| Description |
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. I 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.
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