On the TeV Halo Fraction in gamma-ray bright Pulsar Wind Nebulae

Abstract

The discovery of extended TeV emission around the Geminga and PSR B0656+14 pulsars, with properties consistent with free particlepropagation in the interstellar medium (ISM), has sparked considerable discussion on the possible presence of such halos in othersystems. Here we make an assessment of the current TeV source population associated with energetic pulsars, in terms of size andestimated energy density. Based on two alternative estimators we conclude that a large majority of the known TeV sources haveemission originating in the zone energetically and dynamically dominated by the pulsar (i.e. thepulsar wind nebula), rather than froma halo of particles diffusing in to the ISM. Furthermore, whilst the number of established halos will surely increase in the future, wefind that it is unlikely that such halos contribute significantly to the total TeVγ-ray luminosity from electrons accelerated in PWN