TITLE: GCN CIRCULAR NUMBER: 7328 SUBJECT: Swift-XRT improved positions, and new redshift limits. DATE: 08/02/25 11:40:47 GMT FROM: Phil Evans at U of Leicester P.A. Evans, A.P. Beardmore, J.P. Osborne and M.R. Goad (U. Leicester), J. Kennea and D.N. Burrows (PSU) and N. Gehrels (NASA/GSFC) report on behalf of the Swift-XRT team: The Swift-XRT team announces the availability of two new products: significantly improved `enhanced' XRT positions, including promptly available positions; and prompt XRT data analysis, including nH estimates and redshift limits. 1) Improved XRT positions. The technique of using the UVOT as a super-star-tracker to improve XRT positions of GRBs was presented by Goad et al. (2007, A&A, 476, 1401). We have made improvements to this process, reducing the typical position uncertainty by a further 30-50% in radius. We now find positions with 90% confidence error radii <2" 90% of the time. The percentage of Swift-observed GRBs with enhanced XRT positions has also risen from ~55% to ~70%. To achieve this we use more UVOT filters and more Swift observations per burst, and other algorithm improvements (see http://www.swift.ac.uk/xrt_positions/changes.php). The table of XRT GRB positions at http://www.swift.ac.uk/xrt_positions has been updated using the new software. The old table of positions is still available at http://www.swift.ac.uk/xrt_positions/v1/, this will not be updated with new GRBs. The new software can also be used to enhance the promptly available ground-calculated XRT positions: those produced within ~20 minutes of a trigger from the TDRSS single pixel Photon Counting mode event (SPER) data, and reported in the first GCN circular from the Swift team and via the GCN Notices system. These SPER positions are not as precise as those described above which use the full ground-based data set (available ~2-3 hours post-trigger), however the 90% confidence position error radius is <3" 90% of the time which represents a significant improvement on the `standard' positions available at this early time. A full table of prompt positions is available at http://www.swift.ac.uk/sper, this table updates automatically as soon as new positions are deduced. SPER-derived positions are distributed automatically as GCN Position Notices, and the positions determined from ground data will be announced in automatic GCN Circulars (see Osborne et al., GCN Circ. 6726, for details). Full documentation of the enhancement process, including verification of the error-circle radius, is available at http://www.swift.ac.uk/xrt_positions/docs.php and for the prompt positions further information is available at http://www.swift.ac.uk/sper/docs.php. 2) Prompt data analysis. We have developed tools to produce light curves and spectra from the promptly available SPER data. The light curve will be used by the XRT team to rapidly determine whether a source is fading or variable; the spectra will be used to measure the column density towards the GRB. If the column density is in excess of the Galactic value we use the relation given in Grupe et al. (2007, AJ, 133, 2216) to derive an upper limit on the redshift of the GRB. We use the 90% confidence lower limit on the excess column. If this provides a limit in the range 2-5, as it does for 18% of GRBs, we will quote it in the initial GCN circular. A summary page for each GRB with prompt event data, showing the prompt position, light curve and spectrum, can be found at http://www.swift.ac.uk/sper/ which also shows the XRT-derived redshift limits. Full documentation of the prompt data analysis and verification of the results is here as well.