Concordance: In-flight Calibration of X-ray Telescopes without Absolute References

TL;DR

This paper introduces a statistical method for in-flight calibration of X-ray telescopes' effective areas using common targets, without relying on traditional standard candles, by applying shrinkage estimation and accounting for systematic uncertainties.

Contribution

It extends previous calibration techniques by incorporating priors on systematic uncertainties and correlations across energy bands, improving cross-calibration accuracy.

Findings

Effective area correction factors were successfully determined for multiple X-ray telescopes.

The method improved agreement between flux estimates from different instruments.

Demonstrations showed the approach's robustness across various datasets.

Abstract

We describe a process for cross-calibrating the effective areas of X-ray telescopes that observe common targets. The targets are not assumed to be "standard candles" in the classic sense, in that we assume that the source fluxes have well-defined, but {\it a priori} unknown values. Using a technique developed by Chen et al. (2019, arXiv:1711.09429) that involves a statistical method called {\em shrinkage estimation}, we determine effective area correction factors for each instrument that brings estimated fluxes into the best agreement, consistent with prior knowledge of their effective areas. We expand the technique to allow unique priors on systematic uncertainties in effective areas for each X-ray astronomy instrument and to allow correlations between effective areas in different energy bands. We demonstrate the method with several data sets from various X-ray telescopes.