ORBITAL-PHASE-DEPENDENT γ-RAY EMISSIONS FROM THE BLACK WIDOW PULSAR

We report on evidence for orbital phase dependence of the γ-ray emission from the PSR B1957+20 black widow system using data from the Fermi Large Area Telescope. We divide an orbital cycle into two regions: one containing the inferior conjunction and the other containing the rest of the orbital cycle. We show that the observed spectra for the different orbital regions are fitted by different functional forms. The spectrum of the orbital region containing the inferior conjunction can be described by a power law with an exponential cutoff (PLE) model, which also gives the best-fit model for the orbital phase without the inferior conjunction, plus an extra component above ∼2.7 GeV. The emission above 3 GeV in this region is detected with a ∼7σ confidence level. The γ-ray data above ∼2.7 GeV are observed to be modulated at the orbital period at the ∼2.3σ level. We anticipate that the PLE component dominant below ∼2.7 GeV originates from the pulsar magnetosphere. We also show that inverse Compton scattering of the thermal radiation of the companion star off a ''cold'' ultrarelativistic pulsar wind can explain the extra component above ∼2.7 GeV. The black widow pulsar PSR B1957+20 may be a member of a new class of object, in the sense that the system is showing γ-ray emission with both magnetospheric and pulsar wind origins.