Observation and simulation on the variable Gamma-ray emission from PSR J2021+4016

Abstract

The Fermi Gamma-Ray Large Area Telescope launched in 2008 provides valuable observations on gamma-ray radiations from a variety of astronomical objects. One of the brightest gamma-ray sources in the space is pulsar. Pulsars are rapidly spinning and strongly magnetized neutron stars, which can emit a wide energy range of electromagnetic beam. When pulsars rotate, the radiation beam sweeps through the line of sight from observers and is seen as pulsed signals. The rotation of a pulsar is highly stable, with a gradual spin-down over a long time scale due to the loss of radiation. Yet, there are some occasional events that increase the rotational speed of a pulsar. This phenomenon is known as pulsar glitch. Currently, the exact causes of glitch and the processes happening are not well understood. It is generally believed that the coupling between the normal matter and the superfluid component, and the starquake are the common causes of glitch. In this study, one famous glitching pulsar, PSR J2021+4026, is investigated. PSR J2021+4026 is first discovered in gamma-ray energy range by the Fermi satellite and it is the first variable gamma-ray pulsar observed. Through analyzing the gamma-ray data, 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, theoretical models are applied. The cause of the glitch is hypothesized to be the rearrangement of surface magnetic field by the crustal plate tectonic activities on the pulsar which is triggered by the starquake. In this glitch event, the inclination angle of the magnetic dipole axis is slightly shifted. This proposition is then tested by numerical pulsar simulations. A three-dimensional two-layer outer gap model is used to simulate the emission features of radiations. The simulation results indicate that the modification on the inclination angle can affect the pulse profile and the spectral properties, which can explain the observation changes before and after the glitch.