GCN Circular 42488
Subject
GRB 251023B: Swift-UVOT Upper Limits
Event
Date
2025-10-27T17:38:43Z (5 days ago)
From
noelklin@umbc.edu
Via
Web form
Swift-UVOT has performed follow-up observations of the EP-detected burst GRB 251023B (Wu et al. GCN 42415), collecting 1.7 ks of observations between T0+6.6 ks and T0+26.9 ks. No source was detected in the UVOT data at the position of the optical afterglow (Mandarakas et al., 42404; He at al., GCN 42418; Adami et al., 42421; Méndez et al. 42424).
3-sigma upper limits (AB) using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are:
Filter T_start(s) T_stop(s) Exp(s) Mag
U 6573 26943 1737 >21.92
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.416 in the direction of the burst (Schlegel et al. 1998).
We note that in the first snapshot (1531.80 s worth of data), the optical afterglow's position landed on a UVOT detector patch of small scale sensitivity inhomogeneity. In the U-band this can cause a source's flux to appear 5-16% lower than it actually is. The above limits are not corrected for this. See https://swift.gsfc.nasa.gov/analysis/uvot_digest/sss_check.html for more information.