EP260626a
GCN Circular 45051
Subject
EP260626a: further Liverpool Telescope optical upper limits
Date
2026-06-27T17:25:38Z (12 days ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of EP260626a (Chen et al., GCN 45047; Chen et al., GCN 45050) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 8x150s exposures in SDSS r and z filters, starting at 2026-06-27 01:59:23 UT, approximately 20.57 hours after EP trigger.
The stacked exposures were subtracted from PanSTARRS reference imaging using custom PSF-matching image subtraction code utilising Swarp, SourceExtractor, and PSFex. We do not detect any new sources in the error EP/WXT region (Chen et al., GCN 45047), including at the position of the uncatalogued X-ray source reported by Chen et al. (GCN 45047), down to 3 sigma upper limits of r> 23.44 and z> 22.94.
This is consistent with upper limits from Eyles-Ferris et al. (GCN 45048) and Magnani et al. (GCN 45049).
The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 45050
Subject
EP260626a: EP-FXT follow-up observation
Date
2026-06-27T15:28:42Z (12 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
J. P. Chen (SYSU), K. J. Zhang, D. Zhu (YNU), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of EP260626a (J. P. Chen et al., GCN 45047) approximately 10.8 hours after the WXT detection, with an exposure time of 6.1 ks. An uncatalogued X-ray source was detected within the WXT error circle at R.A. = 277.4479, Dec. = 21.2498 (J2000), with a positional uncertainty of 10.0 arcsec (radius, 90% C.L. statistical and systematic). The FXT spectrum can be fitted with an absorbed powerlaw with a hydrogen column density fixed at the Galactic value of 2.04 x 10^21 cm^-2 and a photon index of 2.13 (+0.6, -0.6). The derived unabsorbed 0.5-10 keV flux is approximately 9.3 (+4.5, -2.1) x 10^-14 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
No optical counterpart has been detected (R. A. J. Eyles-Ferris et al., GCN 45048; Francesco Magnani et al., GCN 45049).
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 45049
Subject
EP260626a: COLIBRÍ optical upper limit
Date
2026-06-27T10:31:22Z (12 days ago)
Edited On
2026-06-29T14:12:14Z (10 days ago)
From
F. Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Francesco at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>
Via
Web form
Francesco Magnani (CPPM), Fredd Sánchez Álvarez (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report:
We imaged the field of the EP 260626a (J.P. Chen et al., GCN Circ. 45047) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-27T07:56:30 to 09:19:26 UTC (from 26.5 to 27.9 hours after the trigger) and obtained 64 minutes of simultaneous exposure in the r, z filters.
The data were reduced, coadded, and analysed with the COLIBRÍ 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.
In the stacked image, we do not detect any new source at the WXT source position (J.P. Chen et al., GCN Circ. 45047) down to the following 3-sigma limit:
r > 23.87
z > 23.15
This upper limit is consistent with the one reported by R. A. J. Eyles-Ferris et al., GCN Circ. 45048.
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 45048
Subject
EP260626a: Liverpool Telescope upper limits
Date
2026-06-27T08:25:50Z (12 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), J. Sanchez-Sierras (Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud), report on behalf of a larger collaboration:
We observed the field of the Einstein Probe fast X-ray transient EP260626a (Chen et al., GCN 45047) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 6x150 s exposures in each of the SDSS r’ and z’ filters starting at 2026-06-27 02:24:23 UT, approximately 21 hours after the X-ray detection.
We performed image subtraction on the stacked images using reference images from Pan-STARRS and also compared the stacked and reference images manually. Within the EP/WXT localisation, we detect no new sources in any of the bands.
We derive the following 3-sigma upper limits for our stacked images. Our photometry is calibrated to PanSTARRS and not corrected for Galactic extinction.
| t_mid (hours) | Filter | AB magnitude |
|---|---|---|
| 21.13 | r’ | >22.89 |
| 21.44 | z’ | >22.37 |
GCN Circular 45047
Subject
EP260626a: EP-WXT detection of a fast X-ray transient
Date
2026-06-26T15:13:06Z (13 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
J.P. Chen (SYSU), K. J. Zhang, D. Zhu (YNU), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient, designated EP260626a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was identified in the telemetry data. The transient was detected in the WXT observation starting at 2026-06-26T05:25:04 (UTC). A significant flare was detected in the source light curve, which started before the beginning of the observation and ended approximately 100 seconds after the start of the observation. The position of the transient is R.A. = 277.4417 deg, Dec. = 21.2403 deg (J2000), with an uncertainty radius of 2.7 arcmin in radius (90% confidence level, including both statistical and systematic uncertainties). There are no known X-ray sources in the WXT error circle.
The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 2.04×10^21 cm^-2 and a photon index of 1.93 (-0.4/+0.42). The derived average unabsorbed 0.5-4 keV flux is 8.02 (-1.7/+2.1) x 10^(-11) erg cm^-2 s^-1. The uncertainties are at the 90% confidence level for the above parameters.
Follow-up observations by EP-FXT will be arranged.
Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 44732
Subject
EP260526a: SVOM/VT optical upper limits
Date
2026-05-29T02:52:08Z (a month ago)
Edited On
2026-06-02T13:06:29Z (a month ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
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J. R. Xu, H. L. Li, C. Wu, Y. L. Qiu, L. P. Xin, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) and J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations to the field of EP260526a triggered by Einstein Probe (Yang et al., GCN 44712). The observation started at 2026-05-27T05:44:54 UTC, 20.022 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
No uncatalogued sources were detected in stacked images within the error box of EP-FXT (Yang et al, GCN 44721) and EP-WXT (Yang et al, GCN 44712), compared to the Legacy Survey. The 3 sigma limit magnitudes are derived as follows:
Mid time | Band | Exposure Time | 3 sigma upper limit
21.53 h VT_B 57*50 s > 23.5 mag
21.53 h VT_R 57*50 s > 23.3 mag
Our non-detection is consistent with the observations from COLIBRÍ (Becerra et al., GCN 44710) and LCO (van Hoof et al., GCN 44713).
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.