X-Ray Timing Measurements of NGC 5408 X-1 With XMM-Newton and Swift/XRT: Implications for the Mass of a ULX

I present new X-ray timing measurements of the ULX NGC 5408 X-1 with XMM-Newton, and long term monitoring with the XRT onboard SWIFT. This object is one of only two ULXs in which quasi-periodic oscillations (QPO) have been detected. Recent observations with XMM-Newton confirm the presence of QPOs in this object. Most interestingly, two QPOs are again detected, as in an earlier observation, however, the QPO frequencies are different, now appearing at 10 and approximately 14 mHz. We discuss the implications of the QPO frequencies and spectra at these two different epochs for the mass of the black hole in NGC 5408 X-1. We also present strong evidence for long term flux variations based on the SWIFT monitoring campaign.