GCN Circular 45964
GRB 261010A: Pan-STARRS z-band imaging and upper limit
From: James Gillanders at University of Oxford
Issued: 2026-10-16 16:13 UTC
Type: GRB, Optical Transient
J. H. Gillanders (Oxford), M. Huber, K. C. Chambers, D. Jones, A. S. B. Schultz (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith, (Oxford/QUB), S. Srivastav (Oxford), M. Nicholl, D. Young (QUB), T.-W. Chen (NCU, Taiwan) T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).
We observed the field of GRB 261010A, discovered by the Fermi GBM Team (GCN 45888) and also detected by Swift/BAT (Lien et al., GCN 45889), Konus-Wind (Frederiks et al., GCN 45925), GRID (Yu et al., GCN 45940) and Glowbug (Cheung et al., GCN 45950), with the Pan-STARRS1 telescope (Chambers et al., 2016, arXiv e-prints, 1612.05560) in an effort to constrain the presence of transient flux at the position of the afterglow, as discovered by O’Neill et al. (GCN 45890), and subsequently detected by Aceituno et al. (GCN 45891), Corcoran et al. (GCN 45912), Castro-Tirado et al. (GCN 45928), Marchesini et al. (GCN 45952) and Rastinejad et al. (GCN 45957).
Our observation commenced on MJD 61327.22 (exposure midpoint; corresponding to 3.41 days post-GRB) and consisted of 6x300s exposures in z-band. We processed our images with the standard Pan-STARRS pipeline, whereby the images were astrometrically and photometrically calibrated before stacking (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). No difference imaging was applied as the PS1 3Pi reference stack is significantly shallower than this 1800s stack.
We inspected this stacked target image and no detection of a source at the position of the afterglow is visible. We estimate a z-band 3-sigma upper limit, m_z > 22.5. At the redshift of the purported nearby host galaxy (z = 0.070; Corcoran et al., GCN 45912), this corresponds to a limit of M_z = -15 (assuming a distance modulus of 37.5, and ignoring extinction effects). At 3.4 days post-burst, AT2017gfo possessed m_z = 18.74 +/- 0.02 (Smartt et al., 2017, Nature, 551, 75), which, rescaled to z = 0.070, corresponds to m_z ~ 23. Thus, we would not expect to recover a signal for an AT2017gfo-like kilonova at this redshift. Of course this is also the case for an association with the alternative proposed host galaxy at z = 0.785 (Castro-Tirado et al., GCN 45928).
The Pan-STARRS system consists of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters.
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: Pan-STARRS z-band imaging and upper limit
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