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

Abstract

We report on evidence for orbital phase-dependence of the gamma-ray emission\nfrom PSR B1957+20 black widow system by using the data of the Fermi Large Area\nTelescope. We divide an orbital cycle into two regions: a region containing the\ninferior conjunction, and the other region containing rest of the orbital\ncycle. We show that the observed spectra for the different orbital regions are\nfitted by different functional forms. The spectrum of the orbital region\ncontaining inferior conjunction can be described by a power-law with an\nexponential cutoff (PLE) model, which gives the best-fit model for the orbital\nphase that does not contain the inferior conjunction, plus an extra component\nabove ~2.7 GeV. The emission above 3 GeV in this region is detected with a\n~7-sigma confidence level. The gamma-ray data above ~2.7 GeV are observed to be\nmodulated at the orbital period at the ~2.3-sigma level. We anticipate that the\nPLE component dominating below ~2.7 GeV originates from the pulsar\nmagnetosphere. We also show that the inverse-Compton scattering of the thermal\nradiation of the companion star off a "cold" ultra-relativistic pulsar wind can\nexplain the extra component above ~2.7 GeV. The black widow pulsar PSR B1957+20\nmay be the member of a new class of object, in the sense that the system is\nshowing gamma-ray emission with both magnetospheric and pulsar wind origins.\n