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Transient Alerts » in reply to EP261015a — X-ray Transient, Optical Transient

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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: AZT-20 Assy-Turgen optical observations
GCN Circulars —

GCN Circular 46003
EP261015a: AZT-20 Assy-Turgen optical observations
From: Nicolai Pankov at IKI
Issued: 2026-10-21 06:17 UTC
Type: X-ray Transient, Optical Transient

N. Pankov (IKI), M. Krugov (FAI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of the Einstein Probe fast X-ray transient EP261015a (Li et al., GCN 45956) with the 1.5-m AZT-20 telescope of the Assy-Turgen Observatory. Observations started on 2026-10-19 at 19:20:07 UT, 4.276 days after the EP/WXT trigger. A series of frames was obtained in the r' filter.

We clearly detect the optical counterpart (Corcoran et al., GCNs 45955 and 45972; Li et al., GCNs 45956 and 45969; 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; Shivaprasad et al., GCN 46000) at coordinates (J2000) 02:57:19.83, -13:21:17.3. The residual source is detected after subtraction of a DESI Legacy Survey DR10 r-band template. Preliminary photometry of the residual source is given below.

Date UTstart t-T0 Exp. Filter OT Err. UL
(mid, days) (s) (3sigma)
2026-10-19 19:20:07 4.2764 67*60 r' 22.59 0.06 23.8

The photometry was calibrated against nearby Pan-STARRS1 stars

RA (J2000) Dec (J2000) r
02:57:33.73 -13:21:44.6 16.891
02:57:25.43 -13:24:41.2 19.263

and is not corrected for Galactic extinction.

The residual magnitude is consistent with the Legacy Survey-subtracted measurements of Fortin et al. (GCN 45991; r = 22.58 +/- 0.09 at 3.7 d) and Shivaprasad et al. (GCN 46000; r = 22.52 +/- 0.28 at 4.37 d).

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.

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