Polarisation leakage due to errors in track reconstruction in gas pixel detectors

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
e-mail: niccolo.bucciantini@inaf.it
² Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, Sesto F.no (FI), Italy
³ INFN – Sezione di Firenze, via G. Sansone 1, Sesto F.no (FI), Italy
⁴ Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA
⁵ INFN – Sezione di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa (PI), Italy
⁶ Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa (PI), Italy
⁷ University of Maryland, Baltimore County, Baltimore, 620 W. Lexington St., MD 21250, USA
⁸ NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
⁹ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
¹⁰ NASA Marshall Space Flight Center, 4600 Rideout Rd., Huntsville, AL 35812, USA

Received: 21 December 2022
Accepted: 1 February 2023

Abstract

X-ray polarimetry based on gas pixel detectors (GPDs) has reached a high level of maturity thanks to the Imaging X-ray Polarimeter Explorer (IXPE) providing the first-ever spatially resolved polarimetric measurements. However, as this a new technique, a few unexpected effects have emerged in the course of in-flight operations. In particular, it was almost immediately found that, the unpolarized calibration sources on-board were showing radially polarized halos. The origin of these features was recognized in a correlation between the error in reconstructing the absorption point of the X-ray photon and the direction of its electric field vector. Here, we present and discuss this effect in detail, showing that it is possible to provide a simple and robust mathematical formalism to handle it. We further show its role and relevance for the recent IXPE measures as well as for the use of GPD-based techniques in general. We also illustrate how to model it in the context of studying extended sources.