine ASCA observations. Long-term variability occurred on a timescale of days, with a change in flux by a factor of up to ∼4, corresponding to a point-source luminosity of L_2-10 keV_∼6x10⁴⁰ ergs.s^–1. Short-term variability with an amplitude of ∼1.4 on a timescale of hours was observed during the longest observation. This demonstrates that a large fraction of the hard X-ray emission of M82 (depending on the flux state) is from a compact region and is probably due to an accreting source. The 2-10 keV luminosity of the source is a lower limit to its Eddington luminosity, implying a black hole mass of at least ∼460 M_☉ or a mass intermediate to that of normal active galactic nuclei and stellar-mass black hole candidates.