EP260812b
GCN Circular 45386
Subject
EP260812b: GTC/OSIRIS+ tentative spectroscopic redshift z = 2.034
Date
2026-08-15T13:59:19Z (17 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), D. B. Malesani, (DAWN/NBI and Radboud), P. G. Jonker (Radboud), F. M. Perez Toledo (GTC) report on behalf of a larger collaboration:
We observed the optical counterpart (Malesani et al., GCN 45367; Ma et al., GCN 45370; Gill et al., GCN 45371; Starling et al., GCN 45374; Eyles-Ferris et al., GCN 45375; Lee et al., GCN 45383) of the fast X-ray transient EP260812b (Li et al., GCN 45368) using the Gran Telescopio Canarias (GTC) located on the island of La Palma (Canary Islands, Spain), equipped with the OSIRIS+ spectrograph. A sequence of 4x1200 s observations was obtained using grism R1000B, which covers the wavelength range 3600-7730 AA. The observation started on 2026-08-14 at 00:11:08 UT (~24.59 hr after the EP/WXT trigger).
In the acquisition image (taken 24.52 hr after trigger), we measure an AB magnitude r = 23.13 +/- 0.12 for the optical counterpart, calibrated against the Pan-STARRS catalog, and not corrected for Galactic extinction. The delivered seeing was 0.8".
A faint continuum is detected across most of the covered wavelength range of the spectrum. A clear absorption line is detected at 4700 AA. This is consistent with the (resolved) C IV doublet at redshift z = 2.033. At the same redshift, we also identify the Si IV doublet. By heavily rebinning the spectrum, a drop in the very blue end, around 3730 AA, roughly matches the expected position of the Lyman break at this redshift. We note however that none of the usually observed strong low-ionization lines are detected, with the possible exception of Fe II 1608. We thus suggest that EP260812b lies at z = 2.033, in an environment unusually rich in high-ionization species.
The analysis of this spectrum was carried out with the help of the zHunter (doi:10.5281/zenodo.15189495) and GRBSpec (http://grbspec.eu; de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774) tools.
GCN Circular 45383
Subject
EP260812b: Optical upper limit with Kinder
Date
2026-08-14T14:33:12Z (18 days ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
Y.-H. Lee, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), A. Sankar.K, K. N.-T. Ho, A. Dutta, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, W.-J. Hou, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), K. W. Smith, H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP260812b (Li et al., GCN 45368) using the 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations in r-band started at 18:18 UTC on the 13th of August 2026 (MJD 61265.7625), 18.71 hr after the EP-WXT detection.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. In the stacked image, we did not detect the optical counterpart reported by Malesani et al., (GCN 45367); Ma et al., (GCN 45370); Gill et al., (GCN 45371); Starling et al., (GCN 45374) and Eyles-Ferris et al., (GCN 45026).
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 61265.7625 | 18.71 | 300 * 6 | >22.4 | 1".2 | 2.08
Given the reported r-band magnitude of ~21.6 mag (Gill et al., GCN 45371) for observations starting at +4.55 hr post-EP-WXT detection, our observed limit is consistent.
The presented magnitudes are calibrated using the field stars from the Pan-STARRS1 catalogue. The reported magnitude is not corrected for an expected galactic extinction of A_r = 0.15 mag, in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 45375
Subject
EP260812b: Liverpool Telescope observations
Date
2026-08-13T14:50:30Z (19 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD) and R. L. C. Starling (Leicester) report on behalf of a larger collaboration:
We observed the field of the Einstein Probe fast X-ray transient EP260812b (Li et al., GCN 45368) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 5x150 s exposures in each of the SDSS r’ and z’ filters starting on 2026-08-12 at 23:59:13 UT, approximately 0.39 hr after the X-ray detection.
We detect the optical counterpart (Malesani et al., GCN 45367; Ma et al., GCN 45370; Gill et al., GCN 45371; Starling et al., GCN 45374) in both bands and measure the following photometry, calibrated to Pan-STARRS and not corrected for Galactic extinction:
| t_mid - t0 (min) | Filter | AB magnitude |
| ------------- | ------ | ------------ |
| 31.8 | r’ | 20.33 +/- 0.03 |
| 47.5 | z’ | 20.08 +/- 0.04 |
Our photometry is consistent with that of Malesani et al. (GCN 45367) at a comparable time.
GCN Circular 45374
Subject
EP260812b: GOTO detection of the optical counterpart
Date
2026-08-13T12:51:47Z (19 days ago)
From
Rhaana Starling at U of Leicester <rlcs1@leicester.ac.uk>
Via
Web form
R. Starling, D. O'Neill, B. P. Gompertz, G. Ramsay, S. Belkin, A. Kumar, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, M. Pursiainen, on behalf of the GOTO collaboration
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the Einstein Probe detected X-ray transient EP260812b (ID: 01709278129, Li et al. 2026, GCN 45368).
