[vsnet-grb-info 3200] GRB060428B: possible detection of an optical counterpart by Swift/UVOT.

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5035
SUBJECT: GRB060428B: possible detection of an optical counterpart by Swift/UVOT.
DATE:    06/04/29 16:13:07 GMT
FROM:    Massimiliano de Pasquale at MSSL-UCL  <mdp at mssl.ucl.ac.uk>

  M. De Pasquale (UCL-MSSL), S. Campana (INAF-OAB) report on
  behalf of the Swift/UVOT team:

  The Swift/UVOT began taking data on the field of GRB060428B
at 08:57:52 UT on 2006-04-28, 194  s after the BAT trigger
(Campana et al., GCN 5017).
At the position of the optical afterglow claimed by Price et
al. (GCN 5019) and Li et al. (GCN 5027), inside the XRT error
circle (Troja et al., GCN 5031), we find a source near the
detection limits in the V band and in the White filter, not
present in the USNO B1.0 catalogue. The source is present in
the first ~1000 seconds of observations, with a significance
of ~3 sigma. It is not detected later.

    Filter   T_range(s)   Exp(s)        Mag

     V        194-1233     356     19.5 +0.5 -0.3
     V       5630-16900   1081          >20.4

    White     211-960      227     19.7 +/- 0.3
    White    5247-18579   1135          >20.7

  The upper limits quoted above are at the 3 sigma level.
The source is not detected in any of the other filters. The
fading behaviour may indicate that it is the GRB060428B optical
afterglow, however we do caution that detections are marginal
and the results suffer from a contamination by an extended
source, the galaxy quoted by Helpern et al. in GCN 5034. An
association with the GRB cannot be excluded.

  No other afterglow candidate was detected at the refined XRT
position in summed images from any of the filters down to the
following three-sigma upper limits:

    Filter   T_range(s)   Exp(s)   3sigma U.L.

     V        194-16900   1441       20.5
     B        413-17812   1308       21.4
     U        390-16900   1357       21.1
     UW1      367-19996   1227       20.6
     UM2      443-12022   1358       20.6
     UW2      323-6894     383       20.2
    White     212-18579   1363       20.8


    These upper limits are uncorrected for the estimated
  Galactic reddening of E_{B-V} = 0.01 mag (Schlegel et al. 1998).