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A. Ptak, R. Griffiths (CMU)
We report on the detection of hard (2-10 keV) variability in the starburst galaxy M82 over the course of 9 ASCA observations. Long-term variability occurred on a time scale of days, with a change in flux by a factor of up to ~ 4, corresponding to a point-source luminosity of L2-10\rm \ keV ~6 \times 1040 \rm \ ergs \ s-1. Short-term variability with an amplitude of ~ 1.8 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 due to an accreting source. The 2-10 keV luminosity of the source is a lower limit to its Eddington luminosity, implying a blackhole mass of at least ~460M\odot, or a mass intermediate to that of normal AGN and solar-mass blackhole candidates. With very limited statistics, the spectral (energy) index of the hard flux was observed to vary from \alpha = 0.7-1.7, although the spectral variability does not appear to be correlated with the intensity fluctuations. The hard and soft flux are positionally coincident, showing that the variable source lies within the nucleus of M82 and the hard flux can probably be associated with the variable nuclear source discovered in ROSAT observations.
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