Accurate early positions for Swift GRBS: enhancing X-ray positions with UVOT astrometry

TL;DR

This paper presents an autonomous method to improve early X-ray positions of Swift-detected GRBs by leveraging UVOT astrometry, reducing systematic errors and enhancing localization accuracy within hours of detection.

Contribution

The authors introduce a novel technique combining UVOT and XRT data to significantly improve prompt GRB localization accuracy, outperforming previous methods at early times.

Findings

Achieves a factor of 2 improvement in localization accuracy within hours.

Provides superior early-time positions compared to uncorrected XRT images.

Achieves accuracy comparable to deep XRT imaging at later times.

Abstract

Here we describe an autonomous way of producing more accurate prompt XRT positions for Swift-detected GRBs and their afterglows, based on UVOT astrometry and a detailed mapping between the XRT and UVOT detectors. The latter significantly reduces the dominant systematic error -- the star-tracker solution to the World Coordinate System. This technique, which is limited to times when there is significant overlap between UVOT and XRT PC-mode data, provides a factor of 2 improvement in the localisation of XRT refined positions on timescales of less than a few hours. Furthermore, the accuracy achieved is superior to astrometrically corrected XRT PC mode images at early times (for up to 24 hours), for the majority of bursts, and is comparable to the accuracy achieved by astrometrically corrected X-ray positions based on deep XRT PC-mode imaging at later times (abridged).