Recently a bright X-ray point source has been observed some 200 pc from the center of the starburst galaxy M82 (Matsumoto1999, Matsumoto et al. 2001, Kaaret et al. 2001). Shining at >10^{41} erg/s, it is too bright for an ordinary X-ray binary, while its off-center location in the galaxy argues against a supermassive black hole. The luminosity is consistent with an accreting compact object of at least 350 solar mass; intermediate between stellar-mass and supermassive black holes. The discovery of $54.4\pm0.9$\,mHz quasi-periodic oscillations (Strohmayer & Mushotzky 2003) supports this case. The X-ray source is apparently located in the star cluster MGG-11. Curiously, its more massive neighbor MGG-9 shows no X-rays. Based on recent observations of both clusters (McCrady et al. 2003), we have performed realistic star-by-star simulations of each cluster. For MGG-11, where the mass-segregation time scale is comparable to the lifetime of the massive stars, we typically find a runaway collision, leading to the formation of a 800--3000 solar mass black hole. No such runaway occurs in MGG-9, because the mass-segregation time is too large. We have thus identified a natural explanation for the observed X-ray emission from MGG-11 and its absence in MGG-9.


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