Effects of Irradiation on Companions in Spider Pulsar Binaries

Abstract

Millisecond “spider” pulsars irradiate and ablate their low mass binary companion stars, but the impact of prolonged high energy irradiation remains uncertain. Black widows and redbacks are the two main types of spider system. In black widows the companion is extremely low mass, while in redbacks it is larger. This project investigates the effects of such irradiation on the companion using a coupled workflow, beginning with a Monte Carlo model of gamma-ray propagation (sampling the Fermi-LAT spectral energy distribution of PSR B1957+20). Using the resultant depths of gamma-ray penetration, several MESA simulations were run to assess the companion’s response on both main types of spider system, black widows and redbacks. Focussing on Compton scattering, the depth-dependent energy deposition was used to inform power injection into the envelope of the companion stars using MESA. The Monte Carlo results show that the majority of the gamma-ray deposition occurs in the outer 1000 km of both black widows and redbacks. In the black widow case, continuous deposition at this radius increased radius by ≈15% and increased luminosity and effective temperature by ≈2500% and ≈110% respectively. 95% of the injected power re-emerges very quickly as radiation, indicating that there is minimal deep storage. In contrast, a redback with the same intercepted power shows only ≈2.4% radius bloating, a slight effective temperature decrease, and a luminosity boost of only ≈4.6%. Only around ≈18% of the injected power re-emerges as radiation, consistent with thicker envelope storing the heat. To stand in for inherently 3D, anisotropic wind and shock heating within a 1D model, a spherically averaged “wind” experiment was performed. This deposited power in the outer 0.1% of a black widow companion’s mass and reduced the convective efficiency to trap the heat. This saw inflation to near Roche lobe fill (≈1.96 ×105 km, which corresponds to ≈212% increase from the no injection case) and log(g) ≈4.3 in a PSR J1555−2908-like companion. This suggests that wind/shock input is likely the main driver of radial bloating since the gamma-ray simulations yielded only modest expansion (≈15% in the black widow case, and ≈2.4% in the redback case). Limitations and caveats of all stages of the project are discussed.

Date of Award	22 Dec 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Rene Breton (Main Supervisor) & Laura Wolz (Co Supervisor)