GRB 260623B, EP260623a
GCN Circular 45149
Subject
EP260623a: KAIT optical observations
Date
2026-07-13T17:04:57Z (a month ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and
Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of EP260623a (Wang et al.,
GCN 45020, 45035; Svinkin et al., GCN 45046) at 5.85 hours after
the burst, and again at 1.34 days after the burst. A set of clear
(roughly R) filter images were obtained. We clearly detected the
optical afterglow (Li et al., GCN 45021; Izzo et al., GCN 45022;
Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al.,
GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee
et al., GCN 45030; Saikia et al., GCN 45034; Bochenek & Perley,
GCN 45040; Busmann et al., GCN 45042; Brivio et al., GCN 45045)
in our coadd images. We measure its brightness decayed from
18.8 +/- 0.1 mag (Vega) to 22.2 +/- 0.2 mag with a power-law
decay index of ~2.0.
GCN Circular 45046
Subject
Konus-Wind detection of GRB 260623B / EP260623a
Date
2026-06-26T15:11:51Z (2 months ago)
From
Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>
Via
Web form
D. Svinkin, D. Frederiks, A.Lysenko, A. Ridnaia, A. Tsvetkova,
M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report:
The long-duration GRB 260623B
was detected by Konus-Wind (KW) in the waiting mode
at about T0 = T0(EP) = 10445 s UT (02:54:05).
A Bayesian block analysis of the KW waiting mode data in
the 20-300 keV band reveals a ~10 sigma count-rate increase in
the interval from T0-259 s to T0+327 s.
The burst overlaps in time with EP260623a (EP-WXT detection: Wang et al., GCN 45020;
Wang et al., GCN 45035) and the KW ecliptic latitude response is consistent with the EP260623a localization.
The positional and temporal coincidence of GRB 260623B with
the EP260623a supports the conclusion that both events have a common origin.
The KW light curve of this burst is available
at http://www.ioffe.ru/LEA/GRBs/GRB260623B
The total burst fluence is 8.2(-1.2,+1.2)x10^-6 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0+3.366 s,
is 0.7(-0.3,+0.4)x10^-7 erg/cm^2/s.
(both in the 20 keV - 1500 keV energy range).
The time-integrated spectrum of the burst,
measured from T0-258.650 s to T0+327.206 s,
can be described by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha > -1.16 and Ep = 76(-8,+28) keV.
The spectrum near the maximum count rate
(measured from T0-2.522 to T0+38.694 s)
can be described with the CPL model with alpha > -1.42 and Ep = 87(-15,+83) keV.
Assuming the redshift z=0.703 (Izzo et al., GCN 45022)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 1.13(-0.16,+0.24)x10^52 erg,
the peak luminosity L_iso is 1.71(-0.72,+0.97)x10^50 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 130(-14,+47) keV and the spectrum near the maximum count rate
Ep,p,z is 148(-26,+143) keV.
With the obtained estimates, GRB 260623B is inside the 68% prediction band for
the 'Amati' and inside the 90% prediction band for the 'Yonetoku' relations derived for the sample of >300 long
KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB260623B/GRB260623B_rest_frame.pdf
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.
GCN Circular 45045
Subject
EP260623a: TNG NIR afterglow detection
Date
2026-06-25T15:50:24Z (2 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio (INAF-OAB), M. Ferro (INAF-OAB), L. Izzo (INAF-OACN and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), A. Melandri (INAF-OAR), V. Lorenzi (INAF-TNG), A. G. de Gurtubai Escudero (INAF-TNG) report on behalf of the CIBO collaboration:
We observed the field of EP260623a detected by EP/WXT (Wang et al., GCN 45020) with the Italian 3.6m TNG telescope equipped with the near-infrared camera NICS to follow up its afterglow. A series of images were obtained with the K filter starting on 2026-06-25 at 01:29:13 UT (i.e. 1.94 days after the trigger).
The afterglow (Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee et al., GCN 45030; Saikia et al., GCN 45034; Bochenek & Perley, GCN 45040; Busmann et al., GCN 45042) is detected in the co-added image with a preliminary magnitude of
K = 19.9 +/- 0.2 mag (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 1.96 days after the trigger.
GCN Circular 45042
Subject
EP260623a: FTW optical and NIR observations
Date
2026-06-25T08:19:33Z (2 months ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Julius Gassert (LMU/CMU), Xander Hall (CMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the counterpart of EP260623a (Wang et al., GCNs 45020 and 45035; Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee et al., GCN 45030; Saikia et al., GCN 45034; Bochenek & Perley, GCN 45040) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 20 x 180 s starting at 2026-06-24T23:13:34 UT (1.85 days after the trigger). We detect the counterpart in all bands and measure an r-band magnitude of
r = 23.93 ± 0.22 AB mag.
