GCN Circular 46009
EP261015a: GSP multiband optical follow-up observations
From: fufu Glow
Issued: 2026-10-22 15:15 UTC
Type: X-ray Transient, Optical Transient
J. F. Gou, W. X. Li, S. J. Xue (NAOC); M. Andrews, D. A. Howell, C. McCully, J. Ridenbaugh and K. Wynn (Las Cumbres Observatory) report:
We observed the field of the Einstein Probe fast X-ray transient EP261015a (Li et al., GCNs 45956 and 45969) using MuSCAT3 on the Las Cumbres Observatory (LCO) 2-m Faulkes Telescope North at Haleakala, Hawaii. Observations were obtained simultaneously in g, r, i, and z_s from 2026 October 20, 14:06:51 to 14:29:01 UTC, with four 300-s exposures in each band, approximately 5.0 days after the EP/WXT event onset.
Following subtraction of same-band archival Legacy Survey DR10 templates with HOTPANTS, we detect residual emission at the position of the optical counterpart reported by Corcoran et al., GCNs 45955 and 45972; Fortin et al., GCN 45991; Pawar et al., GCNs 45998 and 46006; Shivaprasad et al., GCN 46000; Pankov et al., GCN 46003. Preliminary PSF photometry using AutoPhOT (Brennan & Fraser 2022), calibrated against nearby Pan-STARRS DR2 stars, gives:
Magnitude (AB)
g 23.51 ± 0.22
r 22.26 ± 0.06
i > 22.34 (5 sigma)
z > 22.14 (5 sigma)
These observations were taken as part of the Global Supernova Project (GSP).
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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: GSP multiband optical follow-up observations
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