GCN Circular 6726

J. P. Osborne (U Leicester), M. R. Goad (U Leicester), P. A. Evans (U 
Leicester), A. P. Beardmore (U Leicester), L. G. Tyler (U Leicester),
D. N. Burrows (PSU), N. Gehrels (GSFC) and S. D. Barthelmy (GSFC), report on 
behalf of the Swift Team:

The Swift XRT team will from now be providing better positions earlier for 
most GRBs by automatically issuing: 1) an early position update notice 
based on ground processing; 2) a circular giving an enhanced XRT position 
and error derived from a rectification of the UVOT field with USNO B1. See 
http://www.swift.ac.uk/xrtpos.php.

1) Position Update Notices

GCN Swift XRT position update notices will soon be routinely distributed.
The position and error will be based upon fully automated ground processing
of SPER data (TDRSS-downlinked Single Pixel Event Report data in Photon
Counting mode), fit using a PSF model with the dead columns to give a
more reliable position than is possible on-board. This position update notice
will typically be sent within 15 minutes of the start of the XRT observation.
Robotic telescopes may want to adapt their reaction strategy.

SPER data come only during the first snap-shot observation of a burst,
it is distributed to the XRT team at ~5 minute intervals; the notice will be 
generated from the combined first and second SPER datasets, as this offers the 
best position improvement for the shortest delay (notice distribution will be 
forced if a second SPER is not received within 9 minutes of the first).  If this 
is not the first XRT_Position notice for a burst then the notice UPDATE_FLAG 
keyword is set, the email subject line will have 'UPDATE' added, and in the 
packets to socket sites the 2^25 bit will be set in the PKT_MISC field 
(http://gcn.gsfc.nasa.gov/gcn/sock_pkt_def_doc.html).

Subsequent SPER data may allow further improvement of the early position 
information. This will not be distributed by GCN notice, but will be 
immediately available at http://www.swift.ac.uk/spertable.php.  A link from that 
page describes the derivation of positions from SPER data in detail.  Over 80% 
of bursts can have a position derived in this way; for these, the 90% position 
error radius is <3.9" half of the time.

2) Enhanced Position Circulars

GCN Swift XRT position circulars will soon be routinely distributed when a 
first UVOT-enhanced XRT position becomes available.  Use of the XRT-UVOT 
boresight alignment and the UVOT-USNO B1 astrometric registration for the 
bursts simultaneously covered by the XRT PC mode and UVOT V band gives more 
precise and accurate XRT positions than those based on star tracker data 
previously distributed.  Around 60% of XRT bursts can have their positions 
improved by this method; such UVOT-enhanced XRT burst positions have a 90% 
confidence error radius <2.1" half of the time, and are typically available 
within 2-3 hours of the burst (Goad et al. A&A submitted, arXiv:0708.0986,
http://www.swift.ac.uk/xrt_positions/Goad.pdf).  These rapidly available 
enhanced positions have an accuracy comparable to that obtainable in one to 
several days using serendipitous X-ray sources (eg Butler AJ 133, 1027, 2007).

Improvement of a UVOT-enhanced XRT position is possible as more data become 
available after the initial enhanced position circular.  Such subsequently 
improved positions will not be announced via the GCN but will be immediately 
available at http://www.swift.ac.uk/xrt_positions/index.php

It is not yet possible to enhance a larger fraction of positions due to the 
need for simultaneous V band and XRT PC mode data when Swift is still and for 
at least 10 XRT counts. We hope to improve this in the future.