[vsnet-grb-info 6759] GRB081011: UVOT detection of an optical afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8359
SUBJECT: GRB081011: UVOT detection of an optical afterglow
DATE:    08/10/11 17:57:02 GMT
FROM:    Patricia Schady at MSSL/Swift  <ps at mssl.ucl.ac.uk>

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

The Swift/UVOT observed the field of GRB 081011 (Grupe et al., GCN Circ. 
8355) with settled exposures starting 99s after the trigger. A new fading 
source is detected in the white and u-band finding chart (fc) exposures 
and marginally in a co-added b-band exposure at
RA(J2000) = 14:41:22.627 (220.34428)
Dec(J2000) = +33:32:37.12 (+33.54364)
consistent with the XRT position (Kennea et al., GCN Circ. 8358). These 
detections imply an upper limit on the redshift of z < ~3.8. The source is 
decaying with an approximate optical decay rate of alpha~1.1 (where the 
afterglow flux is proportional to t^-alpha).

The detections and 3 sigma upper limits in the UVOT photometric system 
(Poole et al. 2008, MNRAS, 383, 627) are:

Filter      T_start T_stop  Exp     Mag or 3-sigma UL
            (s)     (s)     (s)
========================================================
white (FC)Â Â 99Â  Â  Â Â 249Â Â Â Â Â 145.8Â   19.25 +/- 0.08
white       537     1878    283.5   20.33 +/- 0.13
u (FC)Â Â Â Â Â Â 257Â Â  Â  507Â Â Â Â  245.8Â Â  18.62 +/- 0.10
u           660     2003    155.6   19.91 +/- 0.30
v           586     7371    548.8   > 20.29
b           512     6756    351.4   21.13 +/- 0.39
uvw1Â Â Â Â Â Â Â Â 636Â Â Â Â  7781Â Â   548.8Â Â  > 20.96
uvm2Â Â Â Â Â Â  Â 611Â Â  Â  7576Â  Â  548.9Â   > 20.74
uvw2Â Â Â Â Â  Â  562Â Â Â Â  7167Â Â   548.8Â Â Â > 21.06

The values quoted above are not corrected for the expected Galactic 
extinction corresponding to a reddening of E(B-V) = 0.01 mag in the 
direction of the burst (Schlegel et al. 1998).