Ramachandran, Varsha, Oskinova, Lidia M., Hamann, Wolf-Rainer, Sander, Andreas, Todt, Helge, Pauli, Daniel, Shenar, Tomer, Torrejon, Jose M., Postnov, Konstantin, Blondin, John M., Bozzo, Enrico, Hainich, Rainer, Massa, Derck
Phase-resolved spectroscopic analysis of the eclipsing black hole X-ray binary M33 X-7: System properties, accretion, and evolution
Astronomy & Astrophysics. 2022, 667: A77. https://doi.org/10.1051/0004-6361/202243683
URI: http://hdl.handle.net/10045/129634
DOI: 10.1051/0004-6361/202243683
ISSN: 0004-6361 (Print)
Abstract: 
M33 X-7 is the only known eclipsing black hole high mass X-ray binary. The system is reported to contain a very massive O supergiant donor and a massive black hole in a short orbit. The high X-ray luminosity and its location in the metal-poor galaxy M33 make it a unique laboratory for studying the winds of metal-poor donor stars with black hole companions and it helps us to understand the potential progenitors of black hole mergers. Using phase-resolved simultaneous HST- and XMM-Newton-observations, we traced the interaction of the stellar wind with the black hole. We observed a strong Hatchett-McCray effect in M33 X-7 for the full range of wind velocities. Our comprehensive spectroscopic investigation of the donor star (X-ray+UV+optical) yields new stellar and wind parameters for the system that differ significantly from previous estimates. In particular, the masses of the components are considerably reduced to ≈38 M⊙ for the O-star donor and ≈11.4 M⊙ for the black hole. The O giant is overfilling its Roche lobe and shows surface He enrichment. The donor shows a densely clumped wind with a mass-loss rate that matches theoretical predictions. An extended ionization zone is even present during the eclipse due to scattered X-ray photons. The X-ray ionization zone extends close to the photosphere of the donor during inferior conjunction. We investigated the wind-driving contributions from different ions and the changes in the ionization structure due to X-ray illumination. Toward the black hole, the wind is strongly quenched due to strong X-ray illumination. For this system, the standard wind-fed accretion scenario alone cannot explain the observed X-ray luminosity, pointing toward an additional mass overflow, which is in line with our acceleration calculations. The X-ray photoionization creates an He II emission region around the system emitting ∼1047 ph s−1. We computed binary evolutionary tracks for the system using MESA. Currently, the system is transitioning toward an unstable mass transfer phase, possibly resulting in a common envelope of the black hole and the O-star donor. Since the mass ratio is q ≳ 3.3 and the period is short, the system is unlikely to survive the common envelope, but will rather merge.
Keywords:Stars: black holes, Stars: massive, Stars: winds, Outflows, X-rays: binaries, Stars: fundamental parameters, Stars: evolution
EDP Sciences
info:eu-repo/semantics/article