[vsnet-grb-info 14037] GRB 131014A: XRT position information

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15351
SUBJECT: GRB 131014A: XRT position information
DATE:    13/10/18 13:30:03 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

P.A. Evans and J.P. Osborne (U. Leicester) report on behalf of the
Swift-XRT team

The Swift-XRT localisation of GRB 131014A has unusually significant
uncertainty, which is not included in the positions previously 
circulated (GCNs 15335, 15339), or on the website. The afterglow was 
detected towards the edge of two of the initial tiled pointings, and 
near the centre of a dedicated follow-up pointing. Positions derived 
from these individual pointings vary by up to 4" when the Swift star 
tracker attitude is used. If we instead create enhanced positions (using 
the UVOT to derive the astrometry) we find astrometric corrections of 
~5-8", although we find significant variation between these corrections. 
This suggests that there is some degeneracy in the aspect solution.

Given this variation we suggest that the XRT position should be assumed
to have an uncertainty of 6", rather than the values previously
announced or posted online. Thus the best available XRT position is
RA, Dec=100.3033, -19.0971, which is equivalent to:
RA:   06h 41m 12.80s
Dec: -19° 05′ 49.6′′
with an uncertainty of approx 6". This is the enhanced position derived
from the follow-up observation, which was centred on the afterglow.

Some low-level scatter in the aspect solutions is seen in all enhanced 
positions, and is calibrated into our enhanced position errors. GRB 
131014A shows an exceptionally large scatter in the aspect solutions 
which is not currently understood. Futhermore, as this is a late-time 
tiled follow up, where the source is faint and individual exposures are 
short, the normal mechanisms which remove outlying aspect solutions 
cannot be applied.