Transient Alerts — HabitableZone

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.

GRB261010A: Keck/MOSFIRE detection of a near-IR source
GCN Circulars —

GCN Circular 45957
GRB261010A: Keck/MOSFIRE detection of a near-IR source
From: Jillian Rastinejad at University of Maryland
Issued: 2026-10-16 06:42 UTC
Type: GRB, Optical Transient

Jillian Rastinejad (UMD), Maggie Li, and Mansi Kasliwal (Caltech) report on behalf of a larger collaboration:

We observed the location of GRB 261010A (Fermi GBM Team, GCN 45888; Lien et al., GCN 45889, Frederiks et al. GCN 45925) using the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) mounted on the Keck I Telescope. We obtained 73 x 40-sec imaging in J-band at a mid-time of 05:17 UT on 2026 Oct 16 (5.4 days post-burst) at seeing of ~0.7”.

We detect a near-IR source at >3 sigma significance that is consistent with the position of the optical afterglow (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891; Corcoran et al., GCN 45912). Precise photometry is complicated by contamination from the nearby galaxy, but we roughly measure a J-band magnitude that is consistent with the reported LBT measurement of H = 24.0 +/- 0.3 AB mag (Marchesini et al. GCN 45952). We observe a slight offset (< 0.5”) to the North between the reported optical afterglow position (O’Neill et al., GCN 45890) and the near-IR source in our MOSFIRE imaging. Given the lack of variability, we are unable to conclude if this emission is due to the counterpart of GRB 261010A or is due to an underlying galaxy.

We thank the Keck staff, including Julie Renaud-Kim and Josh Walawender, and the Keck classical night observers, including Gillian Willson for their co-operation with the ToO observations.

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

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