Letter to the Editor

Formation and nature of “Huntsman” binary pulsars

¹ Instituto de Astrofísica de La Plata, IALP, CCT-CONICET-UNLP, Argentina and Facultad de Ciencias Astronómicas y Geofísicas de La Plata, Paseo del Bosque S/N, (1900) La Plata, Argentina
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
³ Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, R. do Matão, 1226, Cidade Universitária, 05508-090 São Paulo SP, Brazil

^⋆ Corresponding author: obenvenu@fcaglp.unlp.edu.ar

Received: 14 March 2025
Accepted: 7 May 2025

Abstract

Context. “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 along the secondary evolutionary track.

Aims. We present here explicit evolutionary tracks that support the Huntsman nature recently suggested, and also discuss how the whole class of spiders emerge 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.

Methods. We employed our binary evolution code to compute a suite of binary systems formed by a donor star together with a neutron star for different initial orbital periods, for the case of solar composition and also for Z = 10⁻³. Although many models do not consider IFB, we also present the evolution with IFB for one system as an example.

Results. We found that the recently suggested association of Huntsman pulsar with the evolutionary stage where (as consequence of the dynamics of HSBD) the system remains detached for a few million years is truly plausible. However, this feature alone is unable to account for the occurrence of the 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, the latter were implicit in our earlier calculations, but not addressed explicitly. That is, Huntsman systems were already present as an “implicit prediction” in these former works.

Conclusions. We conclude that Huntsman is an expected stage of these spider systems under quite general conditions. This is another step towards a unified picture of spider pulsars as a group.