The stellar and wind parameters of six prototypical HMXBs and their evolutionary status

Abstract

Context. High-mass X-ray binaries (HMXBs) are exceptional astrophysical laboratories that offer a rare glimpse into the physical processes that govern accretion on compact objects, massive-star winds, and stellar evolution. In a subset of the HMXBs, the compact objects accrete matter solely from winds of massive donor stars. These so-called wind-fed HMXBs are divided in persistent (classical) HMXBs and supergiant fast X-ray transients (SFXTs) according to their X-ray properties. While it has been suggested that this dichotomy depends on the characteristics of stellar winds, they have been poorly studied. Aims. With this investigation, we aim to remedy this situation by systematically analyzing donor stars of wind-fed HMXBs that are observable in the UV, concentrating on those with neutron star (NS) companions. Methods. We obtained Swift X-ray data, HST UV spectra, and additional optical spectra for all our targets. The spectral analysis of our program stars was carried out with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. Results. Our multi-wavelength approach allows us to provide stellar and wind parameters for six donor stars (four wind-fed systems and two OBe X-ray binaries). The wind properties are in line with the predictions of the line-driven wind theory. Based on the abundances, three of the donor stars are in an advanced evolutionary stage, while for some of the stars, the abundance pattern indicates that processed material might have been accreted. When passing by the NS in its tight orbit, the donor star wind has not yet reached its terminal velocity but it is still significantly slower; its speed is comparable with the orbital velocity of the NS companion. There are no systematic differences between the two types of wind-fed HMXBs (persistent versus transients) with respect to the donor stars. For the SFXTs in our sample, the orbital eccentricity is decisive for their transient X-ray nature. The dichotomy of wind-fed HMXBs studied in this work is primarily a result of the orbital configuration, while in general it is likely that it reflects a complex interplay between the donor-star parameters, the orbital configuration, and the NS properties. Based on the orbital parameters and the further evolution of the donor stars, the investigated HMXBs will presumably form Thorne-Żytkow objects in the future.

Effects of radiative losses on the relativistic jets of high-mass microquasars

“…In binaries, the situation is more complex. Hainich et al (2020) present an observation based study of winds in HXRBs and basically confirm that the wind-driving can be put on the same general ideas as in single hot massive stars. However, the situation is more complicated due to the presence of a secondary radiation source, either another star or a compact object and its environment.…”

The origin of the high-mass X-ray binary 4U 2206+54/BD +53 2790

“…Taking into account the bolometric correction (BC = -3.2 mag, see, e.g., Pecaut & Mamajek 2013) for an O9.5V spectral type star we estimated the mass to be M= 23.5 +14.5 −8.0 M using the luminosity-mass relation for main-sequence stars selected from the components of detached eclipsing spectroscopic binaries in the solar neighborhood (Eker et al 2018, log L = (2.726 ± 0.203) × log M + (1.237 ± 0.228)). Note, the estimate of spectroscopic mass M=27 +67 −23 M (Hainich et al 2020) of BD+53 2790 exhibits 35% larger than its evolution mass, i.e. the mass of an object, which exhibits the current stellar and wind parameters, that has evolved like a single star.…”

“…Currently, the 4U 2206+54 is the only known HMXB system hosting a accreting magnetar with or without a fallback disk (Alpar et al 2013;Özsükan et al 2014). The donor star does not meet the criteria for a classical Be V star, but rather is a peculiar O9 V star with higher than normal helium abundance (Blay et al 2006) and a double peaked Hα emission line, as typical for the decretion disks (Hainich et al 2020). With an orbital period of 9.5 days, 4U 2206+54 exhibits one of the shortest orbital periods among known HMXBs.…”

Radiography in high mass X-ray binaries

“…Reig et al (1996) estimated the terminal wind speed to be 1 200 km/s, such that if the stellar radius is R * = 25R , we deduce from N H,0 a stellar mass loss rateṀ ∼ 7 • 10 −7 M yr −1 , very similar to the inferred mass loss rate of the donor star in Vela X-1 (Gimenez- Garcia et al 2016). Spectral characterization in the optical and UV waveranges of the stellar atmosphere with non local thermodynamical equilibrium analysis can bring additional constrains to break the degeneracy and disentangle between the different parameters which come into play in the value of N H,0 (Hainich et al 2020).…”

“…The amount of material involved in an observed flare is thus an upper limit on the mass of a clump: a flare might be triggered by the capture of a clump, as noticed by Bozzo et al (2015), but likely involves additional material which has been previously piling up at the outer rim of the NS magnetosphere due to the magneto-centrifugal gating mechanism (or propeller effect, Illarionov & Sunyaev 1975;Grebenev & Sunyaev 2007;Bozzo et al 2008), in a quasi-spherical hot shell on top of the NS magnetosphere (Shakura et al 2013) or in a disk-like structure (El Mellah et al 2019;Karino et al 2019). Also, Hainich et al (2020) showed that there was no systematic difference between the stellar properties in classic SgXBs and in SFXTs, hinting at similar wind micro-structures (Driessen et al 2019). Finally, Pradhan et al (2018) concluded that the higher variability of the emission in SFXTs compared to SgXBs was likely due to mechanisms inhibiting accretion near the accretor, rather than direct clump capture, although flares in SFXTs have recently been found by Ferrigno et al (2020) to be associated with massive structures approaching the accretor.…”