Observation and Simulation of the Variable Gamma-ray Emission from PSR~J2021+4026

Abstract

Pulsars are rapidly spinning and highly magnetized neutron stars, with highly stable rotational period and gradual spin-down over a long timescale due to the loss of radiation. Glitches refer to the events that suddenly increase the rotational speed of a pulsar. The exact causes of glitches and the resulting processes are not fully understood. It is generally believed that couplings between the normal matter and the superfluid components, and the starquakes, are the common causes of glitches. In this study, one famous glitching pulsar, PSR~J2021+4026, is investigated. PSR~J2021+4026 is the first variable gamma-ray pulsar observed by Fermi. From the gamma-ray observations, it is found that the pulsar experienced a significant flux drop, an increase in the spin-down rate, a change in the pulse profile and a shift in the spectral cut-off to a lower energy, simultaneously around 2011 October 16. To explain these effects on the high-energy emissions by the glitch of PSR~J2021+4026, we hypothesized the glitch to be caused by the rearrangement of the surface magnetic field due to the crustal plate tectonic activities on the pulsar which is triggered by a starquake. In this glitch event, the inclination angle of the magnetic dipole axis is slightly shifted. This proposition is then tested by numerical modeling using a three-dimensional two-layer outer gap model. The simulation results indicate that a modification of the inclination angle can affect the pulse profile and the spectral properties, which can explain the observation changes after the glitch.

arXiv comment20 pages, 6 figures. Accepted for publication in ApJ

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• New pulse profile variability associated with a glitch of PSR J0738-4042
• Pulse profile variability associated with the glitch of PSR J1048$-$5832
• Multiwavelength observations of PSR J2021+4026 across a mode change reveal a phase shift in its X-ray emission
• X-ray study of Variable Gamma-ray Pulsar PSR J2021+4026
• An Investigation of the state changes of PSR J2021+4026 and the Vela pulsar
• A renewed search for radio emission from the variable $\gamma$-ray pulsar PSR J2021$+$4026
• The persistent shift in spin-down rate following the largest Crab pulsar glitch rules out external torque variations due to starquakes
• Search for gamma-ray variability around Fermi-LAT pulsar glitches
• On the challenges of Angular Momentum Transfer Mechanism of Pulsar Glitches