Recovering Swift-XRT energy resolution through CCD charge trap mapping

TL;DR

This paper presents a calibration method to recover the degraded energy resolution of the Swift-XRT detector caused by space radiation damage, significantly improving its spectroscopic performance.

Contribution

The authors developed a charge trap mapping and correction technique that restores most of the lost spectral resolution in Swift-XRT data.

Findings

Substantial recovery of XRT spectral resolution achieved.

Implementation of position-dependent energy corrections.

Enhanced calibration improves scientific data quality.

Abstract

The X-ray telescope on board the Swift satellite for gamma-ray burst astronomy has been exposed to the radiation of the space environment since launch in November 2004. Radiation causes damage to the detector, with the generation of dark current and charge trapping sites that result in the degradation of the spectral resolution and an increase of the instrumental background. The Swift team has a dedicated calibration program with the goal of recovering a significant proportion of the lost spectroscopic performance. Calibration observations of supernova remnants with strong emission lines are analysed to map the detector charge traps and to derive position-dependent corrections to the measured photon energies. We have achieved a substantial recovery in the XRT resolution by implementing these corrections in an updated version of the Swift XRT gain file and in corresponding improvements to the Swift XRT HEAsoft software. We provide illustrations of the impact of the enhanced energy resolution, and show that we have recovered most of the spectral resolution lost since launch.