Energy Dissipation and Gamma-ray Emission in Young Pulsars

Pulsars are rapidly rotating neutron stars with plasma-filled magnetospheres that radiate their rotational energy in the form of Poynting flux, also referred to as the spin-down power. Some of the young pulsars exhibit rotation-modulated gamma-ray emission, captured by the Fermi observatory. The luminosity of this emission suggests that a significant fraction (0.1-10%) of the spin-down power is dissipated in the magnetosphere and reradiated as high-energy photons. This fraction is referred to as the gamma-ray efficiency of the pulsar. First-principles global plasma simulations of pulsar magnetospheres during the past decade enabled us to self-consistently model this process by capturing both the microscopic plasma physics and the global structure of the magnetosphere. These models show that the spin-down power is dissipated in the equatorial current sheet due to magnetic reconnection, and particles accelerated in the reconnection process emit synchrotron photons, providing the explanation for the observed gamma-ray emission. In this work we examine the reconnection process and its radiative signatures in detail using global 3D particle-in-cell simulations of pulsar magnetospheres with synchrotron cooling. We show that the fraction of the spin-down power dissipated in the magnetospheric current sheet is uniquely controlled at microphysical plasma scales and only depends on the pulsar inclination angle. We demonstrate that the maximum energy and the distribution function of accelerated pairs is controlled by the available magnetic energy per particle: plasma magnetization parameter. Ultimately, the shape and the extent of the plasma distribution is imprinted in the observed high-energy emission. While the cutoff energy in gamma-rays is dictated by the synchrotron emission from the highest energy pairs, we show that the peak of the emission is also sensitive to the interplay between the efficiency of synchrotron cooling and the particle acceleration rate. We show that there are two separate parameter regimes applicable to young pulsars with low and high spin-down powers. In the former case the synchrotron cooling is dynamically weak, and the peak of the emission is close to the cutoff energy in 1-10 GeV range (e.g., Vela). In pulsars with higher spin-down power the cooling is dynamically important, resulting in broader spectral shapes which peak at lower energies (e.g., for Crab pulsar the peak lies in the MeV band). This picture naturally explains why pulsars with higher spin-down power have lower gamma-ray efficiency in the 0.1 to 100 GeV band of the Fermi satellite.