Repeated State Change of Variable Gamma-Ray Pulsar PSR J2021+4026

PSR J2021+4026 is a radio-quiet gamma-ray pulsar and the first pulsar that shows state change of the gamma-ray emission and spindown rate. The state change of PSR J2021+4026 was first observed at 2011 October, at which the pulsar changes the state from high gamma-ray flux/low spindown rate state to low gamma-ray flux/high spindown rate state. In 2014 December, PSR J2021+4026 recovered the state before the 2011 state change over a timescale of a few months. We report that the long-term evolution of the gamma-ray flux and timing behavior suggests that PSR J2021+4026 changed the state near 2018 February 1 and entered a new low gamma-ray flux/high spindown rate state. At the 2018 state change, the averaged flux dropped from (1.29 0.01) × 10−6 cts cm−2 s−1 to (1.12 0.01) × 10−6 cts cm−2 s−1, which is behavior similar to that of the 2011 event. The spindown rate has increased by ∼3% in the new state since the 2018 state change. The shapes of pulse profile and spectrum in GeV bands also changed at the 2018 event, and they are consistent with behavior at the 2011 state change. Our results probably suggest that PSR J2021+4026 is switching between different states with a timescale of several years, like some radio pulsars (e.g., PSR B1828-11). PSR J2021+4026 will provide a unique opportunity to study the mechanism of the state switching.