Identifying the formation mechanism of redback pulsars

Abstract

We analyse the evolution of close binary systems containing a neutron star that lead to the formation of redback pulsars. Recently there has been some debate on the origin of such systems and the formation mechanism of redbacks may still be considered as an open problem. We show that the operation of a strong evaporation mechanism, starting from the moment when the donor star becomes fully convective (or alternatively since the formation of the neutron star by accretion induced collapse), produces systems with donor masses and orbital periods in the range corresponding to redbacks with donors appreciably smaller than their Roche lobes, i.e., they have low filling factors (lower than 0.75). Models of redback pulsars can be constructed assuming the occurrence of irradiation feedback. They have been shown to undergo cyclic mass transfer during the epoch at which they attain donor masses and orbital periods corresponding to redbacks, and stay in quasi-Roche lobe overflow conditions with high filling factors. We show that, if irradiation feedback occurs and radio ejection inhibits further accretion onto the neutron star after the first mass transfer cycle, the redback systems feature high filling factors. We suggest that the filling factor should be considered as a useful tool for discriminating among those redback formation mechanisms. We compare theoretical results with available observations, and conclude that observations tend to favour models with high filling factors.

Black widow formation by pulsar irradiation and sustained magnetic braking

“…It is instructive to compare our evolutionary tracks to previous studies that computed similar tracks (Chen et al 2013;Benvenuto et al 2015;De Vito et al 2020). In those studies, companions are also initially kept at stable Roche-lobe overflow, but the angular momentum loss rate is calculated using an extrapolation of the Rappaport et al (1983) formula, whereas we derive an appropriate rate for evaporating spiders (Section 3.1.2).…”

“…A key feature in previous black widow and redback calculations is the abrupt shutoff of magnetic braking once the companion's radiative core vanishes and it becomes fully convective, at about 0.3M ⊙ on the main sequence (Chen et al 2013;De Vito et al 2020). This procedure, following Rappaport et al (1983), is based on the assumption that the magnetic field is generated at the inner radiative-convective boundary (the 'tachocline').…”

“…Over the last decade, the number of eclipsing millisecond radio pulsars has greatly increased, thanks to dedicated surveys and followup of Fermi γ-ray sources (Keith et al 2010;Bates et al 2011;Ray et al 2012;Roberts 2013). These systems are often divided into 'black widows' -with companions a few per cent the mass of the sun M ⊙ , and 'redbacks' -with companions more massive than about 0.1M ⊙ (Chen et al 2013;Roberts 2013;De Vito et al 2020).…”

“…The companions of black widows and redbacks (collectively known as 'spiders') are believed to be the remnants of main sequence stars that lost orbital angular momentum and were driven to Roche-lobe overflow by magnetic braking -spinning up their host pulsars to millisecond periods in the process (Phinney et al 1988;Benvenuto et al 2012;Chen et al 2013). It is usually assumed that magnetic braking halts when the companion becomes fully convective, and from then onward it loses mass primarily through ablation by the pulsar's high-energy irradiation (Chen et al 2013;De Vito et al 2020). The ablated wind, which is ultimately powered by the pulsar's spin down, may explain the observed eclipses in these systems (Kluzniak et al 1988;Phinney et al 1988).…”

“…A key ingredient in our calculation is the suppression of the companion's cooling by the optically deep radiative layer induced by the pulsar's incident flux. This effect has been found to play an important role in the evolution of hot Jupiters (Guillot et al 1996;Arras & Bildsten 2006), and related effects were also considered in the context of spider pulsars (Benvenuto et al 2012(Benvenuto et al , 2014De Vito et al 2020).…”