[vsnet-grb-info 5327] Swift UVOT upper limits to GRB071028

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7016
SUBJECT: Swift UVOT upper limits to GRB071028
DATE:    07/10/29 17:12:39 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 observed the field of GRB 071028 (GCN 7012; Racusin et al.) 
beginning 128 seconds after the BAT trigger. No afterglow is detected in 
any of the UVOT filters within the XRT error circle (Racusin et al., GCN 
7014). As already noted by Xin et al. (GCN 7015), the refined XRT position 
(Racusin et al., GCN 7014) lies next to a very bright star (V<10 mag), 
severely limiting the detection of an optical afterglow.

In the case of the white band and optical filters (v,b,u), contamination 
from the nearby, bright star is causing the exposures to be affected by 
coincidence loss to a degree that is outside the photometrically 
calibrated range of UVOT. This limits the depth of our observations at the 
XRT position in these filters to white > 13.86, v > 11.46, b > 12.68 and u 
> 11.91.

The 3-sigma upper limits at the XRT position in co-added UV exposures are as 
follows:

Filter   Tmid  Exp   Magnitude
          (s)   (s)   (3-sigma UL)
uvw1     5044  412    > 19.89
uvm2     4847  413    > 20.32
uvw2     5014  213    > 19.67

where Tmid is the mid time of the observations. The values quoted above are not 
corrected for the expected Galactic extinction corresponding to a reddening of 
E_{B-V} = 0.06 mag in the direction of the burst (Schlegel et al. 1998).