{
  "bibcode": "2025GCN.40509....1O",
  "body": "S. R. Oates (Lancaster U.) and R. A. J. Eyles-Ferris (U. Leicester)\nreport on behalf of the Swift/UVOT team:\n\nThe Swift/UVOT began settled observations of the field of GRB 250520A\n127 s after the BAT trigger (Eyles-Ferris et al., GCN Circ. 40491).\nNo optical afterglow consistent with the enhanced XRT position (Goad et al., GCN Circ. 40494)\nor the SVOM/VT optical candidate (Xin et al., GCN Circ. 40500) is detected in\nthe initial UVOT exposures.\n\nPreliminary 3-sigma upper limits using the UVOT photometric system\n(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first\nfinding chart (FC) exposure and subsequent exposures are:\n\n\nFilter   T_start(s)   T_stop(s) Exp(s)     Mag/3sigUL\n\nwhite        149         298      147        >20.9\nwhite        149         1373     392        >21.3\n     v        127         1251     110        >18.9\n     b        481         1349      97        >19.7\n     u        456         1324      97        >19.3\nuvw1        432         1300      97        >19.1\nuvm2        407         1275      78        >18.6\nuvw2        358         1394     112        >19.2\n\nWe caution that the photometry may not be accurate, as the source region\nused to determine the optical/UV limits, is on an area of the UVOT\ndetector with low sensitivity. The magnitudes in the table are not corrected\nfor the Galactic extinction due to the reddening of E(B-V) = 0.754 in the\ndirection of the burst (Schlegel et al. 1998).\n",
  "subject": "GRB 250520A: Swift/UVOT Upper Limits",
  "submitter": "Samantha Oates at University of Birmingham <samantha.oates@alumni.ucl.ac.uk>",
  "circularId": 40509,
  "createdOn": 1747832573857,
  "submittedHow": "email",
  "eventId": "GRB 250520A"
}