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 \sim 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 ) .