In 240 days of X-ray monitoring of M82 , we have discovered an X-ray periodicity at 62.0 \pm 2.5 days with a peak to peak amplitude corresponding to an isotropic luminosity of 2.4 \times 10 ^ { 40 } erg s ^ { -1 } in M82 and an X-ray flare reaching a peak luminosity of 9.8 \times 10 ^ { 40 } erg s ^ { -1 } .
The periodicity and flare likely originate from the ultraluminous X-ray source ( ULX ) in M82 which has been identified as a possible intermediate mass black hole .
We suggest that the 62 day modulation is due to orbital motion within an X-ray binary with a Roche-lobe overflowing companion star which would imply that the average density of the companion star is near 5 \times 10 ^ { -5 } g cm ^ { -3 } and is therefore a giant or supergiant .
Chandra observations just after the flare show an energy spectrum that is consistent with a power-law with no evidence of a thermal component or line emission .
Radio observations made with the VLA during the flare allow us to rule out a blazar identification for the source and place strong constraints on relativistically beamed models of the X-ray emission .
The Chandra observations reveal a second X-ray source reached a flux of 4.4 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } in the 0.3-7 keV band which is dramatically higher than any flux previously seen from this source and corresponds to an isotropic luminosity of 1.1 \times 10 ^ { 40 } erg s ^ { -1 } .
This source is a second ultraluminous X-ray source in M82 and may give rise to the QPOs detected from the central region of M82 .