[vsnet-grb-info 4444] GRB070318: Swift UVOT indicates unusual optical/UV afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6220
SUBJECT: GRB070318: Swift UVOT indicates unusual optical/UV afterglow
DATE:    07/03/21 21:08:42 GMT
FROM:    Mat Page at MSSL/Swift  <mjp at mssl.ucl.ac.uk>

M.J. Page (MSSL-UCL), F.E. Marshall (NASA/GSFC) S. Immler (GSFC/USRA) S. 
T. Holland (CRESST/GSFC/USRA) and J.R. Cummings (GSFC/UMBC) report on 
behalf of the Swift/UVOT team:

The optical/UV afterglow of GRB 070318 (Cummings et al, GCN 6210) 
decayed to magnitude of 21.5 in the UVOT white filter in the first 24 
hours after the BAT trigger, and has remained at approximately constant 
brightness for at least 48 hours since that time. Examination of the 
Supercosmos scan of the UK Schmidt sky survey plate at this location 
reveals no evidence for a host galaxy (the plate limit is Bj>22 mag). 
For the flattening of the lightcurve to be due to the host galaxy, the 
host galaxy would have to be very luminous (Mb ~ -22) and have an 
unusually blue spectrum (e.g. peaking in the U band, observer frame). It 
therefore seems more likely that this is an afterglow with a very 
unusual lightcurve.

Swift continues to monitor the afterglow. Further ground-based 
observations are encouraged.