A 62 Day X‐Ray Periodicity and an X‐Ray Flare from the Ultraluminous X‐Ray Source in M82

Abstract

In 240 days of X-ray monitoring of M82, we have discovered an X-ray periodicity at 62.0 ? 2.5 days with a peak-to-peak amplitude corresponding to an isotropic luminosity of 2.4 ? 1040 ergs s-1 in M82 and an X-ray flare reaching a peak luminosity of 9.8 ? 1040 ergs 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 ? 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 that a second X-ray source reached a flux of 4.4 ? 10-12 ergs 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 ? 1040 ergs 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.