Modelling of the γ -ray pulsed spectra of Geminga, Crab, and Vela with synchro-curvature radiation

Abstract

γ-ray spectra of pulsars have been mostly studied in a phenomenological way, by fitting them to a cut-off power-law function. Here, we analyse a model where pulsed emission comes from synchro-curvature processes in a gap.We calculate the variation of kinetic energy of magnetospheric particles along the gap and the associated radiated spectra, considering an effective particle distribution. We fit the phase-averaged and phase-resolved Fermi-LAT (Large Area Telescope) spectra of the three brightest γ -ray pulsars: Geminga, Crab, and Vela, and constrain the three free parameters we leave free in the model. Our best-fitting models well reproduce the observed data, apart from residuals above a few GeV in some cases, range for which the inverse Compton scattering likely becomes the dominant mechanism. In any case, the flat slope at low energy (≲ GeV) seen by Fermi-LAT both in the phase-averaged and phase-resolved spectra of most pulsars, including the ones we studied, requires that most of the detected radiation below ~GeV is produced during the beginning of the particle trajectories, when radiation mostly come from the loss of perpendicular momentum.