EP260920a
GCN Circular 45679
Subject
EP260920a: VLT/HAWK-I near-infrared candidate
Date
2026-09-21T10:34:38Z (19 hours ago)
From
Valerio D'Elia at ASDC <valerio.delia@ssdc.asi.it>
Via
Web form
J. An (NAOC), D. Xu (NAOC), V. D’Elia (ASI/SSDC), M. Garnichey (LUX-Paris Obs.) G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), N. R. Tanvir (U. Leicester), A. L. Thakur (INAF/IAPS), S. D. Vergani (LUX-Paris Obs.), M. Garnichey (LUX-Paris Obs.), report on behalf of the Stargate collaboration:
We observed the field of the EP260920a (Ni et al., GCN 45649) using the ESO/VLT UT4 equipped with the HAWK-I instrument. Observation started at 2026-09-21 02:43:54 UT, i.e., 1.11 days after the trigger, and 20 x 60 s of J-band frames were obtained.
We detect an uncatalogued source in the stacked image at the following coordinates, located within the EP/FXT error circle:
R.A. (J2000) = 01:46:06.91 (26.5288 deg)
Dec. (J2000) = -13:58:49.83 (-13.9805 deg)
with an uncertainty of ~ 0.5 arcsec.
The source had J ~ 20.72+/-0.08 mag (Vega) at a median time of ~ 26.87 hr post-trigger, calibrated with 2MASS and not corrected for Galactic extinction. The target position has no known detections in either the Legacy or Vista Hemisphere Survey historical survey images. No NEO would be at the above position at the observational time by checking MPC. We thus think the source could be the candidate of EP260920a, and this hints at its high-redshift nature.
We acknowledge excellent support from the ESO staff in Paranal.
GCN Circular 45660
Subject
EP260920a: NOT early time optical upper limits
Date
2026-09-20T16:14:12Z (2 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radboud), J. N. D. van Dalen (Radboud), T. Pursimo (NOT, Aarhus University), A. Krzyzaniak (NOT) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260920a (Ni et al., GCN 45649; Ni et al., GCN 45659) using the ALFOSC camera on the Nordic Optical Telescope (NOT) in rapid response mode (RRM). Observations consisted of a sequence of 5 x 180 s exposures in SDSS r-band followed by 9 x 150 s exposures in SDSS z-band. Observations started at 2026-09-20 00:19:12 UT (~18.3 minutes after the refined EP/WXT trigger time (Ni et al., GCN 45659)).
Within the refined EP-FXT position we identify no new sources down to the following limits:
r > 23.5 (Mid time 2026-09-20 00:27:36 UT, ~26.7 minutes after the refined start time)
z >22.6 (Mid time 2026-09-20 00:50:53 UT, ~50.0 minutes after the refined start time)
Our photometry was calibrated using nearby stars in the PanSTARRS catalog and is not corrected for Galactic extinction. Our non-detection is consistent with the later observations of Dornic et al. (GCN 45654), Li et al. (GCN 45657) and Zhu et al. (GCN 45659). We detect the candidate proposed by Liu et al. (GCN 45650) in both bands at magnitudes consistent with those catalogued in the Legacy Survey DR11 implying it is not the counterpart in agreement with Liu et al. (GCN 45653).
GCN Circular 45659
Subject
EP260920a: NOT optical upper limits
Date
2026-09-20T12:04:25Z (2 days ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Via
Web form
Z.-P. Zhu, X. Liu, D. Xu, J. An, L.-B. He, S.-Q. Jiang (NAOC), S.-Y. Fu, L. Lei (HUST), J.P.U. Fynbo (DAWN/NBI), T. Pursimo, A. Krzyzaniak (NOT) report on behalf of a large group:
We observed the field of EP260920a detected by the Einstein Probe (EP) (Ni et al., GCN 45649 ), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 2x300 s frames in each of SDSS g-, r-, and i- filters as well as 4x180s in the z filter, starting on 2026-09-20 at 02:41:57 UTC, i.e., 2.6 hrs after the EP trigger.
No new source is detected in our stacked images within or at the border of the refined EP-FXT error circle (Ni et al., GCN 45655), down to the following 3-sigma limits:
g > 23.8
r > 23.2
i > 22.9
z > 22.1
calibrated using nearby stars from the Pan-STARRS catalog and are not corrected for Galactic extinction.
Our results are consistent with previous reports by LCO (Liu et al., GCN 45653), COLIBRI (Dornic et al., GCN 45654), and SVOM/VT (Li et al., GCN 45657).
Given the deep optical upper limits shortly after the EP trigger, EP260920a would be a good candidate of high redshift, and thus near-infrared follow ups are strongly encouraged.
GCN Circular 45657
Subject
EP260920a: SVOM/VT optical upper limits
Date
2026-09-20T10:35:29Z (2 days 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 EP260920a (Ni et al., GCN 45649, GCN 45655). The observation started at 2026-09-20T00:48:38 UTC, about 43 minutes 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-FXT (Ni et al., GCN 45655) 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 | 3-sigma upper limit
1.65 hr VT_B 72*50 sec > 24.0 mag
1.81 hr VT_R 93*50 sec > 24.0 mag
Our results are consistent with the non-detections from LCO (Liu et al., GCN 45653) and COLIBRÍ (Dornic et al., GCN 45654) .
