Gamma-Ray Modes in a Transitional Pulsar

Abstract Transitional millisecond pulsars (tMSPs) exhibit a unique subluminous disk state, at the crossroads between accretion and rotation power, where they switch between two distinct X-ray modes. We present the discovery of gamma-ray modes in PSR J1023+0038, the first confirmed tMSP, from stacking Fermi-LAT data during the modes (which we identify using simultaneous X-ray observations). Surprisingly, we find that gamma rays and X-rays are anticorrelated during this mode switching: the gamma-ray flux is higher in the X-ray low mode, and vice versa. This contradicts the state-of-the-art model, which predicts bright gamma rays only in the X-ray high mode, from the interaction between the pulsar wind and surrounding disk via synchrotron and inverse Compton processes. Because the pulsar wind is likely absent in the gamma-ray high (X-ray low) mode, which is also brighter in the radio band, we suggest that jet emission may be dominant in GeV gamma rays. We also discuss an alternative interpretation where the gamma-ray high mode is powered by self-synchrotron Compton emission in the propeller state.