TITLE:   GCN CIRCULAR
NUMBER:  6263
SUBJECT: SUBJECT: GRB 070406: Astrometrically Refined XRT Position, Unfading Source
DATE:    07/04/09 19:53:43 GMT
FROM:    Nat Butler at MIT/CSR  <nrbutler@space.mit.edu>

N. R. Butler and J. S. Bloom (UC Berkeley) report:

Using 53.6 ksec of exposure by the Swift XRT on the field of GRB070406
(Cummings et al.; GCN 6247), we derive a refined astrometrically-corrected
position for the X-ray afterglow candidate (Troja et al.; GCN 6255) of

RA= 13 15 51.51 , Dec= +16 30 48.1  +/- 2.7" (90% Conf.)

(http://astro.berkeley.edu/~nat/swift/xrt_pos.html, DSS position v1.3).

The position is within 1.4" of (and the X-ray source is likely associated
with) the bright optical source reported by Kann (GCN 6256) and found
to be a quasar by Berger et al. (GCN 6262).

Examining the arrival times for 70 source counts in comparison to the detector
on-times, we find a KS-test probability that the source rate is steady
and non-fading of 80%.  It is thus very unlikely that this X-ray source
and the associated possible optical counterparts are the afterglow of
GRB070406.