Gamma‐Ray Emission from Rotation‐powered Pulsars

Abstract

Using a simplified model of cascade pair creation over pulsar polar caps presented in two previous papers, we investigate the expected gamma-ray output from pulsars' low altitude particle acceleration and pair creation regions. We divide pulsars into several categories, based on which mechanism truncates the particle acceleration off the polar cap, and give estimates for the expected luminosity of each category.We find that inverse Compton scattering above the pulsar polar cap provides the primary gamma rays which initiate the pair cascades in most pulsars. This reduces the expected γ-ray luminosity below previous estimates which assumed curvature gamma ray emission was the dominant initiator of pair creation in all pulsars. Even for the brightest pulsars where curvature radiation sets the height of the pair formation front (PFF), we find predicted luminosities too low to explain the EGRET pulsars, suggesting that the source of that emission is an outer magnetosphere accelerator. The predicted polar cap luminosities are large enough, however, to be observable by upcoming γ-ray instruments, which provides a firm test for this theory.

Pair production induced by quantum electrodynamic vacuum polarization in pulsars

“…For the polar‐gap models, the accelerating electric field will be screened by the created pairs in a thin layer known as the ‘pair formation front’ (PFF), behind which a secondary pair plasma is formed. For most pulsars, the altitude of the PFF from the star surface is small compared with the stellar radius R * (Ruderman & Sutherland 1975; Hibschman & Arons 2001b; Hibschman 2002). The density of accelerated primary particles is assumed to be n pr ∼ n GJ .…”

“…Strong radiation in the bands of X‐rays and γ‐rays with luminosity L X ≈ 10 31 –10 35 ergs −1 and L γ ≈ 10 33 –10 36 erg s −1 has been observed from a few pulsars (Ulmer 1994). In the polar gap models, high‐energy emissions can occur in lower sites near the PFF at an altitude h ≈ 10 −2 R * from the stellar surface (Ruderman & Sutherland 1975; Hibschman 2002). Therefore the value of the magnetic field is B 0 ≈ B s , and the electric fields E 0 ≈ 10 6 –10 8 esu for X‐rays and E 0 ≈ 10 7 –10 9 esu for γ‐rays are estimated near the PFF.…”

Pulsars: Progress, Problems and Prospects

“…Sensitivity of the EGRET and GLAST telescopes to low altitude (below and just above the PFF) gamma ray emission at energies ε > 100 MeV, in a simple dipole model for the low altitude geometry (which affects the magnetic curvature and therefore the maximum photon energy that can escape) and unidirectional space charge limited flow with current density J ≃ cηR, not the force-free current given (approximately) by ( 9). The different symbols refer to the different major contributors to gamma ray emission and absorption -see Hibschman (2002) for the details.…”

RADIO AND Γ-Ray CONSTRAINTS ON THE EMISSION GEOMETRY AND BIRTHPLACE OF PSR J2043+2740

“…Apart from arousing observational interest, this pulsar's physical properties are also attractive in theoretical investigations. In terms of γ -ray observability, L γ /D 2 , PSR J2043 + 2740 has been previously considered as a strong candidate for γray emission, in the framework of both polar cap (PC) and outer gap (OG) models (Rudak & Dyks 1998;Hibschman 2002;Cheng & Zhang 1998;McLaughlin & Cordes 2000). It should be noted, however, that the majority of Fermi pulsar observations to date have produced γ -ray spectra that disagree with the predicted super-exponential cutoffs of PC models (e.g., Abdo et al 2009Abdo et al , 2010b.…”