Lower Bound on the Magnetic Field Strength of a Magnetar from Analysis of SGR Giant Flares

TL;DR

This paper investigates the neutrino cooling processes in magnetar giant flares and derives a lower bound on the magnetic field strength necessary to explain observed flare energies, challenging existing magnetar models.

Contribution

It provides a detailed analysis of neutrino emission mechanisms and establishes a lower magnetic field limit that exceeds previous estimates, questioning the magnetar model's sufficiency.

Findings

Neutrino processes dominate energy losses in the plasma.

A lower magnetic field bound is higher than previous upper limits.

Explains the energy release in giant flares requires stronger magnetic fields.

Abstract

Based on the magnetar model, we have studied in detail the processes of neutrino cooling of an electron--positron plasma generating an SGR giant flare and the influence of the magnetar magnetic field on these processes. Electron--positron pair annihilation and synchrotron neutrino emission are shown to make a dominant contribution to the neutrino emissivity of such a plasma. We have calculated the neutrino energy losses from a plasma-filled region at the long tail stage of the SGR 0526--66, SGR 1806--20, and SGR 1900+14 giant flares. This plasma can emit the energy observed in an SGR giant flare only in the presence of a strong magnetic field suppressing its neutrino energy losses. We have obtained a lower bound on the magnetic field strength and showed this value to be higher than the upper limit following from an estimate of the magnetic dipole losses for the magnetars being analyzed in a wide range of magnetar model parameters. Thus, it is problematic to explain the observed energy release at the long tail stage of an SGR giant flare in terms of the magnetar model.