标题：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
摘要：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 (ν∞ =
1500 km s-1 in IGR J17544-2619 and ν∞ = 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 to
propeller regimes when combined with other parameters.
