GCN Circular 45997
EP261015a: Terskol Zeiss-2000 optical upper limit
From: Alina Volnova at IKI RAS
Issued: 2026-10-20 15:07 UTC
Type: X-ray Transient
A. Volnova (IKI), D. Andreev (INASAN), A. Pozanenko (IKI), A. Moskvitin (SAO), N. Pankov (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of Einstein Probe fast X-ray transient EP261015a (Li et al., GCN 45956) with the Zeiss-2000 2-meter telescope of the Terskol Observatory on Oct. 17, 2026, starting at 21:30 UT and taking several exposures in R-band.
The optical counterpart reported previously (Corcoran et al., GCNs 45955, 45972; Fortin et al., GCN 45991) is not detected in the stacked frame.
Preliminary photometry and observational details are the following:
Date UT start t-T0 Exp, Filter Obj Err UL Site/Telescope
(mid,days) (n*s) (3sigma)
2026-10-17 21:30:24 2.32810 20*180 R n/d n/d 22.8 Terskol/Z2000
The photometry is based on several nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction.
Filter: only the top 10% of the past year's X-ray transients are posted, ranked by the number of GCN Circulars written about them (now ≥ 11). Thresholds are recalculated monthly. Only events first reported within the past 7 days can start a thread.
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: Terskol Zeiss-2000 optical upper limit
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