Escape of cosmic rays from perpendicular shocks in the circumstellar magnetic field

Abstract

We investigate the escape process of cosmic rays (CRs) from perpendicular shock regions of a spherical shock propagating to a circumstellar medium with the Parker-spiral magnetic field. The diffusive shock acceleration in perpendicular shocks of supernova remnants (SNRs) is expected to accelerate CRs up to PeV without upstream magnetic field amplification. Red supergiants (RSGs) and Wolf-Rayet (WR) stars are considered as progenitors in this work. We perform test particle simulations to investigate the escape process and escape-limited maximum energy without magnetic field amplification in the upstream region, where the magnetic field strength and rotation period expected from observations of RSGs and WR stars are used. We show that particles escape to the far upstream region while moving along the equator or poles and the maximum energy is about $10-100~{\rm TeV}$ when SNRs propagate to free wind regions of RSGs and WR stars. In most cases, the escape-limited maximum energy is given by the potential difference between the equator and pole. If progenitors are oblique rotators and SNRs are in the early phase just after the supernova explosion, the escape-limited maximum energy is limited by the half wavelength of the wavy current sheet. In addition, for RSGs, we show that the luminosity of CRs accelerated in the wind region is sufficient to supply the observed CR flux above $10~{\rm TeV}$ if a strong magnetic field strength is sustained in most RSGs. In terms of the CR luminosity, SNRs propagating to the free wind of WR stars can contribute to PeV CRs. As long as no magnetic field amplification works around SNR shocks, the maximum energy is decided by the magnetic field strength in the wind region, which depends on the rotation period, stellar wind, and surface magnetic field of RSGs and WR stars. Therefore, we need to observe these quantities to understand the origin of CRs.