A 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 that is possibly related to transitions between states in redbacks and black widow systems. The transitions are characterized by sudden and significant changes in the gamma-ray flux that persist on a timescale that is 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, which we present here. 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 further study of whether similar phenomenology has occurred in other sources. Although we have not found any evidence of a state transition for most of the studied pulsars, we note two black-widow systems, PSR J2234+0944 and PSR J1446−4701, whose apparent variabilities are 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.