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 pm 2.5 days with a peak to peak amplitude corresponding to an isotropic luminosity of $2.4 times 10 40 rm erg s -1 in M82 and an X-ray flare reaching a peak luminosity of $9.8 times 10 40 rm 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 rm 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 rm 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 rm 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.