On the polarization properties of magnetar giant flare pulsating tails

TL;DR

This paper models the atmosphere and polarization of magnetar giant flare tails, revealing that polarization degree varies with viewing angle and energy band, providing insights into the magnetic environment of magnetars.

Contribution

It introduces a detailed calculation of the atmosphere structure and polarization properties of the trapped fireball in magnetar flare tails, using Monte Carlo simulations in a highly magnetized plasma.

Findings

Polarization degree can reach up to 30% in 1-30 keV band.

Polarization degree can be around 10% in 30-100 keV band.

Polarization depends on the viewing angle relative to the magnetic axis.

Abstract

Three giant flares have been detected so far from soft gamma-ray repeaters, each characterized by an initial short hard spike and a pulsating tail. The observed pulsating tails are characterized by a duration of $\sim100\,\rm{s}$, an isotropic energy of $\sim 10^{44}\,\rm{erg}$, and a pulse period of a few seconds. The pulsating tail emission likely originates from the residual energy after the intense energy release during the initial spike, which forms a trapped fireball composed of a photon-pair plasma in a closed field line region of the magnetars. Observationally the spectra of pulsating tails can be fitted by the superposition of a thermal component and a power-law component, with the thermal component dominating the emission in the early and late stages of the pulsating tail observations. In this paper, assuming that the trapped fireball is from a closed field line region in the magnetosphere, we calculate the atmosphere structure of the optically-thick trapped fireball and the polarization properties of the trapped fireball. By properly treating the photon propagation in a hot, highly magnetized, electron-positron pair plasma, we tally photons in two modes (O mode and E mode) at a certain observational angle through Monte Carlo simulations. Our results suggest that the polarization degree depends on the viewing angle with respect to the magnetic axis of the magnetar, and can be as high as $\Pi\simeq30\%$ in the $1-30\,\rm{keV}$ band, and $\Pi\simeq10\%$ in the $30-100\,\rm{keV}$ band, if the line of sight is perpendicular to the magnetic axis.