A LUMINOUS X-RAY BINARY IN M82

Optical CCD imaging at the position of a bright x-ray point source just off the visible disk of the nearby irregular galaxy M82 constrain it to be an x-ray binary with a 0.2-4 keV x-ray luminosity of greater-than-or-equal-to 9 x 10(38) ergs/s. Assuming that the x-ray emission is due to a single object, then by assuming an Eddington limit appropriate for pure electron scattering this luminosity requires a minimum primary mass of greater-than-or-equal-to 15 M.; thus, this x-ray source is a black hole candidate. If the brightest optical object detected inside the x-ray error circle at B almost-equal-to 22.2 is the secondary to the x-ray emitter, it is most likely an early type star making this system the most luminous massive x-ray binary system known.