Revisiting the archetypical wind accretor Vela X-1 in depth

Abstract

Context. The Vela X-1 system is one of the best-studied X-ray binaries because it was detected early, has persistent X-ray emission, and a rich phenomenology at many wavelengths. The system is frequently quoted as the archetype of wind-accreting high-mass X-ray binaries, and its parameters are referred to as typical examples. Specific values for these parameters have frequently been used in subsequent studies, however, without full consideration of alternatives in the literature, even more so when results from one field of astronomy (e.g., stellar wind parameters) are used in another (e.g., X-ray astronomy). The issues and considerations discussed here for this specific, very well-known example will apply to various other X-ray binaries and to the study of their physics. Aims. We provide a robust compilation and synthesis of the accumulated knowledge about Vela X-1 as a solid baseline for future studies, adding new information where available. Because this overview is targeted at a broader readership, we include more background information on the physics of the system and on methods than is usually done. We also attempt to identify specific avenues of future research that could help to clarify open questions or determine certain parameters better than is currently possible. Methods. We explore the vast literature for Vela X-1 and on modeling efforts based on this system or close analogs. We describe the evolution of our knowledge of the system over the decades and provide overview information on the essential parameters. We also add information derived from public data or catalogs to the data taken from the literature, especially data from the Gaia EDR3 release. Results. We derive an updated distance to Vela X-1 and update the spectral classification for HD 77518. At least around periastron, the supergiant star may be very close to filling its Roche lobe. Constraints on the clumpiness of the stellar wind from the supergiant star have improved, but discrepancies persist. The orbit is in general very well determined, but a slight difference exists between the latest ephemerides. The orbital inclination remains the least certain factor and contributes significantly to the uncertainty in the neutron star mass. Estimates for the stellar wind terminal velocity and acceleration law have evolved strongly toward lower velocities over the years. Recent results with wind velocities at the orbital distance in the range of or lower than the orbital velocity of the neutron star support the idea of transient wind-captured disks around the neutron star magnetosphere, for which observational and theoretical indications have emerged. Hydrodynamic models and observations are consistent with an accretion wake trailing the neutron star. Conclusions. With its extremely rich multiwavelength observational data and wealth of related theoretical studies, Vela X-1 is an excellent laboratory for exploring the physics of accreting X-ray binaries, especially in high-mass systems. Nevertheless, much room remains to improve the accumulated knowledge. On the observational side, well-coordinated multiwavelength observations and observing campaigns addressing the intrinsic variability are required. New opportunities will arise through new instrumentation, from optical and near-infrared interferometry to the upcoming X-ray calorimeters and X-ray polarimeters. Improved models of the stellar wind and flow of matter should account for the non-negligible effect of the orbital eccentricity and the nonspherical shape of HD 77581. There is a need for realistic multidimensional models of radiative transfer in the UV and X-rays in order to better understand the wind acceleration and effect of ionization, but these models remain very challenging. Improved magnetohydrodynamic models covering a wide range of scales are required to improve our understanding of the plasma-magnetosphere coupling, and they are thus a key factor for understanding the variability of the X-ray flux and the torques applied to the neutron star. A full characterization of the X-ray emission from the accretion column remains another so far unsolved challenge.

Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1

“…The neutron star is thus embedded in the dense wind of the supergiant companion, which has a mass loss rate of ∼10 −6 M yr −1 (Watanabe et al 2006). The source is located at 1.99 +0.13 −0.11 kpc (Kretschmar et al 2021) and is therefore one of the brightest persistent point sources in the X-ray sky despite a moderate mean luminosity of 5×10 36 erg s −1 (Fürst et al 2010). The mass of the neutron star is estimated to be ∼1.7-2.1 M (Kretschmar et al 2021).…”

“…Vela X-1 is an eclipsing high-mass X-ray binary (HMXB) that consists of a B0.5 Ib supergiant, HD 77581 (Hiltner et al 1972), and an accreting neutron star with a pulse period of ∼283 s (McClintock et al 1976) in an ∼8.964 d orbit around this supergiant (Kreykenbohm et al 2008). A thorough review of the Vela X-1 system is given by Kretschmar et al (2021), who also discussed the role of this system in understanding HMXB systems in general. Here, we limit ourselves to points that are of immediate relevance for this work.…”

X-ray emission from BH+O star binaries expected to descend from the observed galactic WR+O binaries

“…However, in Cygnus X-1, the wind-captured disk is permanent and so far, the only wind-fed HMXB where a transient wind-captured disk has been observed is Vela X-1 (Liao et al 2020). In the latter case, the disk formation is believed to be associated with variations at the periastron induced by the slightly eccentric orbital motion, rather than with clump capture (Kretschmar et al 2021). Therefore, including wind clumping is not expected to significantly modify the results obtained in this paper.…”

Variations of cyclotron resonant scattering features in Vela X-1 revealed with Insight-HXMT

“…In Fig. 7, H changes from (3 − 8) × 10 22 cm −2 for the observation (a), and from (1 − 3) × 10 22 cm −2 for (b), while from (7−10)×10 22 cm −2 for (c), these strong variations of column density in Vela X-1 suggest the inhomogeneous structure of winds from the supergiant donor (also see Fürst et al 2010;Sidoli et al 2015;Malacaria et al 2016;Grinberg et al 2017;Kretschmar et al 2021). The dips of the pulsar profiles at phase 0 and 0.5 would be due to the increase of this additional absorption column density (e.g., through accretion flow matter).…”