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

¹ Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Nikolaus-Fiebiger-Str. 2, 91058 Erlangen, Germany
² Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy
³ Università di Pisa, Dipartimento di Fisica E. Fermi, I-56127 Pisa, Italy
⁴ Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA

^⋆ Corresponding authors: giovanni.cozzolongo@fau.de; alessio.fiori@pi.infn.it; massimiliano.razzano@unipi.it

Received: 29 April 2025
Accepted: 14 August 2025

Abstract

Context. Pulsars are the most numerous class of Galactic γ-ray sources detected by the Fermi Large Area Telescope (LAT). Young pulsars occasionally experience sudden timing discontinuities called glitches, which are characterized by rapid changes in their rotational parameters, and are usually followed by a return to their regular rotation behavior. PSR J2021+4026 is a unique Fermi-LAT pulsar in that it exhibits peculiar, quasiperiodic switches between two states of varying spin-down rates, approximately every 3–4 years. The mode changes in PSR J2021+4026 are correlated with sudden changes in the γ-ray emission features of the pulsar.

Aims. The goal of this study was to search for variability in the emission features of γ-ray pulsars correlated with the occurrence of glitches. We introduce a novel approach to analyzing LAT γ-ray pulsars, focusing on a systematic examination of variability associated with changes in pulsar spin-down rates.

Methods. We tracked the changes in the rotation and γ-ray emission for a selection of seven glitches that we deemed promising based on the observed changes in the spin-down rates. We conducted a binned likelihood spectral analysis using 14 years of Fermi-LAT data. We specifically analyzed windows of data collected around the epoch of the selected glitches. We improved the precision on the best-fit parameters by incorporating likelihood weights, which we calculated based on a model of the diffuse background, thereby accounting for systematic error contributions.

Results. The phase-averaged flux and spectral parameters of the pulsars analyzed in this study do not vary significantly across any of the glitches investigated. The 95% upper limits on the relative flux change from this work indicate that the flux of the Vela pulsar is unchanging, with a 0.5% upper limit on the relative change, making it a promising candidate for further searches. The connection between glitch dynamics and γ-ray emission in pulsars remains unclear, and PSR J2021+4026 remains unique in terms of its gamma-ray variability properties. We conclude that a comprehensive investigation of glitches with the goal of further unraveling their underlying mechanisms is warranted.