Thefalse widowlink between neutron star X-ray binaries and spider pulsars

Abstract

The discovery of transitional millisecond pulsars (tMSPs) provided conclusive proof that neutron star (NS) low-mass X-ray binaries (LMXBs) comprise part of the evolutionary pathway towards binary millisecond pulsars (MSPs). Redback and black widow ‘spider’ pulsars are a sub-category of binary MSPs that ‘devour’ their companions through ablation - the process through which material is lifted from the stellar surface by a pulsar wind. In addition to reducing the companion star’s mass, ablation introduces observable characteristics like extended, energy-dependent and asymmetric eclipse profiles in systems observed at a sufficiently high inclination. Here, we present a detailed study and comparison of the X-ray eclipses of two NS LMXBs; Swift J1858.6−0814 and EXO 0748−676. Some of the X-ray eclipse characteristics observed in these two LMXBs are similar to the radio eclipse characteristics of eclipsing redback and black widow pulsars, suggesting that they may also host ablated companion stars. X-ray irradiation or a pulsar wind could drive the ablation. We conduct orbital phase-resolved spectroscopy for both LMXBs to map the column density, ionization and covering fraction of the material outflow. From this, we infer the presence of highly ionized and clumpy ablated material around the companion star in both systems. We term LMXBs undergoing ablation, false widows, and speculate that they may be the progenitors of redback pulsars under the assumption that ablation begins in the LMXB stage. Therefore, the false widows could provide a link between LMXBs and spider pulsars. The detection of radio pulsations during non-accreting states can support this hypothesis.

XRBcats: Galactic Low Mass X-ray Binary Catalogue

“…We also do not include rotation-powered pulsars even if they are members of binary systems and emit in X-rays, unless the observed emission is accretion powered. That is, "spider"-type pulsars including Black Widows (BW) and Redbacks (RB; see, e.g., Chen et al 2013;Roberts 2013) are omitted unless they are also classified as transitional milliseconds pulsars (tMSP; see, e.g., Archibald et al 2009;Papitto et al 2013;Knight et al 2023). All tMSPs and their candidates included in the catalogue are listed in Table 1 in Sect.…”

A Light Redback Companion of PSR J1622–0315 and Irradiation Power in Spider Systems

“…Best-fit Results with 1σ Uncertainties Halpern 2014), are important to constrain the binary evolution model. Recent findings suggest that both NS low-mass X-ray binaries and spider systems tend to have an ionized and clumpy environment(Knight et al 2023), implying that pulsar irradiation and mass loss in the form of winds are common in different evolution stages. Assuming the presence of pulsar irradiation, it is useful to compare the companion star…”