PASHAM R. DHEERAJ

Ultraluminous X-ray sources are bright, pointlike, off-nuclear, extragalactic X-ray sources with apparent X-ray (0.3-10.0 keV) luminosities in the range of 10^39-10^41 ergs/sec. They remain mysterious in the sense that their high luminosities imply either sub-Eddington accretion onto the long-sought intermediate-mass black holes (a few 100-1000 solar masses) or super-Eddington accretion onto stellar-mass black holes (3-30 solar masses). A certain sub-sample of these sources exhibit long-term X-ray modulations: M82 with a period of 62 days and NGC 5408 X-1 with periods of 115 & 243 days. It is currently not clear whether these modulations are due to the orbital motion of the binary or if they represent a super-orbital modulation of the system (e.g., a precessing warped accretion disk). Identifying the source of these X-ray periods can help us better understand the nature of accretion processes operating within these systems. The speaker will discuss various properties of these modulations including their coherence, energy dependence, phase-resolved spectroscopy, time-resolved periodograms that will resolve some of the degeneracies between the various models.