GCN Circular 39644
Subject
GRB 250309B / AT2025dws: Swift-UVOT detection
Date
2025-03-10T02:12:00Z (19 days ago)
From
Robert David Stein at JSI <rdstein@umd.edu>
Via
Web form
Robert Stein (JSI), Tomas Ahumada (Caltech) report,
On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations:
We requested observations of AT2025dws (Stein et al., GCN 39639), the candidate counterpart to GRB 250309B (Preis & Greiner, GCN 39629; McDermott et al., GCN 39635), with the Neil Gehrels Swift Observatory.
Observations were conducted beginning 2025-03-09 20:26:46 UT, ~13 hours after the GRB time (2025-03-09 07:38:30.66 UT). These observations had an exposure time of 30 minutes, in the U-band filter.
We reduced the Swift UVOT data (Roming et al. 2005) using HEASoft. A source is clearly visible at the position of AT2025dws, with an apparent magnitude of m = 21.5 +/- 0.2 [AB Mag]. These observations had a limiting magnitude of m = 22.4 [AB Mag]. While we have not performed host subtraction, the source is marginally detected in archival SDSS imaging with a u-band magnitude of m=24.02. This suggests our detection is dominated by transient light.
AT2025dws has been already been confirmed to be fast-fading by recent optical observations with LCO (Perez-Fournon et al., GCN 39643). While there are no earlier U-band detection of this transient, our UVOT detection is fainter than the optical detections reported in GCNs 39639