On the Spectral Shape of Gamma-ray Pulsars Above the Break Energy

TL;DR

This paper analyzes phase-resolved gamma-ray pulsar spectra above the break energy, revealing harder spectra than expected and proposing a new analytical fit to better model these observations within a physical framework.

Contribution

It demonstrates that gamma-ray pulsar spectra above the break are harder than simple models suggest and introduces a new analytical fit function for improved spectral modeling.

Findings

Spectra above the break are harder than exponential cutoff.

Hard spectra indicate non-stationary or multi-zone emission.

New fit function improves modeling of hard spectral tails.

Abstract

It is well known that, for bright gamma-ray pulsars with high statistics above a few GeV, the phase averaged spectral energy distribution (SED) is harder than a simple exponential cutoff above the break. We perform phase-resolved spectral analyses of bright gamma-ray pulsars and demonstrate that, even over narrow phase ranges, the SEDs of gamma-ray pulsars above the break energy are harder than a simple exponential cutoff. We argue within a radiation-reaction limited curvature framework that this is indicative of non-stationary emission or emission from multiple zones. Further, we address a common problem faced when fitting hard spectral tails with a power-law times a sub-exponential function. Namely, that the sub-exponent parameter does not describe any parameters of physical models of pulsar emission. We introduce a simple analytical fit function to solve this problem.