γ-Ray Absorption for a Beam Model of NP0532

Abstract

THE possibility of γ-ray absorption near the Crab Nebula pulsar NP0532 was pointed out by McBreen¹. Recently Pollack et al.² have shown that the critical energy E_c above which absorption is observed to take place can in principle be used to determine the size of the emitting region in γ-ray sources such as pulsars, quasars and supernovae explosions. Neither of these authors, however, attempted to take account of beaming in pulsars such as NP0532 which has the effect of significantly raising the value of E_c. Without taking account of beaming, there would seem to be a conflict between surface emission models such as that of Chiu et al.³ and recent reports of the detection of pulsed γ-ray emission from the Crab Nebula^4–6. These observations seem to rule out any model involving emission from the surface of the pulsar according to he calculations of Pollack et al. because those calculations indicate that γ-radiation with energy greater than 1 MeV originating at the surface of the pulsar would be strongly attenuated.