The Optical Counterpart of an Ultraluminous X-Ray Object in M81

Abstract

Ultraluminous X-ray objects (ULXs) are off-nucleus point sources with L_X=10³⁹-10⁴¹ ergs s^-1 they are among the most poorly understood X-ray sources. To help understand their nature, we are trying to identify their optical counterparts by combining images from the Hubble Space Telescope and the Chandra X-Ray Observatory. Here we report on an optical counterpart to ULX NGC 3031 X-11, which has an average X-ray luminosity of ~2×10³⁹ ergs s^-1 and has varied by a factor of 40% over the last 20 yr. We find a unique optical counterpart with the magnitude and color of an O8 V star, and we identify this as the secondary in a binary system. The primary is believed to be a black hole of approximately 18 M_solar, based on analyses of ACIS and ASCA spectra. This binary system probably is powered by mass transfer from the O8 V star onto the black hole, with the Roche lobe equal to the stellar radius. This model predicts an orbital period of ~1.8 days, which can be tested by future observations.