The dispersion in pulsar $\gamma$-ray efficiency

TL;DR

This study investigates the wide dispersion in pulsar gamma-ray efficiency by analyzing geometric, distance, and intrinsic factors, concluding that the intrinsic efficiency peaks at 5-15% and largely explains the observed variability.

Contribution

The paper introduces a method to estimate the intrinsic gamma-ray efficiency distribution of pulsars, highlighting its dominant role in observed efficiency dispersion.

Findings

Intrinsic efficiency peaks at 5-15%.

Dispersion is mainly due to intrinsic efficiency.

Weak correlation with spin-down power.

Abstract

The observational efficiency of pulsars, defined as the ratio of the observationally derived isotropic-equivalent luminosity, $4\pi d_{obs}^2 F_{obs}$, where $F_{obs}$ is the average pulsed energy flux of a pulsar and $d_{obs}$ is its estimated distance, to its energy budget, shows a wide range of values. This dispersion is believed to be a combination of beaming effects, different geometries, and case-by-case variability of the emission mechanism efficiency, but it is not clear in what proportion. In this work we focused on the gamma-ray range and analysed the four main ingredients that likely contribute to this dispersion: the geometrical term arising from the anisotropic emission (beaming), viewing and inclination angles, the uncertainty on the pulsar distance, the uncertainty on the moment of inertia, and the intrinsic efficiency of the mechanism producing the gamma-ray emission. Estimating the expected ranges of the moment of inertia and the distance errors, and considering a geometrical and spectral model that we have recently used to fit the light curves and spectra of the entire gamma-ray pulsar population, we estimate the a priori distribution of the first three ingredients in order to obtain the a posteriori distribution of the intrinsic efficiency of the mechanism. We found the latter to peak at $\sim 5-15 \%$ and to have a dispersion of around one order of magnitude. That is, we found the intrinsic efficiency of the mechanism to be the leading factor in the observed dispersion. In addition, we found little sensitivity of these results on different distributions of the estimated pulsar distance errors, and saw that the weak, alleged correlation with the spin-down power can only explain part of the observed dispersion. This methodology can be easily applied to other geometrical models of the emission, to test the sensitivity of these results on the beaming distribution.