Measuring the stellar wind parameters in IGR J17544-2619 and Vela X-1 constrains the accretion physics in supergiant fast X-ray transient and classical supergiant X-ray binaries

Abstract

Context. Classical supergiant X-ray binaries (SGXBs) and supergiant fast X-ray transients (SFXTs) are two types of high-mass X-ray binaries (HMXBs) that present similar donors but, at the same time, show very different behavior in the X-rays. The reason for this dichotomy of wind-fed HMXBs is still a matter of debate. Among the several explanations that have been proposed, some of them invoke specific stellar wind properties of the donor stars. Only dedicated empiric analysis of the donors' stellar wind can provide the required information to accomplish an adequate test of these theories. However, such analyses are scarce. Aims. To close this gap, we perform a comparative analysis of the optical companion in two important systems: IGR J17544-2619 (SFXT) and Vela X-1 (SGXB). We analyze the spectra of each star in detail and derive their stellar and wind properties. As a next step, we compare the wind parameters, giving us an excellent chance of recognizing key differences between donor winds in SFXTs and SGXBs. Methods. We use archival infrared, optical and ultraviolet observations, and analyze them with the non-local thermodynamic equilibrium (NLTE) Potsdam Wolf-Rayet model atmosphere code. We derive the physical properties of the stars and their stellar winds, accounting for the influence of X-rays on the stellar winds. Results. We find that the stellar parameters derived from the analysis generally agree well with the spectral types of the two donors: O9I (IGR J17544-2619) and B0.5Iae (Vela X-1). The distance to the sources have been revised and also agree well with the estimations already available in the literature. In IGR J17544-2619 we are able to narrow the uncertainty to d = 3.0 ± 0.2 kpc. From the stellar radius of the donor and its X-ray behavior, the eccentricity of IGR J17544-2619 is constrained to e < 0.25. The derived chemical abundances point to certain mixing during the lifetime of the donors. An important difference between the stellar winds of the two stars is their terminal velocities ( ∞ = 1500 km s −1 in IGR J17544-2619 and ∞ = 700 km s −1 in Vela X-1), which have important consequences on the X-ray luminosity of these sources. Conclusions. The donors of IGR J17544-2619 and Vela X-1 have similar spectral types as well as similar parameters that physically characterize them and their spectra. In addition, the orbital parameters of the systems are similar too, with a nearly circular orbit and short orbital period. However, they show moderate differences in their stellar wind velocity and the spin period of their neutron star which has a strong impact on the X-ray luminosity of the sources. This specific combination of wind speed and pulsar spin favors an accretion regime with a persistently high luminosity in Vela X-1, while it favors an inhibiting accretion mechanism in IGR J17544-2619. Our study demonstrates that the relative wind velocity is critical in class determination for the HMXBs hosting a supergiant donor, given that it may shift the accretion mechanism from direct accretion...

Multi-wavelength observations of IGR J17544-2619 from quiescence to outburst

“…This confirms previous findings that most of the outbursts displayed by IGR J17544-2619 occur when the neutron star is closer to the supergiant companion and that the orbit of this system could be characterized by a non-negligible eccentricity (see, e.g., Drave et al 2014;Romano 2015, and references therein). Although this eccentricity could help in enhancing the X-ray dynamic range achievable by IGR J17544-2619, it cannot be the only explanation for the extreme behavior displayed by this source, as the system orbital period is relatively short and only a limited eccentricity of 0.2-0.3 can be expected (Walter et al 2015;Giménez-García et al 2016). …”

“…However, precise measurements of the wind properties are still scarce due to the large distances of the SFXTs that make observations challenging, especially in the ultraviolet domain. Furthermore, through the performed observational cam-paigns so far, it was not possible to identify a net difference in the stellar wind properties of SFXTs and classical SgXBs, and thus the tentative discrimination between the proposed theoretical scenarios in the two classes of sources still relies upon the largely unknown values of the neutron star pulsation periods and magnetic field strength (see discussions in Bozzo et al 2016;Giménez-García et al 2016).…”

“…A likely cyclotron line at 17 keV has been discovered in a NuSTAR observation carried out in 2013 (Bhalerao et al 2015), making IGR J17544-2619 the first SFXT for which a direct measurement of the magnetic field is available (B ≃ 1.5 × 10 12 G). The properties of the supergiant companion hosted in IGR J17544-2619 were recently investigated in depth by Giménez-García et al (2016), confirming it to be a O9I supergiant.…”

Spectral evidence of an accretion disc in wind-fed X-ray pulsar Vela X-1 during an unusual spin-up period

“…We adopt a mass of 1.8M with an uncertainty about 20%. Then one get Racc ≈ 10 lt-s for v rel ∼ 400 km s −1 (Giménez-García et al 2016). We looked for the time delay between the Fe Kα line and the continuum (3-6 keV) with the cross-correlation function for light curves in Seg.…”

An Optical and Infrared Time-domain Study of the Supergiant Fast X-Ray Transient Candidate IC 10 X-2

“…In order to test the viability of Equation (5), we use it to compare against the orbital period (P orb =8.95 days) of the wellstudied wind-fed HMXB Vela X-1 (Hiltner et al 1972;Lamers et al 1976;Quaintrell et al 2003). We adopt the Vela X-1 supergiant stellar wind velocity, mass, and mass-loss rate provided by Giménez-García et al (2016) and the X-ray luminosity derived by Sako et al (1999):…”