EP260618a
GCN Circular 45016
Subject
EP260618a: Liverpool Telescope optical observations
Date
2026-06-20T14:22:19Z (4 months ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
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A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of EP260618a (Yang et al., GCN 44978; Jiang et al., GCN 44991) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x150s exposures in SDSS griz filters, starting at 2026-06-19 02:32:19 UT, approximately 17.96 hours after EP trigger.
Some z-band exposures had to be discarded prior to stacking. We detect a source in the images in all filters at the position reported by Becerra et al. (GCN 44988), also Malesani et al. (GCN 44990), Amram et al. (GCN 44994) and He et al. (GCN 45012). The photometry is:
| MJD (mid) | T_mid-T_0 | Filter | Mag. (AB) |
|---|---|---|---|
| 61210.11078 | 18.08 h | g | 22.47 ± 0.24 |
| 61210.12120 | 18.34 h | r | 22.03 ± 0.12 |
| 61210.13221 | 18.59 h | i | 21.62 ± 0.11 |
| 61210.14403 | 18.88 h | z | 21.48 ± 0.26 |
The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 45012
Subject
EP260618a: JinShan optical observations
Date
2026-06-20T06:12:59Z (4 months ago)
Edited On
2026-06-20T14:24:25Z (4 months ago)
From
L. B. He at NAOC <helb@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of L. B. He at NAOC <helb@bao.ac.cn>
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L.B. He, S.Q. Jiang, J. An, X. Liu, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei (HUST), J.Z. Liu (XAO) report:
We observed the field of EP260618a detected by EP/WXT (Yang et al., GCN 44978; Jiang et al., GCN 44991), possibly associated with GRB 260618A (Fermi GBM Team, GCN 44973) using the JinShan 100C telescope located at Altay, Xinjiang, China.
The previously reported optical counterpart (Becerra et al., GCN 44988; Malesani et al., GCN 44990; Amram et al., GCN 44994) is detected in our stacked images with a brightness of r ~ 22.4 mag at a median time of 1.429 days since the EP trigger, i.e. 1.504 days since the Fermi GBM trigger, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.
GCN Circular 44994
Subject
EP260618a: OHP/T193 optical observations
Date
2026-06-19T11:05:46Z (4 months ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Via
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P. Amram (LAM/Pytheas/AMU), C. Adami (LAM/Pytheas/AMU), D. Russeil (LAM/Pytheas/AMU) report on behalf of a larger collaboration:
We observed the field of EP260618a (Yang et al., GCN44978; see also Fermi GBM Team
, GCN44973 and Jiang et al., GCN44991) using the T193cm telescope at Observatoire de Haute-Provence (France) with the MISTRAL spectro-imager. We obtained 2x10 minutes of exposure in the r-band starting at 2026-06-18T21:12:40 UT (12.63 hr after the Yang et al. trigger).
In the stacked images, the optical counterpart detected by Becerra et al. (GCN44988) and Malesani et al. (GCN44990) is visible with preliminary magnitude of:
r = 21.86 +/- 0.09 mag
in good agreement with their measurements.
The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the STDWeb/STDPipe tools (Karpov 2025), is in the AB system, and is not corrected for Galactic extinction.
We acknowledge the excellent support from Observatoire de Haute-Provence and in particular Stephane Favard. We acknowledge the excellent support of J. Schmitt and J.C. Brunel in the recent MISTRAL upgrade.
