EP260917a
GCN Circular 45646
Subject
EP260917a: refined analysis of the EP-FXT observations
Date
2026-09-19T15:44:15Z (10 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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T. Wu (AHNU), Q. J. Huang (PMO, CAS), K. R. Ni (CCNU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We performed a follow-up observation of EP260917a (Huang et al., GCN 45621). The observation started at 2026-09-18T10:38:31 (UTC, T0+25 hours). The effective exposure time of the observation is around 6 ks. On-ground analysis of the FXT data found a weak uncatalogued source at R.A. = 335.1539, DEC = 9.4534 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position (Huang et al., GCN 45621). The average unabsorbed flux is around 5e-14 erg/s/cm^2 in the 0.5-10 keV energy band.
No counterparts were detected by COLIBRÍ (Guelfand et al., GCN 45624), WINTER (Mohan et al., GCN 45628), or SVOM/VT (Li et al., GCN 45642).
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 45642
Subject
EP260917a: SVOM/VT optical upper limits
Date
2026-09-19T09:03:02Z (17 hours ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, Z. H. Yao, J. R. Xu, L. J. Chen, 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 observation on the field of EP260917a (Huang et al., GCN 45621). The observation started at 2026-09-18T03:23:30 UTC, 17.8 hours after trigger in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
No credible uncatalogued candidates were identified in our stacked images within the error box of EP-WXT (Huang et al., GCN 45621) compared to the Legacy Survey.
The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | 5-sigma upper limit
18.0 hr VT_B 23*50 sec > 22.8 mag
18.0 hr VT_R 23*50 sec > 22.7 mag
Our results are consistent with the non-detections from COLIBRÍ (Guelfand et al., GCN 45624) and WINTER (Mohan et al., GCN 45628).
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 45628
Subject
EP260917A: J-band upper limit with WINTER
Date
2026-09-18T11:37:34Z (2 days ago)
From
tanishk.mohan@iitb.ac.in
Via
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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 EP260917A (Huang et al., GCN 45621) 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 on 2026-09-18 at 03:46:54.397 UTC in the J band (18.25 hr after the T0 reported by Huang et al., GCN 45621), 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 do not detect any new source in the WXT localization in our stacked image (Guelfand et al., GCN 45624). We obtain the following 5-sigma upper limit: J = 19.6 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 45624
Subject
EP260917a: COLIBRÍ optical upper limits
Date
2026-09-18T07:22:39Z (2 days ago)
From
Marion G <marion.guelfand@gmail.com>
Via
Web form
Marion Guelfand (CPPM), Enrique Moreno Méndez (UNAM), Rosa L. Becerra (UNAM), 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), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) report:
We imaged the field of the EP260917a (Huang et al. GCN Circ. 45621) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed in two epochs from 2026-08-09 03:25 to 03:48 (from 17.8 to 18.4 hours after the trigger) and restarted our campaign from 2026-08-09 05:45 to 06:24 (from 20.1 to 20.8 hours after the trigger, starting 14 min after the GCN circular). We obtained a total of 1.32 hr of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the 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.
In the stacked images, after performing image subtraction with STDWeb/STDPipe (Karpov 2025), using templates from the DESI Legacy Survey DR10 (Dey et al. 2019), we do not detect any uncatalogued source at the WXT source position (Huang et al. GCN Circ. 45621) down to the following 5-sigma limits:
r > 24.0,
z > 22.5
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 45621
Subject
EP260917a: Einstein Probe detection of an X-ray transient
Date
2026-09-18T05:31:07Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q.J. Huang (PMO, CAS), K. R. Ni (CCNU), T. Wu (AHNU), H. W. Pan (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 EP260917a. The transient triggered EP-WXT (ID: 01709318047) at 2026-09-17T09:37:45 (UTC). The WXT position of the source is R.A. = 335.151 deg, DEC = 9.481 deg (J2000) with an uncertainty of 1.986 arcmin in radius (90% C.L. statistical and systematic).
The refined analysis of the WXT data shows that the event started at T0=2026-09-17T09:32:07 (UTC), and lasted for approximately 1500 seconds, after which the observation was interrupted due to Earth occultation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 5.08 (-1.73/+2.05) × 10^21 cm^-2, and a photon index of 2.31 (-0.65/+0.77). The derived average unabsorbed 0.5-4 keV flux is 5.55 (-0.98/+2.04) × 10^(-10) erg/s/cm^2, with a peak flux of about 2.06 × 10^(-9) erg/s/cm^2.
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).