X-ray study of Variable Gamma-ray Pulsar PSR J2021+4026

TL;DR

This study analyzes X-ray data of the gamma-ray pulsar PSR J2021+4026 before and after a flux relaxation, finding no significant change in X-ray emission properties, and suggests the emission originates from heated polar caps and pulsar wind nebula.

Contribution

First X-ray analysis comparing pre- and post-relaxation states of PSR J2021+4026, revealing stable emission properties and modeling the pulsar's X-ray light curve.

Findings

No significant change in X-ray emission between states

X-ray spectra fit by power-law plus blackbody components

Large pulsed fraction attributed to asymmetric magnetosphere

Abstract

PSR~J2021+4026 showed a sudden decrease in the gamma-ray emission at the glitch that occurred around 2011, October 16, and a relaxation of the flux to the pre-glitch state at around 2014 December. We report X-ray analysis results of the data observed by XMM-Newton on 2015 December 20 in the post-relaxation state. To examine any change in the X-ray emission, we compare the properties of the pulse profiles and spectra at the low gamma-ray flux state and at the post-relaxation state. The phase-averaged spectra for both states can be well described by a power-law component plus a blackbody component. The former is dominated by unpulsed emission and is probably originated from the pulsar wind nebula as reported by Hui et al (2015). The emission property of the blackbody component is consistent with the emission from the polar cap heated by the back-flow bombardment of the high-energy electrons or positrons that were accelerated in the magnetosphere. We found no significant change in the X-ray emission properties between two states. We suggest that the change of the X-ray luminosity is at an order of ~4%, which is difficult to measure with the current observations. We model the observed X-ray light curve with the heated polar cap emission and we speculate that the observed large pulsed fraction is owing to asymmetric magnetospheric structure.