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 spin-down 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 spin-down rate state to low gamma-ray flux/high spin-down rate st\ ate. In December 2014, 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 1st and entered a new low gamma-ray flux/high spin-down rate state. At the 2018 state change, the averaged flux dropped from $(1.29\pm 0.01)\times 10^{-6} {\rm cts~cm^{-2}s^{-1}}$ to $(1.12\pm 0.01)\times 10^{-6} {\rm cts~cm^{-2}s^{-1\ }}$, which has the similar behavior to the case of 2011 event. The spin-down rate has increased by $\sim 3\%$ 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.