The Radiative X-ray and Gamma-ray Efficiencies of Rotation-powered Pulsars

Abstract

We present a statistical analysis of the X-ray luminosity of rotation-powered pulsars and their surrounding nebulae using the sample of Kargaltsev & Pavlov, and we complement this with an analysis of the γ -ray emission of Fermi-detected pulsars. We report a strong trend in the efficiency with which spin-down power is converted to X-ray and γ -ray emission with characteristic age: young pulsars and their surrounding nebulae are efficient X-ray emitters, whereas in contrast old pulsars are efficient γ -ray emitters. We divided the X-ray sample in a young (τc <1.7 × 104 yr) and old sample and used linear regression to search for correlations between the logarithm of the X-ray and γ -ray luminosities and the logarithms of the periods and period derivatives. The X-ray emission from young pulsars and their nebulae are both consistent with LX ∝ ˙ P3/P 6. For old pulsars and their nebulae the X-ray luminosity is consistent with a more or less constant efficiency η ≡ LX/ ˙Erot ≈ 8 × 10−5. For the γ -ray luminosity we confirm that Lγ ∝ √ ˙E rot. We discuss these findings in the context of pair production inside pulsar magnetospheres and the striped wind model. We suggest that the striped wind model may explain the similarity between the X-ray properties of the pulsar wind nebulae and the pulsars themselves, which according to the striped wind model may both find their origin outside the light cylinder, in the pulsar wind zone.