TITLE:   GCN CIRCULAR
NUMBER:  10703
SUBJECT: GRB 100427A: Swift XRT and UVOT Upper Limits
DATE:    10/04/29 12:42:00 GMT
FROM:    Alice Breeveld at MSSL-UCL  <aab@mssl.ucl.ac.uk>

A. Breeveld (MSSL/UCL), A. Rowlinson, N. Lyons and R.L.C. Starling (U. 
Leicester) report on behalf of the Swift/UVOT team:

Swift began settled observations of the field of GRB 100427A 110220 s 
after the BAT rate increase detected in ground analysis (Cummings et 
al., GCN Circ. 10699). No afterglow consistent with the BAT position 
given in GCN Circ. 10699 is detected in the initial XRT or UVOT exposures.

Swift-XRT obtained 5.9 ks of Photon Counting (PC) mode data, from T0 + 
110 ks to T0 + 144 ks. No source is found in the BAT error circle, 
with a 3-sigma upper limit of 1.3 x 10-3 count s-1. For a typical GRB, 
this would correspond to an upper limit on the 0.3 - 10 keV flux of 5 
x 10-14 erg cm-2 s-1.

For the UVOT a preliminary 3-sigma upper limit for the initial summed 
exposures, using the UVOT photometric system (Poole et al. 2008, 
MNRAS, 383, 627),  is:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag
u               110220       143784         5836         >21.2

The values quoted above are not corrected for the Galactic extinction 
due to the reddening of E(B-V) = 0.69 in the direction of the burst 
(Schlegel et al. 1998).