GCN Circular 45887
EP261006a: Kilonova-Catcher optical follow-up and detection
From: Damien Turpin at CEA-Saclay
Issued: 2026-10-10 17:43 UTC
Type: X-ray Transient, Optical Transient
R. Hellot, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of EP261006a detected by Einstein Probe EP/WXT (Lao et al., GCN 45826) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a TEC160FL telescope operated by M. Freeberg and a CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at TGRB+3.3 days and were taken with the Johnson/Cousins BV and sloan gri filters.
In our stacked frames, subtracted from the Pan-STARRS DR2 template image, we clearly detect the optical counterpart reported by many teams (Eyles-Ferris et al., GCN 45831; Jiang et al. GCN 45833; Wu et al., GCN 45852; Bochenek et al., GCN 45853, GCN 45883; Chen et al. GCN 45853; Corcoran et al., GCN 45868; You et al., GCN 45874; Zhang et al., GCN 45877; Kang et al., GCN 45885).
We report part of our preliminary follow-up results in the table below :
| Tmid-TGRB (days) | Exp (s) | Filter | Magnitude | Instrument |
| ------------ | ------------ |------------ |------------ |------------ |
| 3.33 | 5 x 300s | g (AB) | 18.64 +/- 0.04 | CDK17 |
| 3.37 | 15 x 300s | r (AB) | 19.18 +/- 0.04 | CDK17 |
| 3.42 | 10 x 180s | B (Vega) | 18.67 +/- 0.06 | TEC160FL |
| 3.49 | 15 x 180s | g (AB) | 18.44 +/- 0.03 | TEC160FL |
| 3.52 | 15 x 180s | r (AB) | 18.79 +/- 0.08 | TEC160FL |
| 3.55 | 5 x 180s | i (AB) | 19.34 +/- 0.38 | TEC160FL |
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-cousins filters were calibrated using the GAIA DR3 synphot catalog. Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog.
Our photometric measurements are in good agreement with previous magnitudes reported at similar epochs showing that the optical transient is still exhibiting a bright and blue colored (g-r = -0.35) emission.
The KNC network will keep monitoring this transient as long as it is detectable. Further updates will be reported in GCN Circulars.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (Kilonova Catcher Home (kilonovacatcher.in2p3.fr)).
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.
Transient Alerts » in reply to EP261006a — X-ray Transient
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: Kilonova-Catcher optical follow-up and detection
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