Soft X-ray Polarization in Thermal Magnetar Emission

TL;DR

Future X-ray polarization measurements of magnetars can directly constrain their magnetic field strength and geometry, overcoming limitations of current spectral methods by analyzing polarization signals from surface hotspots.

Contribution

The paper develops models of X-ray polarization from magnetar surface hotspots using advanced atmosphere models, highlighting polarization's potential to determine magnetic properties.

Findings

Polarization signals depend strongly on magnetic field strength and geometry.

Phase-resolved polarimetry can help resolve degeneracies in magnetar parameter estimation.

Models show polarization is less sensitive to atmosphere composition and equation of state.

Abstract

Emission spectra from magnetars in the soft X-ray band likely contain a thermal component emerging directly from the neutron star surface. However, the lack of observed absorption-like features in quiescent spectra makes it difficult to directly constrain physical properties of the atmosphere. We argue that future X-ray polarization measurements represent a promising technique for directly constraining the magnetar magnetic field strength and geometry. We construct models of the observed polarization signal from a finite surface hotspot, using the latest NS atmosphere models for magnetic fields B = 4 x 10^13--5 x 10^14 G. Our calculations are strongly dependent on the NS magnetic field strength and geometry, and are more weakly dependent on the NS equation of state and atmosphere composition. We discuss how the complementary dependencies of phase-resolved spectroscopy and polarimetry might resolve degeneracies that currently hamper the determination of magnetar physical parameters using thermal models.