EP260809a
GCN Circular 45349
Subject
EP260809a: VLT/X-shooter spectroscopic redshift z = 3.07
Date
2026-08-10T17:01:42Z (16 days ago)
From
Andrea Saccardi at CEA/Irfu <andrea.saccardi@cea.fr>
Via
Web form
A. Saccardi (CEA/Irfu), B. Schneider (LAM), G. Corcoran (UCD), F.-F. Song (YNAO, CAS), A. L. Thakur (INAF/IAPS), N. R. Tanvir (U. Leicester), K. E. Heintz (DTU), S. D. Vergani (LUX-Paris Obs.) report on behalf of the Stargate collaboration:
We observed the optical counterpart (Adami et al., GCN 45324; Jiang et al., GCN 45325; Basa et al., GCN 45326) of EP260809a (Sun et al., GCN 45321; Sun et al., GCN 45332) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. From the acquisition images (3x60s) we find a magnitude of the optical transient of r = 22.44+/-0.04 at 6.95 hr post-burst.
Our spectra cover the wavelength range 3000-20000 AA, and consist of 4 exposures of 1200 s each. The observation start time was 07:46 UT on 2026 Aug 9 (7.12 hr after the EP/WXT trigger).
We detect afterglow continuum in each arm, with a strong absorption trough around 4950 AA. Interpreting this as Lyman-alpha absorption at z = 3.07, we also detect weak metal lines of Si II, O I, C II, Al II, Fe II, Mg II, along with a series of H_2 Lyman-Werner band lines at the same redshift. From a preliminary analysis we determine a column density of neutral hydrogen to be log10(N(HI))=22.4+/-0.3.
We also detect an intervening system at z = 1.43.
We acknowledge the excellent support from the observing staff in Paranal, in particular Luca Sbordone, Israel Blanchard and Jonathan Smoker. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
GCN Circular 45335
Subject
EP260809a: J-band upper limit with WINTER
Date
2026-08-09T22:10:21Z (17 days ago)
From
tanishk.mohan@iitb.ac.in
Via
Web form
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP260809a (Sun et al., GCN 45321 and GCN 45332) 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 2026-08-09T05:29:44.454 UTC in the J band (4.85 hr after the EP-WXT trigger), consisting of 15*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 do not detect the optical counterpart (Adami et al., GCN 45324; Jiang et al., GCN 45325; Basa et al., GCN 45326) in our stacked image. We obtain the following 5-sigma upper limit: J = 19.37 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 45332
Subject
EP260809a:refined analysis of the EP-WXT and autonomous EP-FXT observation
Date
2026-08-09T17:58:56Z (17 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
X. X. Sun(NAO, CAS), Q. J. Huang (PMO, CAS), K. J. Zhang(YNU), J. P. Chen (SYSU), Z. X. Ling(NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260809a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Sun et al., GCN 45321) and followed by several optical telescopes (Adami et al., GCN 45324; Jiang et al., GCN 45325; Basa et al., GCN 45326 ) . The refined analysis of the WXT data shows that the event started at T0=2026-08-09T00:38:21(UTC) and lasted for 34 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*tbabs*cflux*powerlaw), with the Galactic hydrogen column density fixed at 8.24 × 10^20 cm^-2. The best-fit photon index is 1.74 (-0.63/+0.88), and the additional hydrogen column density is 1.25 (-1.25/+2.23) × 10^21 cm^-2. The derived average unabsorbed 0.5–4 keV flux is 3.32 (-0.77/+0.98) × 10^-9 erg s^-1 cm^-2.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-08-09T01:25:57(UTC), about 47.6 minutes after T0. The exposure time of this observation is 3020 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 328.5791, 17.6054 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). 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 8.24 × 10^20 cm^-2 and a photon index of 2.00 (-0.28/+0.29), and the intrinsic hydrogen column density is 0.08 (-/+0.08) × 10^22 cm^-2. The derived average unabsorbed 0.5-10 keV flux is 1.27(-0.12/+0.14)× 10^(-12) 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 45326
Subject
EP260809a: COLIBRÍ optical observations
Date
2026-08-09T07:27:51Z (17 days ago)
From
Stephane Basa at UAR Pytheas/OHP, LAM <stephane.basa@lam.fr>
Via
Web form
EP260809a: COLIBRÍ optical observations
Stéphane Basa (UAR Pytheas), Edilberto Aguilar-Ruiz (UNAM), William H. Lee (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), Jean-Grégoire Ducoin (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), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM) and Alan M. Watson (UNAM) report:
We imaged the field of the EP260809a (Sun et al., GCN 45321) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-08-09 05:19:54 to 05:49:15 (from 4.683 to 5.172 hours after the trigger) and obtained 20 minutes of exposure in the z filter.
The data were reduced and coadded with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction.
We clearly detected the optical counterpart (Adami et al., GCN 45324 and Jiang et al., GCN 45325) at a preliminary magnitude of:
z = 22.04 +/- 0.19
Further observations 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 45325
Subject
EP260809a: LCO optical observation
Date
2026-08-09T05:04:21Z (17 days ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang, L.B. He, J. An, X. Liu, Z.P. Zhu, Z. Fan, W.X. Li, D. Xu(NAOC), S.Y. Fu (HUST), Y.L. Hua (PMO) report:
We observed the field of EP260809a detected by EP (Sun et al., GCN 45321), using an LCO 1m telescope located at the Teide Observatory (Spain) equipped with the SINISTRO instrument. Observation started at 2026-08-09T03:55:09.359 UT, i.e., 3.27 hr after the trigger, and 3x200 s Sloan r-band frames were obtained.
The optical counterpart (Adami et al., GCN 45324) is marginally detected in our stacked image with a brightness of r ~ 22 at a median time of 3.36 hr since trigger, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.
GCN Circular 45324
Subject
EP260809a: OHP/T193 optical observations
Date
2026-08-09T03:43:07Z (17 days ago)
Edited On
2026-08-09T22:19:07Z (17 days ago)
From
Christophe Adami at LAM <christophe.adami@lam.fr>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Christophe Adami at LAM <christophe.adami@lam.fr>
Via
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C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), B. Schneider (LAM), A. Saccardi (CEA/Irfu), S. Basa (LAM/OHP/Pytheas/AMU) report on behalf of a larger collaboration:
We observed the field of EP260809a (Sun et al., GCN45321) using the T193cm telescope at Observatoire de Haute-Provence (France) with the MISTRAL spectro-imager. We obtained 4x10 minutes of exposure in the r-band under a 2.3arcsec seeing, starting at 20260809_020201UT (1.4 hr after the Sun et al. trigger) and ending at 20260809_025709UT (2.3 hr after the Sun et al. trigger).
In the preliminary stacked images, we detect an object not present in PanStarr images at
RA (J2000): 21:54:19.2
DEC (J2000): +17:36:23
with a preliminar magnitude of:
r = 21.53 +/- 0.10 mag
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.
GCN Circular 45321
Subject
EP260809a: Einstein Probe detection of an X-ray transient
Date
2026-08-09T02:46:04Z (17 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
X. X. Sun(NAO, CAS), Q. J. Huang (PMO, CAS), K. J. Zhang(YNU), J. P. Chen (SYSU), Z. X. Ling(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 EP260809a. The transient triggered EP-WXT (ID: 01709277768) at 2026-08-09T00:38:56 (UTC). The WXT position of the source is R.A. = 328.573 deg, DEC = 17.607 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. = 328.5816 deg, DEC = 17.6072 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).