TITLE:   GCN CIRCULAR
NUMBER:  10570
SUBJECT: GRB 100401A:  Swift/UVOT Upper Limit
DATE:    10/04/03 04:46:02 GMT
FROM:    Margaret Chester at PSU  <chester@astro.psu.edu>

M.M. Chester (PSU) and G. Stratta (ASDC) report on behalf of the
Swift/UVOT team:

Swift began a Target of Opportunity observation of GRB 100401A,
detected by Swift/BAT (Cummings et al., GCN Circ. 10567) and by
Fermi/GBM (Goldstein and Gruber, GCN Circ. 10568), on April 2nd,
2010 at 19:33:04 UT, approximately 36 hours after the Swift/BAT
trigger.

No optical afterglow consistent with the BAT position is detected
in the UVOT exposure. The 3-sigma upper limit using the UVOT
photometric system (Poole et al. 2008, MNRAS, 383, 627) is:

Filter     T_start(s)   T_stop(s)      Exp(s)      3-sig UL (mag)

u          131128       133123         1963         >20.8

The value quoted above is not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.29 in the direction of the burst
(Schlegel et al. 1998).