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 45655
Subject
EP260920a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Date
2026-09-20T08:34:10Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
K. R. Ni (CCNU), S. Q. Zheng, Jin-Rui Liu (BNU), Tian-Yu Liu, G. J. Yang, B. B. Zhang and Z.-X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260920a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-09-20T00:06:04 (UTC) (Ni et al., GCN 45649). Subsequent follow-up observations were reported by Liu et al. (GCN 45650, GCN 45653) and Dornic et al. (GCN 45654). The refined analysis of the WXT data shows that the event started at T0 = 2026-09-20T00:00:54 (UTC), approximately 300 s before the WXT trigger. The WXT light curve shows no significant temporal variability during the observation.
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at NH = 1.0 x 10^21 cm^-2, as inferred from the FXT spectral analysis. The best-fit photon index is 2.61 (-0.64/+0.71), and the derived average unabsorbed 0.5-4 keV flux is 1.94 (-0.50/+0.68) x 10^-10 erg s^-1 cm^-2.
The Follow-up X-ray Telescope (FXT) on board EP observed the source autonomously following the WXT trigger. With the telemetry data received, the refined on-ground analysis gives an X-ray source position of R.A. = 26.5281 deg, Dec. = -13.9806 deg (J2000), with an uncertainty of about 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows an overall declining trend during the observation.
We jointly fitted the FXT-A and FXT-B spectra in the 0.5-10 keV band with an absorbed power-law model. The best-fit hydrogen column density is NH = 1.00 (-0.18/+0.19) x 10^21 cm^-2, and the photon index is 2.00 (-0.06/+0.06). The derived average unabsorbed 0.5-10 keV flux is 5.40 (-0.14/+0.15) x 10^-11 erg s^-1 cm^-2.
An optical transient candidate was reported by Liu et al. (GCN 45650) at R.A. = 01:46:06.71, Dec. = -13:59:06.1 (J2000). This position is about 15 arcsec from the refined FXT position, outside the refined 10-arcsec 90% confidence error circle.
The uncertainties are at the 90% confidence level for the above spectral parameters.
Further multi-wavelength follow-up observations are encouraged to explore the nature of EP260920a.
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 45654
Subject
EP260920a: COLIBRÍ optical upper limits
Date
2026-09-20T08:00:28Z (2 days ago)
From
Rosa L. Becerra at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
Web form
Damien Dornic (CPPM), Ramandeep Gill (UNAM), Rosa L. Becerra (UNAM), 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), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (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), Alan M. Watson (UNAM) report:
We imaged the field of the EP260920a (Ni et al., GCN 45649) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-09-20 05:33 to 06:57 (from 5.4 to 6.9 hours after the trigger) and obtained 61 and 56 minutes of exposure in the r and z filters, respectively.
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 the tentative source reported by Li et al. (GCN Circ. 45650; 45653) down to the following 3-sigma limits:
r > 24.0,
z > 22.9
We agree with Li et al. (GCN Circ. 45653) that the analysis at the position of the candidate is complicated by the presence of the underlying catalogued point source. In particular, image subtraction at this location is sensitive to differences between the science images and the reference templates, including filter and PSF mismatches, which can produce subtraction residuals. After inspecting both the unsubtracted and difference images, we find no convincing evidence for an additional transient component at this position.
We also do not detect any uncatalogued source within the FXT region (Ni et al., GCN Circ. 45649) down to the following 5-sigma limits:
r>23.5,
z>22.3
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 45653
Subject
EP260920a: Update of the LCO optical counterpart candidate
Date
2026-09-20T05:35:25Z (2 days ago)
From
liuxing@nao.cas.cn
Via
Web form
X. Liu, Z.P. Zhu, D. Xu (NAOC) report on behalf of a large group:
We reported a weak optical counterpart candidate in GCN 45650. This candidate is a residual transient at the position of a known point-like source in Legacy Survey (LS), by image subtraction of the LCO image in w-band against the LS image in the r-band. Therefore, we caution that this candidate may be due to the filter difference when doing image subtraction.
GCN Circular 45650
Subject
EP260920a: LCO optical counterpart candidate
Date
2026-09-20T02:33:40Z (2 days ago)
From
liuxing@nao.cas.cn
Via
Web form
X. Liu, D. Xu, J. An, Z. Fan, J.F. Gou, L.B. He, Y.L. Hua, S.Q. Jiang, W.X. Li, R.D. Liang, Q. Zhao, Z.P. Zhu (NAOC), Y.L. Hua (PMO), S.Y. Fu, L. Lei (HUST) report:
We observed the field of EP260920a triggered by Einstein Probe (EP; Ni et al., GCN 45649), using an LCO 1m telescope network. We obtained 3 x 200 s exposures in the w filter.
A residual optical transient (OT) is detected at coordinates
R.A. (J2000) = 01:46:06.71
Dec. (J2000) = -13:59:06.1
after subtracting the archival Legacy Survey image, which is within the EP-FXT error circle (EP; Ni et al., GCN 45649). There is a known source at the above coordinates, which has g = 23.39, r = 22.02, i = 20.68, and z = 20.14, and is classified as Point-like in Legacy Survey. The OT and the known source together has r ~ 21.67 mag in the LCO stacked image at a median time of ~1.174 hr post-trigger, and the OT was decaying in the LCO three images.
We thus think the above OT/known source could be optical counterpart of EP260920a.
GCN Circular 45649
Subject
EP260920a: Einstein Probe detection of an X-ray transient
Date
2026-09-20T01:22:23Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
K. R. Ni (CCNU)，S. Q. Zheng，Jin-Rui Liu(BNU)，Tian-Yu Liu, G. J. Yang, B. B. Zhang and 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 EP260920a. The transient triggered EP-WXT (ID: 01709318515) at 2026-09-20T00:06:04 (UTC). The WXT position of the source is R.A. = 26.557 deg, DEC = -13.967 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. = 26.5298 deg, DEC = -13.9825 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).