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

Abstract

The Swift Gamma Ray Burst satellite routinely provides prompt positions for GRBs and their afterglows on timescales of a few hundred seconds. However, with a pointing accuracy of only a few arcminutes, and a systematic uncertainty on the star-tracker solutions to the World Coordinate System of 3-4 arcseconds, the precision of the early XRT positions is limited to 3-4 arcseconds at best. This is significant because operationally, the XRT detects >95% of all GRBs, while the UVOT detects only the optically brightest bursts, ∼30% of all bursts detected by BAT; thus early and accurate XRT positions are important because for the majority of bursts they provide the best available information for the initial ground-based follow-up campaigns.Here we describe an autonomous way of producing more accurate prompt XRT positions for 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.

The Swift Supergiant Fast X-ray Transients Project: A review, new results and future perspectives

“…We derive astrometrically corrected X-ray positions by using the XRT-UVOT alignment and matching to the U-SNO-B1 catalog (Monet et al, 2003) as described in Goad et al (2007) and Evans et al (2009) 3 . Fig.…”

Swift J2058.4+0516: Discovery of a Possible Second Relativistic Tidal Disruption Flare?

“…14 After correcting the XRT astrometry following Goad et al (2008), we identified a bright point source at (J2000.0) coordinates α = 20 h 58 m 19. s 85, δ = +05…”

The swift x-ray telescope: status and performance

“…The uncertainties in the techniques listed above are dominated by two components: photon statistics, and the uncertainty in the star tracker solutions, which imposes a lower limit of about 3 arcseconds on the XRT position accuracy. We have recently developed a technique 14 that eliminates the latter (star tracker) component by using the UVOT instrument as a "super star tracker", taking advantage of the precise registration achievable between UVOT images and astrometric star catalogs, and of the highly stable Swift Optical Bench, which maintains the relative alignment between the XRT and UVOT boresights to better than an arcsecond in flight. We achieve additional improvements by using a fitting routine to determine XRT source positions in detector coordinates that accounts for the instrument PSF and that correctly and accurately treats the effects of bad pixels and columns.…”

“…In these figures we have simply plotted the difference between the best X-ray and optical positions. Testing of this technique using bursts with UVOT afterglows shows that 90% of the XRT positions are within 2 arcseconds of the UVOT position 14 .…”