Transitional millisecond pulsars (tMSPs) are an elite group of compact binary millisecond pulsars that alternate between a radio pulsar state and an accretion disk state. They exhibit a unique subluminous disk state, characterized by rapid switches between two modes and occasional flares. During the mode switching, the radio emission is anticorrelated with the X-ray emission.
The average gamma-ray emission in the disk state is a factor of ~10 brighter than in the pulsar state. However, its origin is still not clear, and the sensitivity of the Fermi-LAT gamma-ray telescope is not sufficient to detect individual modes directly.
In this work, we study the GeV emission of the prototypical tMSP PSR J1023+0038.
Using available X-ray observations performed with NICER, XMM-Newton, Swift/XRT, and NuSTAR, we identify all high- and low-mode intervals and analyze the Fermi-LAT data corresponding to each mode.
We significantly detect the source in both modes and surprisingly we discover 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 current leading model, which predicts bright gamma-ray emission only in the X-ray high mode.
We suggest that in the gamma-ray high-mode the jet emission may be dominant in the GeV band.
We also discuss an alternative interpretation where the gamma-ray emission is powered by self-synchrotron Compton emission in the propeller regime.
