Hard X-Ray Variability in M82: Evidence for a Nascent Active Galactic Nucleus?

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 L2-10keV~6×1040 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.