Is M82 X-1 Really An Intermediate-Mass Black Hole? X-ray Spectral and   Timing Evidence
  Ultra-luminous X-ray sources (ULXs) with apparent luminosities up to 100's of times the Eddington luminosity for a neutron star have been discovered in external galaxies. The existence of intermediate mass black holes has been proposed to explain these sources. We present evidence for an intermediate-mass black hole in the ULX M82 X-1 based on the spectral features and timing (QPO) properties of the X-radiation from this source. We revisited XMM Newton and RXTE data for M82 X-1 obtained in 2001 and 1997 for XMM and RXTE respectively. We show for these observations that the source is either in transition or in a high/soft state with photon spectral indices 2.1 and 2.7 respectively. We confirm the early determination of the QPO frequency ~ 55 mHz in this source by Strohmayer & Mushotzky and identify this as the low frequency QPO for the source. We apply a new method to determine the BH mass of M82 X-1. The method uses the index-QPO low frequency correlation that has been recently been established in galactic black hole candidates GRS 1915+105, XTE J1550-564, 4U 1630-47 and others. Using scaling arguments and the correlation derived from consideration of galactic BHs, we conclude that M82 X-1 is an intermediate BH with a mass of the order of 1000 solar masses. 
Ralph Fiorito (UMD/GSFC) and Lev Titarchuk (GMU/NRL/GSFC)