Images consisted of 4x90s exposures in the GOTO L-band (400-700 nm) and were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks (Lyman et al. 2026).
We detect the optical counterpart (Malesani et al. GCN 45367; Ma et al. GCN 45370; Gill et al. GCN 45371) at 3-sigma confidence with the following magnitudes
| Time (UT) | t-t0 (min) | ABmag and 1-sigma error |
|---|---|---|
| 2026-08-12 23:46:46 | 10.9 | 20.49 ± 0.22 |
| 2026-08-13 00:55:02 | 79.2 | 20.66 ± 0.26 |
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular 45371
Subject
EP260812b: COLIBRÍ optical observations
Date
2026-08-13T05:15:41Z (20 days ago)
From
Ramandeep Gill <rsgill.rg@gmail.com>
Via
Web form
Ramandeep Gill (UNAM), Massimiliano Lincetto (CPPM), Noémie Globus (UNAM), Enrique Moreno Méndez (UNAM), Dalya Akl (NYUAD), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of EP260812b (Malesani et al., GCN Circ. 45367; Li et al., GCN Circ. 45368; Ma et al., GCN Circ. 45370) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-08-13 04:08 to 04:59 UTC (from 04.55 to 05.40 hours after the trigger) and obtained 38 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
We clearly detect the optical counterpart reported by Malesani et al., GCN Circ. 45367 at preliminary magnitudes of:
r = 21.55 +/- 0.05,
z = 21.40 +/- 0.44,
consistent with the measurements reported by NOT (Malesani et al., GCN Circ. 45367), SVOM/VT (Ma et al., GCN Circ. 45370).
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 45370
Subject
EP260812b: SVOM/VT optical observation
Date
2026-08-13T04:09:25Z (20 days ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, H. L. Li, Y. L. Qiu, C. Wu, L. P. Xin, 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 EP260812b detected by EP/WXT (Li et al., GCN 45368). SVOM/VT began observing the field at 2026-08-13T00:45:07 UTC, 1.157 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data available, the optical counterpart (Malesani et al., GCN 45367) was detected in both VT_B and VT_R bands within EP/FXT error range (Li et al., GCN 45368). The AB magnitudes are:
mid time (h) | exposure time (s) | band | mag (AB)
-------------|-------------------|------|--------------
1.26 | 14*50 | VT_B | 20.98+-0.07
1.26 | 14*50 | VT_R | 20.62+-0.07
Preliminary photometric analysis shows a fading of approximately 0.6 mag from 1.26 h to 2.97 h, with a fading slope of ~0.6.
Our photometry was not corrected for Galactic extinction.
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.
GCN Circular 45368
Subject
EP260812b: Einstein Probe detection of an X-ray transient
Date
2026-08-13T02:29:37Z (20 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
A. Li (BNU), G. L. Huang (IHEP), C. Y. Wang (THU) and W. D. Zhang (NAOC) 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 EP260812b. The transient triggered EP-WXT (ID: 01709278129) at 2026-08-12T23:35:41 (UTC). The WXT position of the source is R.A. = 307.783 deg, DEC = -9.857 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 307.7920 deg, DEC = -9.8600 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). NOT also detected an possible optical counterpart 6.8 arcsec from FXT location (D. B. Malesani et al. 2026, GCN 45367).
Further information will be updated when the telemetry data is received.
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).
GCN Circular 45367
Subject
EP trigger #01709278129 (EP260812b?): NOT optical counterpart
Date
2026-08-13T00:55:59Z (20 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), J. Chacon (PUC), P. G. Jonker (Radboud), V. Vuolteenaho (NOT / University of Oulu), K. Kisielowski (ING / University of Groningen), R. Vieliute (ING / University of St Andrews), report on behalf of a larger collaboration:
We observed the field of the EP/WXT trigger #01709278129 (likely EP260812b) with the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Several observations were taken in the r filter, starting on 2026 Aug 12.98964 UT, that is 9.4 min after the EP/WXT trigger. Observations were carried out in “rapid-response mode”, with both triggering and observations carried out robotically and with minimal intervention of the telescope operator.
In our images, a new object, not visible in the Legacy survey archival images, is detected at coordinates:
RA(J2000) = 20:31:10.26
Dec(J2000) = -09:51:42.3
Calibrated against nearby stars from the Pan-STARRS catalog, we measure for this target preliminary magnitudes (not corrected for Galactic extinction):
r = 20.56 +- 0.03 (t_mean = 10.9 min after trigger);
r = 20.26 +- 0.03 (t_mean = 24.7 min after trigger).
This source is thus observed to rebrighten over the course of our observations. This variability, coupled with the lack of any counterpart in deeper archival imaging, suggest that this is in fact the afterglow of the EP trigger #01709278129. Further observations are encouraged.