Phase-resolved spectroscopic analysis of the eclipsing black hole X-ray binary M33 X-7: System properties, accretion, and evolution

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 ∼ 10 47 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. Based on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. These observations are associated with the GO program 15636. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, these observations are associated with id numbers 0831590201 & 0831590401.

Constraints on the Cosmological Coupling of Black Holes from the Globular Cluster NGC 3201

“…However, such low birth masses are in conflict with BHs in high-mass X-ray binaries (HMXBs), where a compact object accretes matter from the stellar winds of a young and massive (5 M e ) companion. Of the many known HMXBs (>100 in the MW alone; see the recent catalogs of Fortin et al 2023;Neumann et al 2023), three are known to host dynamically confirmed BHs: Cygnus X-1, with a BH mass of 21.2 ± 2.2 M e and an age of ∼4 Myr (Miller-Jones et al 2021); LMC X-1, with a BH mass of 10.91 ± 1.41 M e and an age of ∼5 Myr (Orosz et al 2009); and M33 X-7, with a BH mass of 11.4 M e and an age of ∼5.8 Myr (Ramachandran et al 2022). Because their accretion is fed from the winds of the companion, these BHs likely have masses close to their natal values.…”

Multiwavelength Characterization of the High-mass X-Ray Binary Population of M33

“…Supergiant/giant companion stars have been observed in HMXB systems in other galaxies with both black hole and neutron star accretors. Black hole HMXBs (BH-HMXBs) such as Cyg X-1 (Orosz et al 2011), LMC X-1 (Orosz et al 2009), and M33 X-7 (Orosz et al 2007;Ramachandran et al 2022, which is included in our sample as Source ID 225) all comprise black holes accreting from supergiant companion stars with masses 30 M e , suggesting they are very young systems because 30 M e stars have maximum lifetimes 10 Myr. Neutron star HMXBs with supergiant companion stars are also thought to be very young systems.…”

“…There are also two eclipsing X-ray binaries in M33 (Pietsch et al 2004(Pietsch et al , 2006(Pietsch et al , 2009. One of the two, M33 X-7, comprises a massive stellar-mass black hole in orbit with an O-type stellar companion (Orosz et al 2007;Ramachandran et al 2022). The exact orbital separation and masses of the black hole and companion star in M33 X-7 have been debated, but the system has provided interesting constraints for massive binary stellar evolution models.…”

The Missing Link between Black Holes in High-mass X-Ray Binaries and Gravitational-wave Sources: Observational Selection Effects

“…Orosz et al (2009) estimate it to have a donor mass of (31.8 ± 3.5)M e and a BH accretor mass of (10.9 ± 1.4)M e . Another resolved extragalactic HMXB is M33 X-7, an HMXB in the spiral galaxy M33 estimated to have a BH (Ramachandran et al 2022). Similar to Galactic HMXBs, the population of well-constrained extragalactic sources is small, and in this case, none are found to harbor BH accretors >20 M e .…”