# Mode change of a gamma-ray pulsar, PSR J2021+4026

## TL;DR

This study analyzes a gamma-ray pulsar, PSR J2021+4026, revealing a mode change in its magnetosphere triggered by a glitch, with long-term variations in spin-down rate and gamma-ray flux observed over several years.

## Contribution

It provides the first detailed timing and spectral analysis of PSR J2021+4026's mode change associated with a glitch, highlighting magnetospheric evolution.

## Key findings

- The pulsar experienced a sustained high spin-down rate and low gamma-ray flux for about 3 years post-glitch.
- The spin-down rate and gamma-ray flux gradually returned to pre-glitch levels within months.
- The mode change is likely caused by a reconfiguration of the magnetic field or inclination angle.

## Abstract

A glitch of a pulsar is known as a sudden increase in the spin frequency and spin-down rate (frequency time derivative), and it can be caused by a sudden rel\ ease of the stress built up in the solid crust of the star or pinned vortices in the superfluid interior. PSR J2021+4026 is the first pulsar that shows a significant change in the gamma-ray flux and pulse profile at the glitch that occurred around 2011 October 16. We report the results of timing and spectral analysis of PSR~J2021+4026 using $\sim$ 8~yr Fermi-LAT data. We find that the pulsar stayed at a high spin-down rate ($\sim 4\%$ higher than the pre-glitch value) and a low gamma-ray state ($\sim 18\%$ lower) for about 3~yr after the glitch. Around 2014 December, the spin-down rate and gamma-ray flux gradually returned to pre-glitch values within a time scale of a few months. The phase-resolved spectra and pulse profiles after the relaxation are also consistent with those before the glitch. The observed long-term evolution of the spin-down rate and the gamma-ray flux indicates that the glitch triggered a mode change in the global magnetosphere. We speculate that the glitch changed the local magnetic field structure around the polar cap and/or the inclination angle of the dipole axis, leading to a change in the electric current circulating in the magnetosphere.