Search for gamma-ray variability around Fermi-LAT pulsar glitches

TL;DR

This study investigates whether gamma-ray emission from pulsars varies in connection with rotational glitches using 14 years of Fermi-LAT data. By selecting seven glitches with substantial spin-down-rate changes and applying a binned likelihood analysis with summed likelihood weights and a PLEC4 spectral model, the authors search for flux and spectral variations around glitch epochs. Across all targets, no significant gamma-ray variability is detected, and 95% upper limits on flux changes are established, with the Vela pulsar providing the tightest constraint at about 0.5% relative change. The results imply that glitch-induced gamma-ray variability is rare among LAT pulsars, reaffirming PSR J2021+4026 as a unique case and underscoring the value of larger, targeted glitch studies to unravel magnetospheric emission physics.

Abstract

Pulsars are the most numerous class of Galactic gamma-ray sources detected by the Fermi Large Area Telescope (LAT). Young pulsars occasionally experience sudden timing discontinuities called glitches, characterized by rapid changes in rotational parameters, usually followed by a return to regular rotation. PSR J2021+4026 is unique among Fermi-LAT pulsars, exhibiting quasiperiodic switches between two states with varying spin-down rates approximately every 3-4 years, correlated with sudden changes in gamma-ray emission features. This study searched for gamma-ray emission variability in pulsars correlated with glitch occurrence. We introduced a novel approach to analyzing LAT gamma-ray pulsars through systematic examination of variability associated with spin-down rate changes. We tracked rotation and gamma-ray emission changes for seven promising glitches selected based on observed spin-down rate variations. Using 14 years of Fermi-LAT data, we conducted binned likelihood spectral analysis of data windows around selected glitch epochs. We improved best-fit parameter precision by incorporating likelihood weights calculated from diffuse background models, accounting for systematic error contributions. The phase-averaged flux and spectral parameters of analyzed pulsars showed no significant variation across investigated glitches. The 95% upper limits on relative flux change indicate the Vela pulsar flux is unchanging, with a 0.5% upper limit on relative change, making it promising for further searches. The connection between glitch dynamics and gamma-ray emission remains unclear, and PSR J2021+4026 remains unique in its gamma-ray variability properties. We conclude that comprehensive investigation of glitches is warranted to further unravel their underlying mechanisms.

Figures (1)

• Figure 1: Distribution of $|\Delta\Dot{\nu}/\Dot{\nu}|$ with respect to the 0.1--100 GeV energy flux, according to the 4FGL catalog 2020ApJS..247...33A. The green line indicates the value for J2021. Green dots indicate glitches with a larger jump than J2021; the orange dot corresponds to PSR J0742-2822, the only glitch with a nonzero jump in spin-down.