Intra-Binary Shock Heating of Black Widow Companions

TL;DR

This paper introduces an intra-binary shock heating model for black widow pulsar companions, improving light curve fits and explaining observed asymmetries, with implications for companion evaporation timescales.

Contribution

It develops and implements a simple analytic IBS heating model in light curve analysis, enhancing understanding of heating patterns and X-ray emissions in black widow systems.

Findings

Improved fit to asymmetric optical light curves of PSR J2215+5135.

Model predicts X-ray emission patterns consistent with observations.

Estimated companion evaporation timescale of approximately 150 million years.

Abstract

The low mass companions of evaporating binary pulsars (black widows and their ilk) are strongly heated on the side facing the pulsar. However in high-quality photometric and spectroscopic data the heating pattern does not match that expected for direct pulsar illumination. Here we explore heating mediated by an intra-binary shock (IBS). We develop a simple analytic model and implement it in the popular `ICARUS' light curve code. The model is parameterized by the wind momentum ratio beta and velocity v_Rel v_orb and assumes that the reprocessed pulsar wind emits prompt particles or radiation to heat the companion surface. We illustrate an interesting range of light curve asymmetries controlled by these parameters. The code also computes the IBS synchrotron emission pattern, and thus can model black widow X-ray light curves. As a test we apply the results to the high quality asymmetric optical light curves of PSR J2215+5135; the resulting fit gives a substantial improvement upon direct heating models and produces an X-ray light curve consistent with that seen. The IBS model parameters imply that, at the present loss rate, the companion evaporation has a characteristic timescale tau_evap ~150My. Still, the model is not fully satisfactory, indicating additional unmodeled physical effects.