Very high energy emission as a probe of relativistic magnetic reconnection in pulsar winds

Abstract

The population of gamma-ray pulsars, including Crab observed in the TeV range, and Vela detected above 50 GeV, challenges existing models of pulsed high-energy emission. Such models should be universally applicable, yet they should account for spectral differences among the pulsars. We show that the gamma-ray emission of Crab and Vela can be explained by synchrotron radiation from the current sheet of a striped wind, expanding with a modest Lorentz factor G{cyrillic} ≲ 100 in the Crab case, and G{cyrillic} ≲ 50 in the Vela case. In the Crab spectrum, a new synchrotron self-Compton component is expected to be detected by the upcoming experiment CTA. We suggest that the gamma-ray spectrum directly probes the physics of relativistic magnetic reconnection in the striped wind. In the most energetic pulsars, like Crab, with E3/238/P-2 ≳ 0.002 (where E˙ is the spin-down power, P is the pulsar period, and X = Xi × 10i in CGS units), reconnection proceeds in the radiative cooling regime and results in a soft power-law distribution of cooling particles; in less powerful pulsars, like Vela, particle energization is limited by the current sheet size, and a hard particle spectrum reflects the acceleration mechanism. A strict lower limit on the number density of radiating particles corresponds to emission close to the light cylinder, and, in units of the GJ density, it is ≳0.5 in the Crab wind, and κ ≳ 0.05 in the Vela wind.