Energy-dependent Evolution in IC10 X-1: Hard Evidence for an Extended Corona and Implications

We have analyzed a ~130 ks XMM-Newton observation of the dynamically confirmed black hole + Wolf-Rayet (BH+WR) X-ray binary (XB) IC10 X-1, covering ~1 orbital cycle. This system experiences periodic intensity dips every ~35 hr. We find that energy-independent evolution is rejected at a >5σ level. The spectral and timing evolution of IC10 X-1 are best explained by a compact disk blackbody and an extended Comptonized component, where the thermal component is completely absorbed and the Comptonized component is partially covered during the dip. We consider three possibilities for the absorber: cold material in the outer accretion disk, as is well documented for Galactic neutron star (NS) XBs at high inclination; a stream of stellar wind that is enhanced by traveling through the L1 point; and a spherical wind. We estimated the corona radius (r _ADC) for IC10 X-1 from the dip ingress to be ~10⁶ km, assuming absorption from the outer disk, and found it to be consistent with the relation between r _ADC and 1-30 keV luminosity observed in Galactic NS XBs that spans two orders of magnitude. For the other two scenarios, the corona would be larger. Prior BH mass (M _BH) estimates range over 23-38 M _⊙, depending on the inclination and WR mass. For disk absorption, the inclination, i, is likely to be ~60-80°, with M _BH ~ 24-41 M _⊙. Alternatively, the L1-enhanced wind requires i ~ 80°, suggesting ~24-33 M _⊙. For a spherical absorber, i ~ 40°, and M _BH ~ 50-65 M _⊙.