Phase-resolved polarization properties of the pulsar striped wind synchrotron emission

Abstract

Since the launch of the Fermi telescope more than five years ago, many new gamma-ray pulsars have been discovered with intriguing properties challenging our current understanding of pulsar physics. Observation of the Crab pulsar furnish today a broad band analysis of the pulsed spectrum with phase-resolved variability allowing to refine existing model to explain pulse shape, spectra and polarization properties. The latter gives inside into the geometry of the emitting region as well as on the structure of the magnetic field. Based on an exact analytical solution of the striped wind with finite current sheet thickness, we analyze in detail the phase-resolved polarization variability emanating from the synchrotron radiation. We assume that the main contribution to the wind emissivity comes from a thin transition layer where the dominant toroidal magnetic field reverses its polarity, the so-called current sheet. The resulting radiation is mostly linearly polarized. In the off-pulse region, the electric vector lies in the direction of the projection onto the plane of the sky of the rotation axis of the pulsar. This property is unique to the wind model and in good agreement with the Crab data. Other properties such as a reduced degree of polarization and a characteristic sweep of the polarization angle within the pulses are also reproduced. These properties are qualitatively unaffected by variations of the wind Lorentz factor, the lepton injection power law index, the contrast in hot and cold particle, the obliquity of the pulsar and the inclination of the line of sight.