Precession for the mode change in a gamma-ray pulsar

TL;DR

This paper explains the mode changes in a gamma-ray pulsar through a precession model, linking variations in spin-down rate and gamma-ray flux to precession damping and inclination angle modulation.

Contribution

It introduces a precession-based explanation for pulsar mode changes, incorporating damping effects and magnetospheric interactions, which was not previously modeled.

Findings

Precession explains the correlation between spin-down rate and gamma-ray flux.

Damping of precession reduces variation amplitude and shortens modulation period.

Two types of pulsar modulations are proposed: long-term from precession, short-term from magnetosphere.

Abstract

PSR J2021+4026 is a gamma-ray pulsar having variations in its spin-down rate and gamma-ray flux. Its variations in timing and emission are correlated, e.g., a larger spin-down rate for a low gamma-ray flux. We show that the mode change in PSR J2021+4026 can be understood in the precession scenario. In the precession model, the inclination angle is modulated due to precession. At the same time, the wobble angle may decay with time. This results in damping of the precession. Combined with magnetospheric torque model and the outer gap model, the damped precession can explain: (1) when the inclination angle is larger, the spin-down rate will be larger, accompanied by a lower gamma-ray flux. (2) The variation amplitude of the gamma-ray flux and spin-down rate is smaller than previous results due to the damping of the precession. The modulation period is becoming shorter due to a smaller wobble angle. In the end, we propose that there are two kinds of modulations in pulsars. Long-term modulations in pulsars may be due to precession. Short-term modulations may be of magnetospheric origin.