A possible detection mechanism for pre-low-mass X-ray binaries

Abstract

This work presents a possible detection mechanism for close, detached, neutron star–red dwarf binaries, which are expected to be the evolutionary precursors of low‐mass X‐ray binaries (LMXBs). Although this pre‐low‐mass X‐ray binary (pre‐LMXB) phase of evolution is predicted theoretically, as yet no such systems have been identified observationally. The calculations presented here suggest that the X‐ray luminosity of neutron star wind accretion in a pre‐LMXB system can be expected to exceed the intrinsic X‐ray luminosity of the red dwarf secondary star. Furthermore, the temperature of the radiation emitted from the neutron star wind accretion process is expected, within the confines of a reasonable set of conditions, to lie within the detection range of X‐ray satellites. Sources with X‐ray luminosities greater than that expected for a red dwarf star, but the positions of which coincide with that of a red dwarf star, are then candidate pre‐LMXB systems. These candidate systems should be surveyed for the radial velocity shifts that would occur as a result of the orbital motion of a red dwarf star within a close binary system containing a high‐mass compact object.

Pre-low-mass X-ray binaries containing a black hole: investigating a detection mechanism

“…Bleach [5] considered that a close, detached, neutron star-red dwarf evolutionary phase (or pre-LMXB(NS) phase), is, or at least can be, incorporated into all of the generally accepted formation channels for LMXBs containing a neutron star. This detached evolutionary phase would correspond to the close, detached, binary systems classed precataclysmic variables (pre-CVs) that consist of a white dwarf, or white dwarf precursor, primary star and a late-type, low-mass, main-sequence secondary star and that evolve via angular momentum loss processes into semi-detached CV systems (see e.g.…”

“…Considering the expected occurrence of a pre-LMXB(NS) phase of evolution, Bleach [5] proposed a possible detection mechanism for such systems. The difficulty in the detection of pre-LMXB(NS) systems is that, unlike the compact white dwarfs in pre-CVs, neutron stars fail to emit any observable intrinsic radiation.…”

“…The approach of Bleach [5] was to identify candidate systems from their X-ray emission. The origin of the X-rays would be the accretion of the red dwarf stellar wind by the neutron star.…”

“…Bleach [5] concluded that based on a basic wind accretion model the detection of preLMXBs via neutron star accretion of a red dwarf stellar wind is feasible, albeit within the confines of a set of reasonable conditions.…”

“…Willems & Kolb [7] subsequently examined the detection probability of pre-LMXB(NS) systems according to the detection mechanism proposed by Bleach [5] in the context of a binary population synthesis study. Their calculations suggest that pre-LMXB(NS) systems may provide a non-negligible contribution to the population of discrete lowluminosity X-ray sources in the Galaxy (although uncertainties remain over the ability of neutron stars to undergo surface accretion due to the propeller effect [8,9]).…”