GCN Circular 45826
EP261006a: Einstein Probe detection of an X-ray transient
From: EP Team at NAOC/CAS
Issued: 2026-10-07 07:13 UTC
Type: X-ray Transient
Why this event is posted: it has drawn 11 GCN Circulars, at or above the top-10% threshold of 11 for X-ray transients.
Q.H. Lao (NAO, CAS), L.N. Xu (USTC), H.Y. Liu (NAO, CAS), W.F. Wen (SZTU), X.H. Tang (THU) and C.C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP261006a. The transient triggered EP-WXT (ID: 01709320985) at 2026-10-06T17:49:57 (UTC) . The WXT position of the source is R.A. = 21.806 deg, DEC = 12.9 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The exposure time of WXT observation (ID: 01709320985) is 384 s, and no significant variability was detected during the observation.
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at Galactic NH = 3.84 x 10^20 cm^-2 at the source position. The best-fit photon index is 1.73 (-0.66/+0.77), and the derived average unabsorbed 0.5-4 keV flux is 1.46 (-0.53/+0.78) x 10^-10 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed the source autonomously following the WXT trigger at 2026-10-06T17:53:48(UTC), 231s after the trigger. With the telemetry data received, the refined on-ground analysis gives an X-ray source position of RA=21.7913 deg, DEC=12.8907 deg (J2000) , with an uncertainty of about 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows an overall declining trend during the observation.
We jointly fitted the FXT-A and FXT-B spectra in the 0.5-10 keV band with an absorbed power-law model, with the hydrogen column density fixed at Galactic NH = 3.84 x 10^20 cm^-2. The best-fit photon index is 1.64 (-0.01/+0.01). The derived average unabsorbed 0.5-10 keV flux is 1.17 (-0.02/+0.02) x 10^-10 erg s^-1 cm^-2.
Within the FXT error circle, there is a galaxy, Gaia DR3 2587044864659595392 (Hughes et al. 2022) with a photometric redshift of 0.184 (Duncan et al. 2022). If this transient is located within this galaxy, its estimated 0.5-10 keV luminosity is 1.12×10^46 erg s^-1.
The uncertainties are at the 90% confidence level for the above spectral parameters.
Subsequent follow-up observations were reported by Novotny et al. (GCN 45821) and Shi et al. (GCN 45823). Further multi-wavelength follow-up observations are encouraged to explore the nature of EP261006a.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
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.
In plain words Notable
On 6 October 2026 the Einstein Probe, a space X-ray telescope, spotted a sudden flash of X-rays at the position of a galaxy, with a measured redshift of 0.1853 (very roughly 2-3 billion light-years away). Optical telescopes then found a bright, blue, new light source right at the galaxy's centre that was not there two days earlier and has brightened to about 18th magnitude over several days. Its blue colour and central location fit a tidal disruption event, where a black hole tears apart a passing star, but the spectra show no clear features to confirm this, and a fast-fading exploding star remains possible. The X-rays faded by about a factor of 300 within a day, while the visible light kept rising, so the two may not come from the same process. Radio telescopes (VLA and MeerKAT) detected nothing.
How unusual is it? A measured distance, an optical counterpart at the galaxy's centre and a rising blue light curve make this a well-characterised fast X-ray transient, with a likely tidal disruption event, which is more than the typical alert. It is not exceptional, since such events are being found regularly, and the classification is still unconfirmed; clear emission lines or radio emission would settle the nature, and a different type of explosion would change the picture.
Written by an AI model (claude-sonnet-5-5) from the official reports in this thread, and rewritten as new reports arrived (version 7); last updated . It may contain mistakes: the reports themselves are the authority.
EP261006a — X-ray Transient
The whole thread (20 posts)
- EP261006a — X-ray Transient
- EP261006a / AT 2026aetp: Optical follow-up with the 1.3-m DFOT
- EP261006a : MeerKAT radio observations.
- EP261006a: 10 GHz VLA observations
- EP261006a / AT 2026aetp: Luminance-band optical follow-up with small-aperture telescopes
- EP261006a: Kilonova-Catcher optical follow-up and detection
- Updated explainer: what the latest reports add (AI-written)
- EP261006a / AT 2026aetp: Multiband optical follow-up with the PKU 60cm Telescope
- EP261006a: Liverpool Telescope optical follow-up
- Explainer: what this is and why it matters (AI-written)
- EP261006a: Xinglong 2.16m/BFOSC optical follow-up observations
- EP261006a: Optical follow-up observations with MATCH
- EP261006a / AT 2026aetp: VLT/X-shooter spectroscopic redshift z = 0.1853
- EP261006a: Swift/XRT detection of the fading X-ray counterpart
- EP261006a: Optical follow-up observations with Kinder
- EP261006a: Liverpool Telescope optical observations
- EP261006a: Early Optical Observations by WFST
- EP261006a: NOT spectroscopy of a blue continuum
- EP261006a: LT and NOT discovery of an optical counterpart
- EP261006a: SVOM/VT optical upper limits