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Transient Alerts » in reply to GRB 261010A

In plain words Notable

On 10 October 2026 several gamma-ray satellites (Fermi, Swift, Konus-Wind and others) recorded a burst of gamma rays in the constellation Hercules that lasted about a minute. Swift's X-ray telescope and ground telescopes quickly found a fading X-ray and optical afterglow, which confirms the position. The host galaxy is uncertain: it is either a nearby galaxy about 25 arcseconds away at redshift 0.070, or a fainter galaxy 2 arcseconds away at redshift 0.785, meaning the light left roughly 7 billion years ago. Several teams say the burst's flash pattern and energy look more like those of a merger of two compact stars (such as neutron stars) than the collapse of a massive star, even though the burst was long. No kilonova (the glow from freshly made heavy elements after a merger) has been seen, and radio searches found nothing; the deep, late near-infrared images are too shallow to rule one out.

How unusual is it? A long gamma-ray burst with an optical afterglow, a measured redshift, and several independent analyses pointing to a merger origin is out of the ordinary, but no kilonova has been seen and the host is unconfirmed. It would become exceptional if a kilonova were confirmed, and would drop back toward routine if the burst were shown to come from a collapsing massive star.

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.

Konus-Wind detection of GRB 261010A
GCN Circulars —

GCN Circular 45925
Konus-Wind detection of GRB 261010A
From: Dmitry Frederiks at Ioffe Institute
Issued: 2026-10-12 16:48 UTC

D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 261010A (Swift detection:
Lien, et al., GCN 45889; Barthelmy et al., GCN 45918;
Fermi-GBM detection: The Fermi GBM team, GCN 45888;
Palafox & Jameson, GCN 45911)
triggered Konus-Wind (KW) at T0=69424.522 s UT (19:17:04.522).

The KW triggered on a bright, hard-spectrum, multipeaked pulse,
with a duration of ~3 s, which is superimposed over a broad,
less intense emission pulse that started ~10 s before the trigger,
peaked at ~T0+16 s, and lasted for ~70 s.

The Konus-Wind light curve of this GRB is available at
GRB 20261010_69424 (ioffe.ru)

As observed by Konus-Wind, the burst had
a fluence of (1.72 ± 0.47)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.128 s,
of (6.02 ± 1.08)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The spectrum of the bright pulse (measured from T0 to T0+8.448 s)
is best fitted in the 20 keV - 15 MeV range by a power law with exponential
cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.98 (-0.12, + 0.15) and Ep = 837(-194,+299) keV, PGstat = 81/98 dof.
Fitting this spectrum by a Band GRB function yields the same values
of alpha and Ep, and only an upper limit on the high energy photon
index beta of -2.0

The spectrum of the broad pulse (measured from T0+8.448 s to T0+49.408 s)
is best fitted in the 20 keV - 15 MeV range by a simple PL function
with the PL index of 1.92 (-0.07,+0.08),
PGstat = 103/99 dof.

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.

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

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