Chandra and Hubble Space Telescope confirmation of the luminous and variable X-ray source IC 10 X-1 as a possible Wolf-Rayet, black hole binary

We present a Chandra and Hubble Space Telescope study of IC 10 X-1, the most luminous X-ray binary in the starburst galaxy closest to the Milky Way. Our new hard X-ray observation of X-1 confirms that it has an average 0.5-10 keV luminosity of 1.5 × 10³⁸ ergs s^-1, is strongly variable (a factor of ≈2 in ≲3 ks), and is spatially coincident (within 0″23 ± 0″30) with the Wolf-Rayet (W-R) star [MAC92] 17A in IC 10. The spectrum of X-1 is best fitted by a power law with ≈ 1.8 and a thermal plasma with kT ≈ 1.5 keV, although systematic residuals hint at further complexity. Taken together, these facts suggest that X-1 may be a black hole belonging to the rare class of W-R binaries; it is comparable in many ways to Cyg X-3. The Chandra observation also finds evidence for extended X-ray emission cospatial with the large nonthermal radio superbubble surrounding X-1.