Implementing and Verifying a Fourier Domain Approach to Fast Stochastic X-ray Polarimetry Timing

TL;DR

This paper implements and tests a Fourier domain method for fast, model-independent X-ray polarisation timing analysis using IXPE data, enabling studies of rapid polarisation variability in astrophysical sources.

Contribution

It adapts and applies a Fourier-based technique for short-timescale polarisation analysis to real IXPE data, addressing instrumental effects and validating the method on pulsar observations.

Findings

Successfully recovers known polarisation signals from pulsars.

Enables analysis of stochastic polarisation variability on short timescales.

Facilitates studies of phenomena like QPOs and accretion fluctuations.

Abstract

The launch of the Imaging X-ray Polarimetry Explorer (IXPE), the first space-based polarimeter since 1978, offers a two order of magnitude improvement to the measurement of X-ray polarisation than its predecessor OSO-8, offering unprecedented precision for the measurement of polarisation degree and polarisation angle of X-ray sources. This advancement lends itself to the birth of a number of contemporary techniques to study Galactic compact objects, including X-ray polarimetry-timing, the study of how polarisation properties evolve over short timescales. However, the statistical nature of polarisation measurements poses a challenge for studies on arbitrarily short timescales, as a large number of photons are required to achieve statistically significant measurements of polarisation degree and angle for time-resolved analyses. Furthermore, if the polarisation variability is stochastic, then phase-folding techniques introduce systematic errors in the phase assignment of photons. Ingram and Maccarone presented a model independent Fourier-based technique that circumvents these issues. It can be used on arbitrarily short timescales for any kind of variability, whether aperiodic, quasi-periodic or purely periodic. Here we implement this method on real IXPE data. We address several instrumental effects and test the technique on X-ray pulsars, RX-J0440.9+4431 and Hercules X-1 . We verify that our technique recovers the polarisation variability signal that we already know to be there from typical phase-folding techniques. It will now be possible to study fast stochastic polarisation variability of X-ray sources, with applications including quasi-periodic oscillations, mass accretion rate fluctuations, and reverberation mapping.