GCN Circular 45932
GRB 261010A: a Type I-L (merger-driven) GRB candidate
From: Thomas P.H. Tam at Sun Yat-sen University
Issued: 2026-10-13 05:42 UTC
Type: GRB
Si-Yuan Zhu and Pak-Hin Thomas Tam (SYSU) report:
We analyzed GRB 261010A using publicly available data from Fermi/GBM. The burst was detected by Fermi/GBM at 19:17:03 UT on 2026 October 10, as a long-duration GRB with T90 about 54 s (50-300 keV; Fermi GBM Team, GCN 45888).
The 10-1000 keV light curve of GRB 261010A consists of multiple emission episodes. In particular, we identified a precursor emission (PE) episode from T0-0.128 s to T0+0.384 s, with a duration of T100,PE = 0.512 s. The PE is separated from the main emission (ME, from T0 + 3.856 s to T0 + 5.936 s), by a quiescent interval (QE) lasting T100, QE=3.472 s. We adopted the pulse rise time of the PE as its minimum variability timescale (MVT), T_MVT,PE = 0.152 s. The time-integrated spectrum of the ME is best fitted by a power law with an exponential cutoff, yielding a photon index of alpha = -0.94 +/- 0.04 and a peak energy of Ep,ME=970 +/- 198 keV.
To further classify GRB 261010A, we apply the Ep,ME-precursor index (EPI), a diagnostic that uses prompt emission alone (Zhu et al. 2026; [1]). The EPI separates merger-driven (Type I) from collapsar-driven (Type II) GRBs, with Type I GRBs concentrating at EPI > 6.2 and Type II GRBs at EPI < 6.2. Values in the range 6.0 < EPI < 6.4 fall in a transition region where misclassification is possible, whereas EPI > 6.4 and EPI < 6.0 allow safe classification as Type I and Type II, respectively. We calculate EPI = 6.81, suggesting that GRB 261010A is unambiguously classified as a Type I-L (merger-driven) GRB. (see GRB261010a_epi (tinyurl.com))
The optical afterglow of GRB 261010A (O'Neill et al., GCN 45890; Aceituno et al., GCN 45891) is located at a projected offset of 34.5 kpc from the nearby galaxy LEDA 1781464 at z = 0.070 (Corcoran et al., GCN 45912). GTC/OSIRIS spectroscopy of the red object 2.2 arcsec NE of the afterglow shows a galaxy at z = 0.785 +/- 0.001 (Castro-Tirado et al., GCN 45928), and the afterglow is offset from the centre of that galaxy by 16 kpc in projection.
If LEDA 1781464 at z = 0.070 is the host, the large projected offset of 34.5 kpc supports a merger origin. We obtain Eiso = 5.04E49 erg and Ep,z = 1038 keV of the ME, consistent with the Ep,z-Eiso correlation of Type I (short) bursts. If the galaxy at z = 0.785 is the host. We obtain Eiso = 7.19E51 erg and Ep,z = 1731 keV of the ME, still consistent with the Type I correlation.
Based on our preliminary analysis, GRB 261010A is a Type I-L (merger-driven) GRB candidate. Further analysis of GRB 261010A is ongoing and multi-wavelength follow-up observations are strongly encouraged.
References
[1] Zhu et al., ApJ, 1003, 142, 2026
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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: a Type I-L (merger-driven) GRB candidate
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