Constraining the Pulsar Beaming Fraction with TeV-Selected Galactic Pulsar Wind Nebulae and unidentified TeV Sources

TL;DR

This study estimates pulsar beaming fractions across different observational bands and surveys using TeV-selected PWNe and unidentified sources, revealing survey-dependent differences likely due to selection effects.

Contribution

It introduces a method to estimate pulsar beaming fractions using TeV data and demonstrates a unified model with a time-dependent opening angle consistent with observations.

Findings

Beaming fractions are roughly 0.1-0.3 across surveys and bands.

Discrepancies between surveys are likely due to selection effects.

A unified, time-dependent opening angle model reproduces observed properties.

Abstract

The pulsar beaming fraction is a fundamental quantity for connecting the observed pulsar population to the intrinsic Galactic population and for constraining pulsar emission geometry. In this study, we estimate the beaming fraction in each observational band (radio, $\gamma$-ray, and X-ray) and for each TeV survey (H.E.S.S., HAWC, and LHAASO) using TeV-selected pulsar wind nebulae (PWNe) and unidentified (Unid) TeV sources, assuming that the TeV emission from PWNe is approximately isotropic and that Unid sources are PWNe powered by pulsars whose beams do not intersect our line of sight. Within each survey, the inferred beaming fractions $\sim 0.1-0.3$ are comparable across bands. In contrast, the values differ by more than a factor of two between H.E.S.S. and HAWC/LHAASO. This discrepancy likely reflects survey-dependent selection effects, including differences in angular resolution and energy range, and is also consistent with the possibility that HAWC/LHAASO selected samples preferentially include older pulsars associated with more extended PWNe than those in the H.E.S.S. sample. We further show that the inferred beaming fractions can be reproduced within a unified framework using a time-dependent opening angle, and that this framework remains compatible with the statistical properties of the observed pulsar population.