GCN Circular 44991
Subject
EP260618a: EP-FXT follow-up observation
Date
2026-06-19T09:13:11Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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S. Q. Jiang (NAO, CAS), J. Yang (ZZU), H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the X-ray transient EP260618a (Yang et al., GCN 44978), which is possibly associated with GRB 260618A (Fermi GBM team, GCN 44973). The observation started at 2026-06-18T18:09:47 (UTC), about 9.58 hours after the initial WXT detection, with an exposure time of 3 ks. An uncatalogued source was detected within the WXT error circle at R.A. = 276.6799, DEC = 23.8439 (J2000) with an uncertainty of 10 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 1.32 x 10^21 cm^-2 and a photon index of 2.09 +/- 0.13. The derived unabsorbed 0.5-10 keV flux is 1.28 (+0.13, -0.10) x 10^-12 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The position of the uncatalogued X-ray source is consistent with the optical counterpart (Becerra et al., GCN 44988; Malesani et al., GCN 44990). Further EP-FXT follow-up observations 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 44990
Subject
EP260618a: LCO optical counterpart confirmation
Date
2026-06-19T08:41:05Z (4 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
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D. B. Malesani (DAWN/NBI and Radboud), J. A. Quirola-Vasquez (Radboud), J. Sanchez-Sierras (Radboud), G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud), J. Chacon (PUC), report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260618a (Yang et al., GCN 44978), possibly the afterglow of GRB 260618A (Fermi GBM team, GCN 44973), using an LCO 1m telescope located at the Teide Observatory (Spain) equipped with the SINISTRO instrument. We obtained 6x300 s exposures in the SDSS-r filter starting on 2026-06-18 at 21:56:12 UT (13.56 hr after the trigger).
The object reported by Becerra et al. (GCN 44988) is detected in the stack of our images, with a magnitude of r = 21.93 +/- 0.16 (AB), calibrated against the Pan-STARRS catalogue and not corrected for Galactic extinction.
We note that our magnitude indicates little or no fading compared to the COLIBRÍ observation, which was carried out about 6.5 hr later (Becerra et al., GCN 44988). Further follow-up is encouraged.
GCN Circular 44988
Subject
EP260618a: COLIBRÍ optical counterpart candidate
Date
2026-06-19T05:09:23Z (4 months ago)
From
Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
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Rosa L. Becerra (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), 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), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report:
We imaged the field of the EP260618a (Yang et al., GCN Circ. 44978), possibly associated with GRB 260618A (Fermi GBM team GCN Circ. 44973), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-19 03:44 to 04:55 UTC (from 19.15 to 20.34 hours after the trigger) and obtained 55 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the ASU 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, which is particularly significant in this direction (A_r = 1.1 mag; Schlafly & Finkbeiner 2011) due to the proximity of the field to the Galactic plane.
We detect an uncatalogued source, absent in Pan-STARRS template images and and located within the WXT error region (Yang et al., GCN Circ. 44978) at:
RA(J2000) = 18:26:43.13 = 276.6797 degrees,
Dec(J2000) = +23:50:37.8 = 23.8438 degrees,
with an uncertainty of 0.5 arcsec.
The preliminary magnitudes derived for this source are:
r = 21.90 +/- 0.04,
z = 21.49 +/- 0.08.
We therefore suggest that this source is the optical counterpart of EP260618a.
Further observations and analysis are ongoing.
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 44978
Subject
EP260618a: EP-WXT detection of an X-ray transient
Date
2026-06-18T15:21:15Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
J. Yang (ZZU), S. Q. Jiang (NAO, CAS), H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient, designated EP260618a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission through the ground data analysis. The transient was detected in one WXT observation starting at 2026-06-18T08:34:46 (UTC). No significant variability was detected during the 2.7 ks exposure. The position of the transient is R.A. = 276.676 deg, Dec. = 23.849 deg (J2000), with an uncertainty radius of 2.5 arcmin (90% confidence level, including both statistical and systematic uncertainties). There is no historical X-ray source in the WXT error circle.
The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a fixed Galactic hydrogen column density of 1.32^21 cm^-2 and a photon index of 1.4 (-0.4/+0.5). The derived average unabsorbed 0.5-4 keV flux is 6.9 (-1.7/+2.1) x 10^(-11) erg cm^-2 s^-1.
The position of EP260618a falls within the error circle of GRB 260618A reported by Fermi/GBM (GCN 44973), suggesting that EP260618a may be the X-ray afterglow of GRB 260618A. Follow-up observation with the Follow-up X-ray Telescope (FXT) onboard EP has been scheduled.
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).