GCN Circular 45819
GRB 261005B: Swift/UVOT Upper Limits
From: Alice Breeveld at MSSL-UCL
Issued: 2026-10-06 13:43 UTC
A. A. Breeveld (UCL-MSSL), N. Klingler (NASA-GSFC/UMBC/CRESST II) and T.M. Parsotan (GSFC) report on behalf of the Swift/UVOT team:
The Swift/UVOT started observing the field of GRB 261005B 4424 s after the BAT trigger (Parsotan et al., GCN Circ. 45812).
No optical afterglow consistent with the enhanced XRT position (Page et al., GCN Circ. 45815) 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 data available are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 5451 5651 197 >20.4
b 5247 5446 197 >19.8
u 5040 5239 197 >19.3
v 4424 4624 197 >18.3
uvw1 4835 5035 197 >18.7
uvw2 5658 5836 175 >19.1
uvm2 4629 4829 197 >18.1
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.12 in the direction of the burst (Schlegel et al. 1998).
Every one of the gamma-ray bursts GCN reports is posted here.
Relayed automatically from NASA's General Coordinates Network.
Transient Alerts » in reply to GRB 261005B
In plain words Routine
On the evening of 5 October 2026 (UTC), two space telescopes, NASA's Swift and Fermi, both caught a burst of gamma rays (the most energetic form of light) from a spot in the sky near the border of the constellations Ophiuchus and Hercules. The burst flickered with several peaks and lasted somewhere between about 20 and 90 seconds, depending on which instrument and energy range measured it. Swift's X-ray telescope found a fading X-ray afterglow and pinned its position to within about 2 arcseconds, but ultraviolet, optical and infrared searches hours later found no visible afterglow. No distance has been reported, so how far away it is remains unknown. It looks like an ordinary long gamma-ray burst, the kind usually linked to the death of a massive star.
How unusual is it? This is a typical gamma-ray burst: moderate brightness, an X-ray afterglow found, no visible-light afterglow and no distance measured. A redshift (distance) measurement or a supernova or other counterpart found later would make it more notable.
Written by an AI model (claude-sonnet-5-5) from the official reports in this thread; last updated . It may contain mistakes: the reports themselves are the authority.
GRB 261005B: Swift/UVOT Upper Limits