TITLE: GCN CIRCULAR NUMBER: 6439 SUBJECT: GRB 070520A: Swift UVOT Upper Limits DATE: 07/05/21 13:26:54 GMT FROM: Stefan Immler at NASA/GSFC S. Immler (USRA/GSFC) and P. Romano (Univ. Bicocca & INAF-OAB) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 070520A starting 152 s after the BAT trigger (Romano et al., GCN 6423). We do not find any source in any of the UVOT observations inside the XRT error circle. The 3-sigma upper limits for detecting a source inside the XRT error circle in the co-added frames are: Filter T_start(s) T_stop(s) Exp(s) Mag (3-sigma UL) White 168 4793 345 >20.2 V 152 11162 1288 >19.6 B 4389 6021 393 >20.0 U 4184 16822 1142 >20.3 UVW1 3979 16047 1279 >20.1 UWM2 3774 11805 1021 >20.4 UVW2 4799 10249 1082 >20.5 The values quoted above are not corrected for the expected Galactic extinction corresponding to a reddening of E(B-V) = 0.02 mag towards the direction of the burst (Schlegel et al. 1998). [GCN OPS NOTE(21may07): Per author's request, the typo in the first sentence was corrected (070525A --> 070520A).]