GCN Circular 45967
EP261015a: J-band upper limit with WINTER
From: unknown
Issued: 2026-10-16 23:35 UTC
Type: X-ray Transient
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP261015a (Li et al., GCN 45956) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began on 2026-10-16 at 05:43:44.591 UTC in the J band (16.47 hr after the WXT observation), consisting of 30*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (GitHub - winter-telescope/mirar: Modular Image Reduction and Analysis Resource (MIRAR) (github.com), winter-telescope/mirar: v0.17.0 (doi.org)).
We do not detect the optical counterpart reported by Corcoran et al., GCN 45955; Méndez et al., GCN 45960 and Ma et al., GCN 45963 in our stacked image.
We obtain the following 5-sigma upper limit: J = 19 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
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Relayed automatically from NASA's General Coordinates Network.
Transient Alerts » in reply to EP261015a — X-ray Transient, Optical Transient
In plain words Notable
On 15 October 2026 the Einstein Probe, a space telescope that watches for X-ray flashes, saw a brief flare lasting about two minutes in the southern sky. Telescopes then found a faint, slowly changing point of visible light in the same area, sitting on top of a faint galaxy. It faded a little in the first day and a half, then brightened again around day 4 and stayed roughly steady through day 5. A separate, fainter X-ray source in the same region was found to belong to an unrelated, known quasar (a distant galaxy with a very bright core). The leading idea is that the visible light is a supernova (an exploding star) linked to the X-ray flash, but this is not confirmed, and no distance has been measured yet.
How unusual is it? A visible-light counterpart was found for an X-ray flash, and its rebrightening hints at an accompanying supernova, which is not typical of most such flashes. It is not exceptional because the link is unconfirmed, the source could be an unrelated transient, and no distance is known; a spectrum confirming a supernova or giving a distance would settle the verdict.
Written by an AI model (claude-sonnet-5-5) from the official reports in this thread, and rewritten as new reports arrived (version 4); last updated . It may contain mistakes: the reports themselves are the authority.
EP261015a: J-band upper limit with WINTER
The whole thread (16 posts)
- EP261015a — X-ray Transient, Optical Transient
- Updated explainer: what the latest reports add (AI-written)
- EP261015a: GSP multiband optical follow-up observations
- EP261015a: Further optical observations with the 1.3m DFOT
- EP261015a: AZT-20 Assy-Turgen optical observations
- Explainer: what this is and why it matters (AI-written)
- EP261015a: Wendelstein 43 cm optical observations
- EP261015a: Follow-up with the 1.3-m DFOT
- EP261015a: Terskol Zeiss-2000 optical upper limit
- EP261015a: COLIBRÍ optical observations of a rebrightening
- EP261015a: Tentative fading of the candidate optical counterpart from the NOT
- EP261015a: refined analysis of EP-FXT observations
- EP261015a: J-band upper limit with WINTER
- EP261015a: SVOM/VT optical observation
- EP261015a: COLIBRÍ optical observations
- EP261015a: Einstein Probe detection of a fast X-ray transient