GCN Circular 46000
EP261015a: Wendelstein 43 cm optical observations
From: unknown
Issued: 2026-10-20 18:26 UTC
Type: X-ray Transient, Optical Transient
Surya Shivaprasad (LMU), Malte Busmann (LMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), Julius Gassert (LMU), Xander J. Hall (CMU), and Antonella Palmese (CMU) report:
We observed the counterpart of EP261015a (Corcoran et al., GCNs 45955 and 45972; Li et al., GCNs 45956 and 45969; Moreno Méndez et al., GCN 45960; Ma et al., GCN 45963; Mohan et al., GCN 45967; Fortin et al., GCN 45991; Volnova et al., GCN 45997; Pawar et al., GCN 45998) with the 43 cm telescope at Wendelstein Observatory. Observations were taken in the r band, with 47 × 300 s exposures starting at 2026-10-19T22:15:03 UT (4.37 days after the trigger). Using a template from the DESI Legacy Surveys for difference imaging, we measure an r-band magnitude of
r = 22.52 ± 0.28 AB mag.
This measurement is consistent with the updated report by Fortin et al. (GCN 45991), which reports r = 22.58 ± 0.09 AB mag at ~3.7 d post-trigger, indicating that the source is evolving slowly at this phase.
Magnitudes are calibrated against the PS1 catalog and are not corrected for 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.
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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: Wendelstein 43 cm optical 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