[vsnet-grb-info 1913] GRB 050802: Swift/UVOT optical and UV detections

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3745
SUBJECT: GRB 050802: Swift/UVOT optical and UV detections
DATE:    05/08/03 15:37:09 GMT
FROM:    Katie McGowan at MSSL-UCL  <km2 at mssl.ucl.ac.uk>

K. McGowan (UCL-MSSL), A. Morgan (PSU), K. Mason (PPARC),
T. Kennedy (UCL-MSSL) report on behalf of the Swift/UVOT team:

The Swift/UVOT began observing the afterglow of
GRB 050802 at 2005-08-02T10:15:14, 286 s after the
BAT trigger (Band et al., GCN 3734).  In addition to our
reported detection from the initial V filter observations
(McGowan et al., GCN 3739), we now report that we detect
the afterglow in all six filters.

We extracted data using a 6 arcsec radius aperture
centered on the source position (McGowan et al., GCN 3739)
for the optical filters, a 12 arcsec radius for the W1 and
M2 filters, and 4 arcsec for the W2 filter.  If the
significance of the source detection was less than 2 sigma
we determined a 3-sigma limiting magnitude.

The detection in UVW2 implies that the Lyman Limit must be blueward
of 2000 A, and we estimate that the burst is at redshift z < 1.2.

We obtain the following photometry of the afterglow candidate:

Filter   Exposure (s)  T_mid (s)   Magnitude      Notes

V        100           336         17.07+/-0.09   detection
V        900           12566       20.35+/-0.09   detection

B        10            437         17.66+/-0.20   detection
B        900           36405       21.10+/-0.08   detection

U        10            423         16.82+/-0.24   detection
U        899.79        35498       20.22+/-0.16   detection

W1       10            409         17.08+/-0.29   detection
W1       100           2296        19.22+/-0.23   detection

M2       278           1316        18.79+/-0.26   detection
M2       3753          28197       21.30+/-0.26   detection

W2       268           1240.5      20.73+/-0.27   detection
W2       3481          25890.5     >22.64         3-sig limit

where T_mid is the midpoint of the exposure post-trigger in seconds.

The magnitudes are based on preliminary zero points, measured in
orbit, and will require refinement with further calibration.