Search for transitions between states in redbacks and black widows using seven years of Fermi-LAT observations

Considering about seven years of Fermi-Large Area Telescope (LAT) data, we present a systematic search for variability possibly related to transitions between states in redbacks and black widow systems. Transitions are characterized by sudden and significant changes in the gamma-ray flux that persist on a timescale much larger than the orbital period. This phenomenology was already detected in the case of two redback systems, PSR J1023+0038 and PSR J1227-4853, for which we present here a dedicated study. We show the existence of only one transition for each of these systems over the past seven years. We determine their spectra, establishing high-energy cutoffs at a few GeV for the high gamma-ray state of PSR J1023+0038 and for both states of PSR J1227-4853. The surveying capability of the Fermi-LAT allows studying whether similar phenomenology has occurred in other sources. Although we have not found any hint for a state transition for most of the studied pulsars, we note two black-widow systems, PSR J2234+0944 and PSR J1446-4701, whose apparent variability is reminiscent of the transitions in PSR J1023+0038 and PSR J1227-4853. For the other systems we set limits on potential transitions in their measured gamma-ray light curves.