[vsnet-grb-info 6748] GRB081008: Swift/UVOT observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8348
SUBJECT: GRB081008: Swift/UVOT observations
DATE:    08/10/09 10:30:54 GMT
FROM:    Patricia Schady at MSSL/Swift  <ps at mssl.ucl.ac.uk>

P. Schady (MSSL-UCL) and J. L. Racusin (PSU) report on behalf of the 
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 081008 96 s 
after the BAT trigger (Racusin et al., GCN Circ. 8344), using a newly 
implemented observing sequence. The new sequence takes a 150s white-band 
finding chart exposure, followed by a 250s u-band exposure, after which 
the filter wheel rotates through all filters, taking short 20s exposures.

A new fading source is detected at the ROTSE position (Rykoff et al., GCN 
Circ 8343) in the white, v, b, u and uvw1 filters, consistent with a  
redshift of z=1.967 reported by Cucchiara et al., (GCN Circ 8346). The 
source is decaying at a constant rate of 0.90+/-0.03 for the duration of 
intial UVOT observations, out to ~700s after the BAT trigger. The UVOT 
refined position is
RA(J2000) = 18:39:49.877 (279.95833 deg)
Dec(J2000) = -57:25:52.87 (-57.431111 deg)
with an estimated uncertainty of 0.6 arcsec (radius, 90% confidence).

The detections and 3 sigma upper limits of the fading afterglow are 
reported below, where T_start and T_stop represent the elapsed time since 
the BAT trigger in seconds, and fc indicate finding chart exposures

Filter      T_start(s) T_stop  Exp(s)   Mag or 3 Sigma Limit
white (fc)   96         246     146      14.96 +/- 0.01
white        534        554     19       15.87 +/- 0.03
u (fc)       254        504     246      15.19 +/- 0.02
u            658        678     19       15.81 +/- 0.08
v            584        604     19       15.64 +/- 0.10
b            510        530     19       15.79 +/- 0.05
uvw1         633        653     19       17.03 +/- 0.22
uvm2         608        1601    117      > 19.16
uvw2         559        1717    128      > 19.53

The above magnitudes are not corrected for the Galactic extinction 
corresponding to a reddening of E(B-V) = 0.10 mag (Schlegel et al., 1998, 
ApJS, 500, 525).  The photometry is on the UVOT photometric system 
described in Poole et al. (2008, MNRAS, 383,627).