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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.

GRB 261010A: Swift-BAT refined analysis
GCN Circulars —

GCN Circular 45918
GRB 261010A: Swift-BAT refined analysis
From: Amy
Issued: 2026-10-12 04:10 UTC

S. D. Barthelmy (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha
(GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss
(GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T.
Sakamoto (AGU) (i.e. the Swift-BAT team):

Using the data set from T-120 to T+662 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 261010A (trigger #1516528)
(Lien et al., GCN Circ. 45889). The BAT ground-calculated position is
RA, Dec = 248.767, 26.600 deg which is
RA(J2000) = 16h 35m 04.2s
Dec(J2000) = +26d 36' 01.3"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 83%.

The mask-weighted light curve shows a main pulse that starts and peaks at
~T0 and ends at ~T+3 s. In addition, there are some weak emission that
starts prior to the main pulse at ~T-10 s and lasts till ~T+70 s. T90
(15-350 keV) is 71.50 +- 14.07 sec (estimated error including systematics).

The time-averaged spectrum from T-9.28 to T+76.45 sec is best fit by a
simple power-law model. The power law index of the time-averaged spectrum
is 1.43 +- 0.17. The fluence in the 15-150 keV band is 3.1 +- 0.4 x 10^-6
erg/cm2. The 1-sec peak photon flux measured from T+0.26 sec in the 15-150
keV band is 3.6 +- 0.4 ph/cm2/sec. All the quoted errors are at the 90%
confidence level.

The results of the batgrbproduct analysis are available at
batgrbproduct analysis for trigger=1516528 (swift.gsfc.nasa.gov)

(In memory of Scott Barthelmy, the BAT team dedicates the rest of our
circulars to Scott.)

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

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