  We report a possible detection of a 55-day X-ray modulation for the
ultraluminous accreting pulsar M82 X-2 from archival Chandra observations.
Because M82 X-2 is known to have a 2.5-day orbital period, if the 55-day period
is real, it will be the superorbital period of the system. We also investigated
variabilities of other three nearby ultraluminous X-ray sources in the central
region of M82 with the Chandra data and did not find any evidence of
periodicities. Furthermore, we re-examined the previously reported 62-day
periodicity near the central region of M82 by performing a systematic timing
study with all the archival Rossi X-Ray Timing Explorer and Swift data. Using
various dynamic timing analysis methods, we confirmed that the 62-day period is
not stable, suggesting that it is not the orbital period of M82 X-1 in
agreement with previous work.