The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.
We thank Christoph Ries from the Wendelstein Observatory for obtaining these observations.
GCN Circular 45040
Subject
EP260623a: Liverpool Telescope optical observations
Date
2026-06-24T17:59:07Z (2 months 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 EP260623a (Waang et al., GCN 45020; Wang et al., GCN 45035) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x120s exposures in SDSS griz filters, starting at 2026-06-24 01:15:57 UT, approximately 22.4 hours after EP trigger. Multiple exposures across all filters had to be discarded prior to stacking due to an issue with the telescope rotator.
We detect a source in the images in all filters at the position reported by Li et al. (GCN 45021), also Izzo et al. (GCN 45022); Angulo et al. (GCN 45024); Brivio et al. (GCN 45025); Jiang et al. (GCN 45026); Li et al. (GCN 45027); Jelinek et al. (GCN 45029); Lee et al. (GCN 45030); Saikia et al. (GCN 45034). The preliminary photometry is:
| MJD (mid) | T_mid-T_0 | Filter | Mag. (AB) |
|---|---|---|---|
| 61215.05694 | 22.49 h | g | > 21.44 |
| 61215.06840 | 22.76 h | r | 21.96 ± 0.23 |
| 61215.08921 | 23.26 h | i | 21.59 ± 0.13 |
| 61215.09872 | 23.50 h | z | 21.06 ± 0.16 |
The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 45035
Subject
EP260623a:refined analysis of the EP-WXT and EP-FXT observation
Date
2026-06-24T09:06:30Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B.T. Wang (YNAO, CAS), Y. L. Hua (PMO, CAS) and C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260623a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Wang et al., GCN 45020) and followed by several optical telescopes (Li et al., GCN 45021; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee et al., GCN 45030; Saikia et al., GCN 45034) with the redshift confirmed at z=0.703 (Izzo et al., GCN 45022). The refined analysis of the WXT data shows that the event started at T0=2026-06-23T02:52:27Z (UTC) and lasted for 103 s before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5–4 keV spectrum can be fitted with an absorbed power-law model (tbabs*ztbabs*powerlaw) with a fixed Galactic hydrogen column density of 8.1 × 10^20 cm^-2 and a fixed redshift of z = 0.703. The best-fit photon index is 1.83 (-0.56/+0.63), and the intrinsic hydrogen column density is 2.25 (-0.97/+1.21) × 10^22 cm^-2. The derived average unabsorbed 0.5–4 keV flux is 3.17 (-0.74/+1.39) × 10^-9 erg/s/cm^2. The estimated peak flux is 1.0 × 10^-8 erg/s/cm^2, corresponding to a peak luminosity of 2.2 × 10^49 erg/s.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-06-23T02:56:08 (UTC), about 3.7 minutes after T0. The exposure time of this observation is 3211 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 328.2698, 12.7938 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT light curve displays two pulses structure lasting for around 210 s. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 8.1 × 10^20 cm^-2 and a photon index of 1.96 (-/+ 0.03), and the intrinsic hydrogen column density is 0.94 (-/+ 0.04) × 10^22 cm^-2. The derived average unabsorbed 0.5-10 keV flux is 7.33 (-/+ 0.10) × 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
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 45034
Subject
EP260623a: WINTER J-band observations
Date
2026-06-23T22:37:06Z (2 months ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form
Aditya Pawan Saikia (IITB), Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (NOIRLab), Benjamin Schneider (LAM), Robert Stein (UMD), Viraj Karambelkar (Columbia), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP260623a (Wang et al., GCN 45020) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations began at 2026-06-23T07:00:29 UTC in the J band (4.1 hr after the EP trigger), consisting of 30*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with MIRAR (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We detect a source at the optical counterpart location (Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; Jelinek et al., GCN 45029; Lee et al., GCN 45030), with magnitude J = 17.9 ± 0.1 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 45030
Subject
EP260623a: Optical follow-up observations with Kinder
Date
2026-06-23T16:42:04Z (2 months ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
M.-H. Lee, A. Aryan, T.-W. Chen, W.-J. Hou (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, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, 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 EP260623a (Wang et al., GCN 45020) 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 15:26 UTC on the 23rd of June 2026 (MJD 61214.6431), 12.53 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. Next, we employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. In the difference image, the optical counterpart (Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026; Li et al., GCN 45027; and Jelinek et al., GCN 45029) was clearly detected.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and measured photometry (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 61214.6431 | 12.53 | 300 * 6 | 20.74 +/- 0.07 | 1".97 | 1.91
The presented magnitudes are calibrated using the field stars from ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitude is not corrected for an expected galactic extinction of A_r = 0.32 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 45029
Subject
EP260623a: FRAM-ORM detection of the optical afterglow rise, peak and decay
Date
2026-06-23T14:29:58Z (2 months ago)
From
Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>
Via
email
Martin Jelinek, Jan Strobl and Filip Novotny (ASU CAS Ondrejov, CZ),
Sergey Karpov, Martin Masek, Petr Janecek, Jakub Jurysek, Jan Ebr, Ronan Cunniffe, Petr Travnicek and Michael Prouza (Institute of Physics, Prague, CZ)
report:
The 25cm robotic telescope FRAM-ORM at La Palma (Spain) reacted robotically to the alert of EP260623a (Wang et al. GCN 45020), obtaining a series of unfiltered images starting at 02:59:17 UT, i.e. ~5 min post trigger. The observation consists of a set of 20 x 20 s exposures followed by a longer set of 120 x 60 s exposures.
