The in-flight spectroscopic performance of the Swift XRT CCD camera during 2006-2007

TL;DR

This paper evaluates the in-flight spectroscopic performance of the Swift XRT CCD camera during 2006-2007, focusing on calibration, response modeling, and effects of proton damage on spectral resolution.

Contribution

It provides a detailed calibration and response model update for the Swift XRT CCD, including in-flight experiences and the impact of proton damage on performance.

Findings

Revised response model accurately describes calibration sources.

Negative residuals observed around the Oxygen edge in spectra.

Proton damage increases charge transfer inefficiency over time.

Abstract

The Swift X-ray Telescope focal plane camera is a front-illuminated MOS CCD, providing a spectral response kernel of 135 eV FWHM at 5.9 keV as measured before launch. We describe the CCD calibration program based on celestial and on-board calibration sources, relevant in-flight experiences, and developments in the CCD response model. We illustrate how the revised response model describes the calibration sources well. Comparison of observed spectra with models folded through the instrument response produces negative residuals around and below the Oxygen edge. We discuss several possible causes for such residuals. Traps created by proton damage on the CCD increase the charge transfer inefficiency (CTI) over time. We describe the evolution of the CTI since the launch and its effect on the CCD spectral resolution and the gain.