Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26as..120c..49a&link_type=abstract
Astronomy and Astrophysics Supplement, v.120, p.49-60
Astronomy and Astrophysics
Astrophysics
47
Gamma-Rays, Theory, Pulsars: General, Supernova Remnants, Ism: Individual: Crab Nebula
Scientific paper
Aspects of the physics of rotation powered pulsars as gamma ray sources are discussed. The shock excitation of pulsar powered nebulae (plerions) is discussed, based on recent theoretical work on the structure of relativistic, collisionless magnetosonic shock waves. This theory is used to outline a model in which the γ^-2^ injection spectrum of the Crab Nebula is satisfactorily accounted for. The same theory suggests a model of the ``wisp'' features in the Crab Nebula which accounts for these time variable features in the surface bightness as compressions associated with the magnetic overshoots within the shock structure. It is pointed out that this theory suggests observable variability in the high energy gamma rays from the Crab Nebula (ɛ>50MeV.) The energetics of pulsed gamma ray emission from the six known EGRET pulsars are reviewed and shown to fit a simple efficiency {prop.to}{PHI}_open_^-k^ law, where k~0.8 and {PHI}_open_={OMEGA}_*_^2mu/c^2^=10^13^(˙(P)_15_ /P^3^)^1/2^ is a measure of the total voltage available on a pulsar's open field lines. Here ˙(P)_15_=˙(P)/10^-15^. This result is used to define a criterion for cessation of gamma ray emission in voltage-P space, such that empirically pulsars should stop being gamma ray emitters when the total spindown luminosity falls to ~2x10^32^ergs/sec. A simple result of the same form as the empirical gamma ray emission efficency is derived for the acceleration efficiency of particle beams extracted from the polar cap, and for high voltage pulsars, where curvature radiation reaction is important, equated to the gamma ray efficiency. However, it is also argued that since radio emission from the polar caps continues to lower voltages and spin down luminosities than inferred for the gamma ray emission, that this correspondence is a coincidence and that the EGRET gamma rays come from the outer magnetosphere. The most popular of outer magnetosphere models are shown to be unable to simultanously account for gamma ray efficiencies approaching unity and having most of the gamma ray luminosity in sharp pulses, suggesting that the gamma ray emission has something to do with dense return current boundary layers whose physics has yet to be quantified.
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