[vsnet-grb-info 16850] GRB 150811A: Swift/UVOT further observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18131
SUBJECT: GRB 150811A: Swift/UVOT further observations
DATE:    15/08/12 12:14:43 GMT
FROM:    Paul Kuin at MSSL  <npkuin at gmail.com>

N. P. M. Kuin (UCL-MSSL) and P. A. Evans (U Leicester)
report on behalf of the Swift/UVOT team:

We report further on the Swift/UVOT observations of GRB 150811A which
began 148 s after the BAT trigger (Evans et al., GCN Circ. 18119).
A source consistent with the XRT position (Goad et al. GCN Circ. 18124)
was reported earlier in the optical/ir by Butler et al.(GCN Circ. 18120)
Zheng et al. (GCN Circ. 18121), Kuin et al.(GCN Circ. 12122), Japelj et
al., (GCN Circ. 12123),  Perley and Cenko (GCN. Circ. 12125), Moskivitin
(GCN Circ. 12128). Our initial observations in the white filter show a
rise from the start at 147s around magnitude (white) 21 which continues
in the u band event data and starts leveling off around magnitude 17.8
550s after the trigger, consistent with the report by Perley and Cenko.
The data taken during the next orbit show that  the decay had already
set in. The uv bands showed marginal count rates in all three bands
initially below the 3-sigma level, suggesting the burst is at low redshift.


Preliminary detections and 3-sigma upper limits using the UVOT photometric
system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early
exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white_FC           148          297          147         20.39 +/- 0.23
u_FC               306          549          239         18.19 +/- 0.11
white             4424         4624          197         19.16 +/- 0.07
u                 5449         5854          197         18.83 +/- 0.16
v                 4835         5034          197         18.89 +/- 0.17
b                 4219         4419          197         19.31 +/- 0.14
w1                5244        11888         1082        >20.6
m2                5039        10981         1082        >21.1
w2                4630         6265          393        >20.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.17 in the direction of the burst
(Schlegel et al. 1998).