Practical aspects of X-ray imaging polarimetry of supernova remnants and other extended sources

TL;DR

This paper discusses the use of advanced X-ray polarimetry techniques and simulations to study magnetic fields in supernova remnants, emphasizing the need for deep observations to detect low polarization signals.

Contribution

It introduces a new polarisation mapping method with instrumental corrections and presents the xpolim simulation tool for exploring polarisation measurement schemes.

Findings

Deep ~2 Ms observations are required to detect polarization in supernova remnants.

Simulations show polarization fractions can be as low as 5% in Cas A.

Corrected Stokes maps improve the accuracy of magnetic field studies.

Abstract

The new generation of X-ray polarisation detectors, gas pixel detectors, which will be employed by the future space missions IXPE and eXTP, allows for spatially resolved X-ray polarisation studies. This will be of particular interest for X-ray synchrotron emission from extended sources like young supernova remnants and pulsar wind nebulae. Here we report on employing a polarisation statistic that can be used to makes maps in the Stokes I, Q, and U parameters, a method that we expand by correcting for the energy-dependent instrumental modulation factor, using optimal weighting of the signal. In order to explore the types of Stokes maps that can be obtained, we present a Monte Carlo simulation program called xpolim, with which different polarisation weighting schemes are explored. We illustrate its use with simulations of polarisation maps of young supernova remnants, after having described the general science case for polarisation studies of supernova remnants, and its connection to magnetic-field turbulence. We use xpolim simulations to show that in general deep, ~2 Ms observations are needed to recover polarisation signals, in particular for Cas A, for which in the polarisation fraction may be as low as 5%.