title: The orbital period of the ultraluminous x-ray source in M82.
abstract: The ultraluminous x-ray source (ULX) in the galaxy M82 has been identified as a possible intermediate-mass black hole formed in stellar collisions in the super star cluster MGG 11. The x-ray flux from M82 is modulated, with a peak-to-peak amplitude corresponding to an isotropic luminosity of 2.4 _ 1040 erg s-1 in M82 and a period of 62.0 +/- 2.5 days, which we interpret as the orbital period of the ULX binary. This orbital period implies that the mass-donor star must be a giant or supergiant. Large mass-transfer rates, sufficient to fuel the ULX, are expected for a giant-phase mass donor in an x-ray binary. The giant phase has a short lifetime, indicating that we see the ULX in M82 in a brief and unusual period of its evolution.
authors.affiliation: Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA. philip-kaaret@uiowa.edu
authors.forename: Philip
authors.initials: P
authors.lastname: Kaaret
authors.forename: Melanie G
authors.initials: MG
authors.lastname: Simet
authors.forename: Cornelia C
authors.initials: CC
authors.lastname: Lang