# Opacities for Photon Splitting and Pair Creation in Neutron Star Magnetospheres

## TL;DR

This paper investigates how photon splitting and pair creation affect high-energy photon escape in magnetar magnetospheres, providing constraints on emission regions and implications for future gamma-ray observations.

## Contribution

It offers a detailed analysis of polarization-dependent opacities for photon splitting and pair creation in magnetar magnetospheres, deriving bounds on photon energies and emission locales.

## Key findings

- Attenuation lengths suggest maximum photon energies of a few MeV near the stellar surface.
- Photon splitting constrains emission regions to altitudes within 2-4 stellar radii.
- Predicted spectroscopic and polarimetric signatures are relevant for future MeV missions.

## Abstract

Over the last four decades, persistent and flaring emission of magnetars observed by various telescopes has provided us with a suite of light curves and spectra in soft and hard X-rays, with no emission yet detected above around 1 MeV. Attenuation of such high-energy photons by magnetic pair creation and photon splitting is expected to be active in the magnetospheres of magnetars, possibly accounting for the paucity of gamma-rays in their signals. This paper explores polarization-dependent opacities for these two QED processes in static vacuum dipole magnetospheres of highly-magnetized neutron stars, calculating attenuation lengths and determining escape energies, which are the maximum photon energies for transparency out to infinity. The numerical trajectory integral analysis in flat and curved spacetimes provides upper bounds of a few MeV or less to the visible energies for magnetars for locales proximate to the stellar surface. Photon splitting opacity alone puts constraints on the possible emission locales in their magnetospheres: regions within field loops of maximum altitudes 2-4 stellar radii are not commensurate with maximum detected energies of around 250 keV. These constraints apply not only to magnetar flares but also to their quiescent hard X-ray tail emission. An exploration of photon splitting attenuation in the context of a resonant inverse Compton scattering model for the hard X-ray tails derives distinctive phase-resolved spectroscopic and polarimetric signatures, of significant interest for future MeV-band missions such as AMEGO and e-ASTROGAM.