TITLE: GCN CIRCULAR NUMBER: 40509 SUBJECT: GRB 250520A: Swift/UVOT Upper Limits DATE: 25/05/21 13:02:53 GMT FROM: Samantha Oates at University of Birmingham samantha.oates@alumni.ucl.ac.uk
S. R. Oates (Lancaster U.) and R. A. J. Eyles-Ferris (U. Leicester) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250520A 127 s after the BAT trigger (Eyles-Ferris et al., GCN Circ. 40491). No optical afterglow consistent with the enhanced XRT position (Goad et al., GCN Circ. 40494) or the SVOM/VT optical candidate (Xin et al., GCN Circ. 40500) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL
white 149 298 147 >20.9 white 149 1373 392 >21.3 v 127 1251 110 >18.9 b 481 1349 97 >19.7 u 456 1324 97 >19.3 uvw1 432 1300 97 >19.1 uvm2 407 1275 78 >18.6 uvw2 358 1394 112 >19.2
We caution that the photometry may not be accurate, as the source region used to determine the optical/UV limits, is on an area of the UVOT detector with low sensitivity. The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.754 in the direction of the burst (Schlegel et al. 1998).
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