2015MNRAS.446.1399L

The massive black hole (BH)+Wolf-Rayet (WR) binary IC 10 X-1 was observed in a series of 10 Chandra and two XMM-Newton observations spanning 2003-2012, showing consistent variability around 7x10³⁷ 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 h which together with the optical radial velocity (RV) curve for the companion yields a radius for the eclipsing body of 8-10R_☉ for the allowed range of masses. The orbital separation (a1 + a2) = 18.5-22R_☉ then provides a limiting inclination i > 63° for total eclipses to occur. The eclipses are asymmetric (egress duration ∼ 0.9 h) and show energy dependence, suggestive of an accretion disc hotspot and corona. The eclipse is much ( ∼ 5x) wider than the 1.5-2R_☉ 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 [λλ4686] 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°, 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.