XSPECT on-board XPoSat: Calibration and First Results

TL;DR

This work documents the comprehensive calibration program and first results for XSPECT, the soft X-ray spectro-timing instrument on XPoSat. By combining extensive on-ground thermovac tests with on-board performance verification, the team establishes linear, temperature-dependent gain corrections, a physics-based FWHM scaling, and a well-characterized spectral redistribution function, all validated through observations of Cas A, Tycho, and the Crab. The study also derives precise collimator alignments, tunes the instrument's effective area using Crab data, and develops robust background treatments, including a hybrid model to account for CXB and GCR contributions. Collectively, these calibrations enable XSPECT to perform long-term spectro-temporal studies of X-ray sources with high timing accuracy and pile-up–free spectroscopy, supporting its role in long-duration monitoring and multi-mission cross-calibration, with data and software openly available to the community.

Abstract

XPoSat is India's first X-ray spectro-polarimetry mission, consisting of two co-aligned instruments, a polarimeter (POLIX) and a spectrometer (XSPECT), to study the X-ray emission from celestial sources. Since polarimetry is a photon-hungry technique, the mission is designed to observe sources for long integration times (~ few days to weeks). This provides an unique opportunity, enabling XSPECT to carry out long-term monitoring of sources, and study their spectro-temporal evolution. To ensure that the instrument is able to fulfill its scientific objectives, it was extensively calibrated on-ground. Post launch, these calibrations were validated using on-board observations. Additionally, some aspects of the instrument such as alignment and effective area were also derived and fine-tuned from in-flight data. In this paper, we describe the calibration of XSPECT instrument in detail, including some initial results derived from its data to establish its capabilities.

Figures (19)

• Figure 1: (a) Layout showing the location of the components of XSPECT payload on-board XPoSAT. (b) Image of XPoSat after integration of all subsystems. The locations of the two XSPECT detector packages are marked with arrows.
• Figure 2: (a) Schematic (top view) of the XSPECT test setup inside the thermovacuum chamber. (b) Image of the placement of various components in the test chamber.
• Figure 3: (a) Temperature sweep of the (top) electronics and (bottom) detector packages. (b) (Top) Sample brass spectrum recorded by SCD 14 at the specified T$_{\mathrm{det}}$ and T$_{\mathrm{elec}}$, with the fluorescence peaks marked. (Bottom) Determined peak channels are fitted with a linear function to determine the gain.
• Figure 4: Two-dimensional fit to the variation of gain as a function of T$_{\mathrm{det}}$ and T$_{\mathrm{elec}}$ for SCD 14. The markers represent the data points with their colors representing the measured values of gain. The background colormap represents the surface generated using the best-fit parameters of equation (1) for SCD 14.
• Figure 5: (Top) Variation of FWHM with T$_{\mathrm{det}}$. Spectra have been added across all the detectors. The dashed line is the best fit to the data using Eq. (2). (Bottom) Residuals of the best fit.