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We report on evidence for orbital phase-dependence of the gamma-ray emissionfrom PSR B1957+20 black widow system by using the data of the Fermi Large AreaTelescope. We divide an orbital cycle into two regions: a region containing theinferior conjunction, and the other region containing rest of the orbitalcycle. We show that the observed spectra for the different orbital regions arefitted by different functional forms. The spectrum of the orbital regioncontaining inferior conjunction can be described by a power-law with anexponential cutoff (PLE) model, which gives the best-fit model for the orbitalphase that does not contain the inferior conjunction, plus an extra componentabove ~2.7 GeV. The emission above 3 GeV in this region is detected with a~7-sigma confidence level. The gamma-ray data above ~2.7 GeV are observed to bemodulated at the orbital period at the ~2.3-sigma level. We anticipate that thePLE component dominating below ~2.7 GeV originates from the pulsarmagnetosphere. We also show that the inverse-Compton scattering of the thermalradiation of the companion star off a "cold" ultra-relativistic pulsar wind canexplain the extra component above ~2.7 GeV. The black widow pulsar PSR B1957+20may be the member of a new class of object, in the sense that the system isshowing gamma-ray emission with both magnetospheric and pulsar wind origins.

Orbital Phase Dependent Gamma-ray emissions from the Black Widow Pulsar