Formation and nature of "Huntsman" binary pulsars

Abstract

Spider systems are a class of close binaries in which a neutron star first accretes from a normal companion, and later ablates it in some cases. New observations have expanded this category, with the addition of a Huntsman group, tentatively linked to a short donor phase along the red bump in the secondary evolutionary track. We present explicit evolutionary tracks that support the Huntsman nature recently suggested, and discuss how the whole class of spiders emerges from the full consideration of irradiation and ablating winds. We address the irradiation feedback (IFB) effects and the hydrogen-shell burning detachment (HSBD) simultaneously, and show that they act independently and do not interfere with each other, supporting a physical picture of the Huntsman group. We employ our binary evolution code to compute a suite of binary systems formed by a donor star and a neutron star for different initial orbital periods, assuming solar composition and Z=0.01. Although many models do not consider IFB, we also present the evolution with IFB for one system as an example. We found that the recently suggested association of Huntsman pulsar with the evolutionary stage where (as a consequence of the dynamics of HSBD) the system remains detached for a few million years is plausible. However, this feature alone is unable to account for the occurrence of Redback spider pulsars. Meanwhile, models including IFB, with pulsed mass transfer, display detachment episodes that can be naturally associated with the Redback stage. Irradiation feedback does not preclude or modify HSBD and in fact, Huntsman systems were already present as an implicit prediction in our earlier calculations. We conclude that Huntsman is an expected stage of the spider systems under quite general conditions. This is another step towards a unified picture of spider pulsars as a group.