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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: refined analysis of EP-FXT observations
GCN Circulars —

GCN Circular 45969
EP261015a: refined analysis of EP-FXT observations
From: EP Team at NAOC/CAS
Issued: 2026-10-17 05:09 UTC
Type: X-ray Transient

R.-Z. Li (YNAO), Z.-X. Li, G.-L. Huang (IHEP), M. Nie (YNAO) and H. Sun (NAOC) on behalf of the Einstein Probe (EP) team:

The X-ray transient EP261015a was detected by the Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) mission at 2026-10-15T13:15:36 UTC (Li et al., GCN 45956). Follow-up observations with the Follow-up X-ray Telescope (FXT) onboard EP began at 2026-10-16T04:31:12 UTC, approximately 15.26 hours after the WXT trigger, with an exposure time of 3231 s.

We stacked the FXT-A and FXT-B data. Within the EP/WXT error circle (Li et al., GCN 45956), no X-ray source was detected at the position of the optical candidate (Corcoran et al., GCN 45955; Méndez et al., GCN 45960; Ma et al., GCN 45963; Mohan et al., GCN 45967). The derived 0.5–10 keV flux upper limit is approximately 1.33 × 10^-13 erg cm^-2 s^-1 (90% confidence level).

An uncatalogued X-ray source was detected in the stacked data within the EP/WXT error circle (Li et al., GCN 45956). Its position is R.A. = 44.3459 deg, Decl. = −13.3915 deg (J2000), with a positional uncertainty of 10 arcsec in radius (90% confidence level, including statistical and systematic uncertainties). The 0.5-10 keV FXT spectrum of this source can be fitted with an absorbed power-law model, with the Galactic hydrogen column density fixed at 3.5 × 10^20 cm^-2 and a photon index of 1.55 (+1.45, −1.43). The derived unabsorbed 0.5–10 keV flux is 7.01 (+6.27, −3.45) × 10^-14 erg cm^-2 s^-1. All quoted uncertainties on the spectral parameters and fluxes are at the 90% confidence level.

Launched on January 9, 2024, EP is a space-based X-ray observatory designed to monitor the soft X-ray sky and perform X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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