  We have analyzed all archival RXTE/PCA monitoring observations of the
ultraluminous X-ray source (ULX) M82 X-1 in order to study the properties of
its 62 day X-ray period (Kaaret & Feng 2007). Based on its high coherence it
has been argued that the observed period is the orbital period of the binary.
Utilizing a much longer data set than in previous studies we find: (1) The
phase-resolved X-ray (3-15 keV) spectra -- modeled with a thermal accretion
disk and a power-law -- suggest that the accretion disk's contribution to the
total flux is strongly modulated with phase. (2) Suggestive evidence for a
sudden phase shift--of approximately 0.4 in phase (25 days)-- between the first
and the second halves of the light curve separated by roughly 1000 days. If
confirmed, the implied timescale to change the period is ~ 10 yrs, which is
exceptionally fast for an orbital phenomenon. These two independent pieces of
evidence are consistent with the periodicity being due to a precessing
accretion disk, similar to the super-orbital periods observed in systems like
Her X-1, LMC X-4, and SS433. However, the timing evidence for a change in the
period needs to be confirmed with additional observations. This should be
possible with further monitoring of M82 with instruments such as the Swift
X-ray telescope (XRT).
