GCN Circular 45944
GRB 261010A: a short minimum variability timescale and a distinct pulse evolution compatible with a merger origin
From: ROMAIN LOIC MACCARY at University of Ferrara
Issued: 2026-10-13 15:27 UTC
Type: GRB
R. Maccary, C. Guidorzi, M. Bulla (University of Ferrara) A. Rossi, G. Stratta, (INAF-OAS),S. Klose (TLS Tautenburg) on behalf of a larger collaboration report:
We determined the minimum variability timescale (MVT) of GRB 261010A (Fermi GBM team, GCN 45888) from the Fermi/GBM light curve (NaI detectors 0, 1 and 3) following the prescriptions of Camisasca et al. (2023), i.e. as the minimum full width half maximum (FWHM_min) of all statistically significant pulses, and found FWHM_min = 82 (-21, +28) ms. Combined with the T90~54 s (Fermi GBM team, GCN 45911), in the T90-FWHM_min plot GRB 261010A (green square in the Figure at this link 261010A_mvt_plot.png (drive.google.com)) lies in the region populated by other long-lasting merger candidates, such as also GRB 191019A.
In addition, the waiting times show a distinct temporal evolution (see 4panels.pdf (drive.google.com)), which has been recently identified as an indicator of merger origin (Maccary et al. 2026a,b)
This is in agreement with the prompt-based merger classification made by Zhu & Tam, GCN 45932.
In addition, assuming either a redshift of z= 0.070 (Malesani et al., GCN 45912) or z=0.785 (Castro-Tirado et al., GCN 45928), a fluence of 1.72e-5 erg cm-2 would correspond to either Eiso~2.0e+50 erg or Eiso~2.9e+52 erg. Combined with Ep~837 keV (Frederiks et al., GCN 45925), the two possible positions of GRB261010A in the Epi-Eiso plane seem to be more compatible with Type-I rather than Type-II GRBs (see Figure at this link 261010A_amati.png (drive.google.com)).
We caution that this Figure was obtained using the peak energy computed using the main pulse, the time-integrated peak energy computed on the whole burst could be lower (e.g., Fermi/GBM team, GCN 45911) which would put the point at z=0.785 slightly closer to Type-II GRBs (see 261010A_amati_450kev.png (drive.google.com)) .
References:
- Minimum variability timescale (MVT): Camisasca et al., 2023,https://doi.org/10.1051/0004-6361/202245657
- Maccary et al., 2025,A&A,https://doi.org/10.1051/0004-6361/202555418
- Distinctive pulse evolution: Maccary et al., 2026, A&A, https://doi.org/10.1051/0004-6361/202659083
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 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: a short minimum variability timescale and a distinct pulse evolution compatible with a merger origin
The whole thread (25 posts)
- GRB 261010A
- GRB261010A: Retraction of Keck/MOSFIRE near-IR source
- GRB 261010A: Pan-STARRS z-band imaging and upper limit
- GRB 261010A: VLA radio observations
- GRB261010A: Keck/MOSFIRE detection of a near-IR source
- GRB 261010A: ASKAP non-detection of a radio afterglow
- GRB 261010A: LBT tentative near-infrared detection
- GRB 261010A: Glowbug gamma-ray detection
- GRB 261010A: initial pulse rest-frame energetics from Konus-Wind detection
- GRB 261010A: a short minimum variability timescale and a distinct pulse evolution compatible with a merger origin
- GRID detection of GRB 261010A
- GRB 261010A: J-band upper limit by the SYSU 80cm infrared telescope
- GRB 261010A: Swift-XRT refined Analysis
- GRB 261010A: a Type I-L (merger-driven) GRB candidate
- Updated explainer: what the latest reports add (AI-written)
- GRB 261010A: 10.4m GTC spectroscopy of the galaxy 2" from the afterglow
- Konus-Wind detection of GRB 261010A
- GRB 261010A: Swift-BAT refined analysis
- GRB 261010A: NOT confirmation of the optical afterglow
- GRB 261010A: Fermi GBM Observation
- Explainer: what this is and why it matters (AI-written)
- GRB 261010A: Enhanced Swift-XRT position
- GRB 261010A: possible optical afterglow detected with the 1.5m OSN
- GRB 261010A: NOT possible optical counterpart
- GRB 261010A: Swift detection of a burst