The ultraluminous X-ray source in M82: an intermediate-mass black hole with a giant companion

Abstract The starburst galaxy M82, at a distance of 12 million light years, is the host of an unusually bright 2.4-16 × 1040 erg s−1 X-ray point source, which is best explained by an accreting black hole 102 to 104 times more massive than the Sun. Though the strongest candidate for a so-called intermediate-mass black hole, the only support stems from the observed luminosity and the 0.05-0.1 Hz quasi-periodicity in its signal. Interestingly, the 7-12 Myr old star cluster MGG-11 which has been associated with the X-ray source is sufficiently dense that an intermediate mass black hole could have been produced in the cluster core via collision runaway. The recently discovered 62.0 ± 2.5 d periodicity in the X-ray source X-1 further supports the hypothesis that this source is powered by a black hole several hundred times more massive than the Sun. We perform detailed binary evolution simulations with an accreting compact object of 10-5000 M⊙ and find that the X-ray luminosity, the age of the cluster, the observed quasi-periodic oscillations and the now observed orbital period are explained best by a black hole of 200-5000 M⊙ that accretes material from a 22-25 M⊙ giant companion in a state of Roche-lobe contact. Interestingly, such a companion star is consistent with the expectation based on the tidal capture in a young and dense star cluster such as MGG-11, making the picture self-consistent.

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