Variable Doppler shifts of the thermal wind absorption lines in low-mass X-ray binaries

Abstract

In this paper we address the general applicability of the method pioneered by\n\\citet{Zhang2012} in which the motion of the compact object can be tracked\nusing wind X-ray absorption lines. We present the velocity measurements of the\nthermal wind lines observed in the X-ray spectrum of a few low-mass X-ray\nbinaries: GX 13+1, H 1743$-$322, GRO J1655$-$40 and GRS 1915+105. We find that\nthe variability in the velocity of the wind lines in about all of the sources\nis larger than conceivable radial velocity variations of the compact object. GX\n13+1 provides a potential exception, although it would require the red giant\nstar to be massive with a mass of $\\approx 5-6\\ M_{\\odot}$. We conclude that\nthe variability of the source luminosity occurring on a time scale of\ndays/months can affect the outflow properties making it difficult to track the\norbital motion of the compact object using current observations. Given the\nintrinsic variability of the outflows we suggest that low-mass X-ray binaries\nshowing stable coronae instead of an outflow (e.g. 4U 1254$-$69, MXB 1659$-$29,\n4U 1624$-$49) could be more suitable targets for tracking the orbital motion of\nthe compact object.\n