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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: SVOM/VT optical observation
GCN Circulars —

GCN Circular 45963
EP261015a: SVOM/VT optical observation
From: Yinuo Ma
Issued: 2026-10-16 16:09 UTC
Type: X-ray Transient, Optical Transient

Y. N. Ma, H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, J. R. Xu, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.

SVOM/VT performed a Target of Opportunity observation of EP261015a detected by EP/WXT (Li et al., GCN 45956). SVOM/VT began observing the field at 2026-10-16T02:48:38 UTC, 13.551 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

With X-band data available, the NOT candidate (Corcoran et al., GCN 45955; Méndez et al., GCN 45960) was detected in both VT_B and VT_R bands. The AB magnitudes are:

mid time (h) | exposure time (s) | band | mag (AB)
-------------|-------------------|------|------------
14.20 | 25*50 | VT_B | 22.3+/-0.2
14.20 | 27*50 | VT_R | 22.3+/-0.2

Our photometry was not corrected for Galactic extinction.

There is a faint extended source at the position of this candidate (Corcoran et al., GCN 45955; Méndez et al., GCN 45960), with magnitudes g = 23.09, r = 22.39, i = 22.30, and z = 21.92 in Legacy Survey. Our photometry is contaminated by the source.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

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Relayed automatically from NASA's General Coordinates Network.

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