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Chandra and XMM Monitoring of the Black Hole X-ray Binary IC 10 X-1

The massive black hole + Wolf-Rayet binary IC10 X-1 was observed in a series of 10 Chandra and 2 XMM-Newton observations spanning 2003-2012, showing consistent variability around 7 x10^37 erg/s, with a spectral hardening event in 2009. We phase-connected the entire light-curve by folding the photon arrival times on a series of trial periods spanning the known orbital period and its uncertainty, refining the X-ray period to P = 1.45175(1)d. The duration of minimum-flux in the X-ray eclipse is 5 hr which together with the optical radial velocity curve for the companion yields a radius for the eclipsing body of 8-10 Rsun for the allowed range of masses. The orbital separation of 18.5-22 Rsun then provides a limiting inclination i>63 degrees for total eclipses to occur. The eclipses are asymmetric (egress duration 0.9 hr) and show energy dependence, suggestive of an accretion-disk hotspot and corona. The eclipse is much (5X) wider than the 1.5-2 Rsun WR star, pointing to absorption/scattering in the dense wind of the WR star. The same is true of the close analog NGC 300 X-1. RV measurements of the He II [4686] line from the literature show a phase-shift with respect to the X-ray ephemeris such that the velocity does not pass through zero at mid-eclipse. The X-ray eclipse leads inferior conjunction of the RV curve by 90 degrees, so either the BH is being eclipsed by a trailing shock/plume, or the He II line does not directly trace the motion of the WR star and instead originates in a shadowed partially-ionized region of the stellar wind.