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Transient Alerts » in reply to GRB 261011A — GRB, X-ray Transient

In plain words Notable

On 11 October 2026 two satellites, SVOM and Swift, caught a long gamma-ray burst (a brief flash of high-energy light from a distant exploding star) in the southern sky. It lasted roughly a minute or more, depending on the instrument. Within minutes, telescopes around the world saw a fading visible-light and X-ray afterglow at the same spot. Two large telescopes measured the distance from the light's fingerprints: the burst happened at redshift 2.407, meaning the light left roughly 11 billion years ago. The afterglow faded below detection within about a day, and no host galaxy has been clearly detected. It is most likely an ordinary long burst from the collapse of a massive star. Swift's first report of an unusual light curve was withdrawn after a closer look showed that part of it came from noisy detectors.

How unusual is it? Most gamma-ray bursts have no measured distance, but this one has a confirmed redshift (2.407) from two independent telescopes plus a bright afterglow seen in many wavelengths; otherwise its properties look typical, and the initially reported oddity turned out to be instrumental noise.

Written by an AI model (claude-sonnet-5-5) from the official reports in this thread, and rewritten as new reports arrived (version 9); last updated . It may contain mistakes: the reports themselves are the authority.

GRB 261011A: Swift detection of an unusual burst
GCN Circulars —

GCN Circular 45896
GRB 261011A: Swift detection of an unusual burst
From: Jimmy DeLaunay at PSU
Issued: 2026-10-11 14:20 UTC

S. B. Cenko (GSFC), S. D. Barthelmy (GSFC), S. Dichiara (PSU),
R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 13:46:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 261011A (trigger=1516664). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 21.925, -21.529 which is
RA(J2000) = 01h 27m 42s
Dec(J2000) = -21d 31' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a broad soft pulse
from T0 to T0+130 sec and a shoulder extending out to T0+260 sec for a total
duration of about 260 sec. The peak count rate was ~8000 counts/sec
(15-350 keV), at ~95 sec after the trigger.

The XRT began observing the field at 13:51:57.1 UT, 315.8 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 321 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
RA(J2000) = 01:27:35.39 = 21.89746
DEC(J2000) = -21:33:37.2 = -21.56032
with a 90%-confidence error radius of about 1.10 arc sec. This position is
145.7 arc sec. from the center of the BAT error circle. The estimated magnitude
is 17.78. No correction has been made for extinction.

The BAT lightcurve is very unusual in that it combines a 260 second long
low energy emission mostly below 50 keV, with a ~5 second short peak
structure around T+95 seconds visible at all energies up the to 100-350 keV band.

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: Welcome to the Neil Gehrels Swift Observatory (swift.psu.edu))

Every one of the gamma-ray bursts GCN reports is posted here.
Relayed automatically from NASA's General Coordinates Network.

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