{
  "format": "text/plain",
  "circularId": 40048,
  "bibcode": "2025GCN.40048....1S",
  "eventId": "GRB 250403A",
  "editedOn": 1743783380111,
  "body": "S. P. R. Shilling (Lancaster U.), S. R. Oates (Lancaster U.) and S. Dichiara (PSU) report on behalf of the Swift UVOT team:\n\nSwift/UVOT observed the field of GRB 250403A detected by Fermi GBM (Fermi team, GCN Circ. 40025) and\nSVOM/ECLAIRs (Julakanti et al., GCN Circ. 40026), starting at 17:22:59 UT on 24/04/03, ~2.1 hours after the initial detections.\n\nAn optical afterglow is initially detected in the U-band consistent with the position of the uncatalogued X-ray source detected by Swift/XRT (Dichiara et al., GCN Circ. 40037) and the optical afterglow detected by various facilities (Starling et al., GCN Circ. 40028; Wu et al., GCN Circ. 40029; Du et al., GCN Circ. 40031; Li et al., GCN Circ. 40032; Ghosh et al., GCN Circ. 40039; Jiang et al., GCN Circ. 40041; Pérez-Fournon et al., GCN Circ. 40043; Xin et al., GCN Circ. 40045). At 00:02:02 UT on 25/04/04, ~6.7 hours later, the optical afterglow is no longer detected.\n\nThe preliminary detection and 3-sigma upper limits calculated using the UVOT photometric system\n(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial and subsequent exposures are:\n\nFilter         T_start(h) \t   Exp(s)         Mag\n\nu\t       2.1\t\t   396\t          19.2 +/- 0.2\nu              8.8\t\t   492            >20.3\nb              8.8\t\t   386            >20.5\nv              9.0\t\t   130            >19.0\nw1             12.1\t\t   164            >19.3\nm2             12.0\t\t   245            >19.7\nw2             8.9\t\t   246            >19.8\n\nThe magnitudes given are calculated using the UVOT photometric system\n(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) and are not corrected for the Galactic extinction.",
  "submitter": "Sam Shilling at Lancaster University <shilling.sam@gmail.com>",
  "submittedHow": "web",
  "createdOn": 1743773339037,
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Sam Shilling at Lancaster University <shilling.sam@gmail.com>",
  "version": 2,
  "subject": "GRB 250403A: Swift/UVOT Detection"
}