  M82 X-1 is the brightest ultraluminous X-ray source in starburst galaxy M82
and is one of the best intermediate mass black hole candidates. Previous
studies based on the Rossi X-ray Timing Explorer/Proportional Counter Array
(RXTE/PCA) reported a regular X-ray flux modulation of M82 with a period of 62
days, and attributed this periodic modulation to M82 X-1. However, this
modulation is not necessarily from M82 X-1 because RXTE/PCA has a very poor
spatial resolution of ~1 degree. In this work, we analyzed 1000 days of
monitoring data of M82 X-1 from the Swift/X-ray telescope (XRT), which has a
much better spatial resolution than RXTE/PCA. The periodicity distribution map
of M82 reveals that the 62-day periodicity is most likely not from M82 X-1, but
from the summed contributions of several periodic X-ray sources 4 arcsec
southeast of M82 X-1. However, Swift/XRT is not able to resolve those periodic
sources and locate the precise origin of the periodicity of M82. Thus, more
long-term observations with higher spatial resolution are required.