We clearly detect the optical counterpart reported by other telescopes (Li et al. GCN 45021; Izzo et al. GCN 45022; Angulo et al. GCN 45024; Brivio et al. GCN 45025; Jiang et al. GCN 45026). The FRAM observations cover the rise of the transient, its peak and the subsequent decay. The source peaked at ~T0+24 min at a brightness of r ~ 15.4, after which it decays with a power-law index alpha ~ 1.7, with a few rebrightenings. The photometry is calibrated against the Atlas-REFCAT2 catalogue (Tonry et al. 2018, ApJ, 867, 105) and is not corrected for Galactic extinction.
Extrapolating the observed decay to T0+24h yields a predicted brightness of r ~ 22.0.
GCN Circular 45027
Subject
EP260623a: SVOM/VT optical observations
Date
2026-06-23T12:19:38Z (2 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, C. Wu, Y. L. Qiu, J. R. Xu,, 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, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations to the field of EP260623a triggered by Einstein Probe (Wang et al., GCN 45020). The observation started at 2026-06-10T04:10:37 UTC, about 1.28 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical counterpart (Li et al., GCN 45021; Angulo et al., GCN 45024; Brivio et al., GCN 45025; Jiang et al., GCN 45026) with the redshift of z=0.703 (Izzo et al., GCN 45022) was clearly detected by VT in both channels. The following measurements are in the AB magnitude without correction for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
1.283 h VT_B 50 s 17.00 +/- 0.04 mag
1.283 h VT_R 50 s 16.34 +/- 0.04 mag
3.191 h VT_B 50 s 18.59 +/- 0.07 mag
3.191 h VT_R 50 s 17.85 +/- 0.05 mag
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 45026
Subject
EP260623a: TRT optical observations
Date
2026-06-23T10:40:56Z (2 months ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang, X. Liu (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan, K. Noysena (NARIT), L.B. He, Z.P. Zhu, J. An, Z. Fan, W.X. Li, N.C. Sun, D. Xu (NAOC), S.Y. Fu (HUST) report:
We observed the field of EP260623a detected by EP (Wang et al., GCN 45020) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 04:32:21.6 UTC on 2026-06-23, i.e., 1.638 hrs after the EP trigger, and a series of frames were obtained in Sloan r-band.
The optical counterpart (Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024; Brivio et al., GCN 45025) is clearly detected in our r-band images, with r = 17.13 +/- 0.02 at 1.767 hrs post-burst, calibrated with Panstarrs DR2 stars in the field and not corrected for Galactic extinction.
GCN Circular 45025
Subject
EP260623a: REM optical/NIR afterglow detection
Date
2026-06-23T10:05:07Z (2 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB), L. Izzo (INAF-OACN and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud), A. Melandri (INAF-OAR) report on behalf of the REM team:
We observed the field of EP260623a, detected by EP/WXT (Wang et al., GCN 45020) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile).
The observations were carried out in the g, r, i, z, K bands, started on 2026-06-23 at 03:57:45 UT (i.e. 1.1 hr after the burst) and lasted for about 1 hr.
