A variable Quasi-Periodic Ocillation in M82 X-1. Timing and spectral analysis of XMM-Newton and RossiXTE observations

We report results from a spectral and timing analysis of M82 X-1, one of the brightest known ultraluminous X-ray sources. Data from a new 105 ks {\it XMM-Newton} observation of M82 X-1, performed in April 2004, and of archival {\it RossiXTE} observations are presented. A very soft thermal component is present in the {\it XMM} spectrum. Although it is not possible to rule out a residual contamination from the host galaxy, modelling it with a standard accretion disk would imply a black hole mass of $\approx 10^3 M_{\odot}$. An emission line was also detected at an energy typical for fluorescent Fe emission. The power density spectrum of the {\it XMM} observation shows a variable QPO at frequency of 113 mHz with properties similar to that discovered by Strohmayer & Mushotzky (2003). The QPO was also found in 7 archival {\it RXTE} observations, that include those analyzed by Strohmayer & Mushotzky (2003) and Fiorito & Titarchuk (2004). A comparison of the properties of this QPO with those of the various types of QPOs observed in Galactic black hole candidates strongly suggests an association with the type-C, low frequency QPOs. Scaling the frequency inversely to the black hole mass, the observed QPO frequency range (from 50 to 166 mHz) would yield a black hole mass anywhere in the interval few tens to 1000 $M_\odot$.