Pair cascades in the magnetospheres of strongly-magnetized neutron stars

TL;DR

This paper presents detailed numerical simulations of electron-positron pair cascades in strongly magnetized neutron star magnetospheres, exploring a wide range of magnetic fields and geometries, and incorporating advanced physical processes.

Contribution

It introduces comprehensive simulations that include detailed physical processes like photon splitting and resonant inverse Compton scattering, extending previous work to higher magnetic fields.

Findings

Pair multiplicity varies with magnetic field strength and geometry.

Photon splitting influences cascade development in high magnetic fields.

Results impact understanding of radio pulsar death line and X-ray emissions.

Abstract

We present numerical simulations of electron-positron pair cascades in the magnetospheres of magnetic neutron stars for a wide range of surface fields (B_p = 10^{12}--10^{15} G), rotation periods (0.1--10 s), and field geometries. This has been motivated by the discovery in recent years of a number of radio pulsars with inferred magnetic fields comparable to those of magnetars. Evolving the cascade generated by a primary electron or positron after it has been accelerated in the inner gap of the magnetosphere, we follow the spatial development of the cascade until the secondary photons and pairs leave the magnetosphere, and we obtain the pair multiplicity and the energy spectra of the cascade pairs and photons under various conditions. Going beyond previous works, which were restricted to weaker fields (B < a few x 10^{12} G), we have incorporated in our simulations detailed treatments of physical processes that are potentially important (especially in the high field regime) but were either neglected or crudely treated before, including photon splitting with the correct selection rules for photon polarization modes, one-photon pair production into low Landau levels for the e^+e^-, and resonant inverse Compton scattering from polar cap hot spots. We discuss the implications of our results for the radio pulsar death line and for the hard X-ray emission from magnetized neutron stars.