From preliminary photometry, we detect the optical/NIR counterpart (Li et al., GCN 45021; Izzo et al., GCN 45022; Angulo et al., GCN 45024) with the following magnitudes:
r = 16.35 +/- 0.04 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 1.1 hr after the trigger;
K = 13.51 +/- 0.17 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 1.1 hr after the trigger.
GCN Circular 45024
Subject
EP 260623a: COLIBRÍ optical observations
Date
2026-06-23T07:02:10Z (2 months ago)
Edited On
2026-06-23T09:07:56Z (2 months ago)
From
F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Via
Web form
Camila Angulo (UNAM), Francis Fortin (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), 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 EP 260623a (Wang et al., GCN Circ. 45020) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-23T06:23:26 to 06:37:31 UTC (from 3.49 to 3.69 hours after the trigger) and obtained 4 minutes of exposure in the g, r, i, z, filters and 8 minutes in the y filter.
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.
We detected the optical counterpart reported by Las Cumbres (Li et al., GCN Circ. 45021), at preliminary magnitudes of:
g = 18.73 +/- 0.04
r = 18.30 +/- 0.01
i = 18.00 +/- 0.01
z = 17.78 +/- 0.03
y = 17.55 +/- 0.03
Given the g-band detection and without the presence of a break in our grizy SED, we conclude that this event is likely located at z < 3. This would be consistent with the redshift derived with GTC/OSIRIS+ of z = 0.703 (Izzo et al., GCN Circ. 45022).
We also note that the source is located near the southern edge of the Sh 2-113 (the Flying Dragon Nebula), which may contribute to the moderate extinction observed at its position.
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 45022
Subject
EP260623a: OSIRIS+/GTC redshift z = 0.703
Date
2026-06-23T05:47:48Z (2 months ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
L. Izzo (INAF/OACN and DARK/NBI), A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), S. Geier (GTC), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), G. Lombardi (GTC), A. Martin-Carrillo (UCD), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), F. Pérez Toledo (GTC) and A. Pérez (GTC) report:
We observed the optical counterpart (Li et al. GCN 45021) of EP260623a (Wang et al, GCN 45020) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument.
In the 10-s acquisition image (beginning on 2026-06-23 at 04:44:12 UT, that is 1.83 hr after trigger), the optical afterglow is well detected with a magnitude 17.01 ± 0.05 (AB), calibrated against nearby LS objects, and not corrected for Galactic extinction.
A total of 2 spectra of 600 s were secured, starting on 2026-06-23 at 04:49:58 UT (1.93 hr after EP trigger), using grism R1000B. Continuum is visible over the wavelength range 3615-7800 AA. A number of metal absorption features are detected, which we interpret as due to FeII, MnII, MgII, MgI, as well as multiple FeII fine structure transitions all at a common redshift z = 0.703, which we suggest to be the redshift of EP 260623a.
This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774).
GCN Circular 45021
Subject
EP260623a: Las Cumbres discovery of the optical counterpart
Date
2026-06-23T04:16:44Z (2 months ago)
Edited On
2026-06-23T06:49:50Z (2 months ago)
From
Wenxiong Li at NAOC <liwenxiong1992@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Wenxiong Li at NAOC <liwenxiong1992@gmail.com>
Via
Web form
Wenxiong Li, Runduo Liang (NAOC), Iair Arcavi, Ido Keinan (TAU), David Sand (U of Arizona)
We observed the position of EP260623a (Wang et al. GCN 45020) with a Las Cumbres 1m telescope at Teide Observatory, Tenerife, 8 mins after the Einstein Probe WXT trigger. We took 2x300s exposures in the broad optical w-band.
We find an uncataloged source at RA=328.2698, Dec=12.7939 within the EP-FXT error circle and measure the following preliminary photometry calibrated to the r band:
MJD 61214.12687 Mag 16.54 +- 0.01
MJD 61214.13064 Mag 15.62 +- 0.01
This target brightened by ~0.9 mag within 6 minutes.
The presented magnitudes are calibrated with Pan-STARRS sources in the field and not corrected for Galactic extinction. Additional follow-up is encouraged.
GCN Circular 45020
Subject
EP260623a: Einstein Probe detection of an X-ray transient
Date
2026-06-23T03:52:21Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
B. T. Wang (YNAO, CAS), Y. L. Hua (PMO, CAS) 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 EP260623a. The transient triggered EP-WXT (ID: 01709272000) at 2026-06-23T02:54:05 (UTC). The WXT position of the source is R.A. = 328.262 deg, DEC = 12.813 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. = 328.2725 deg, DEC = 12.7946 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
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).