EP260610b
GCN Circular 44926
Subject
EP260610b: Liverpool Telescope optical upper limits
Date
2026-06-12T17:56:45Z (3 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 EP260610b (Wu et al., GCN 44895, Guo et al., GCN 44912) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 8x200s exposures in SDSS gri filters, starting at 2026-06-11 00:29:50.385 UT, approximately 1.46 days after trigger.
We do not detect any sources at the position of the counterpart in the stacked images (Levan et al., GCN 44897; Xin et al., GCN 44900; Zhu et al., GCN 44902; Burkhonov et al., GCN 44904; Moreno Mendez et al., GCN 44906, van Hoof et al., GCN 44915) down to the three-sigma limiting magnitude of g > 23.2, r > 23.5, i > 22.9. We note that forced PSF photometry on r-band exposures gives a two-sigma detection at 23.7 ± 0.5 mag, at a mid-observation time of 1.52 days post-trigger.
The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards, and was not corrected for extinction.
GCN Circular 44915
Subject
EP260610b: GTC likely spectroscopic redshift z = 1.10
Date
2026-06-11T14:52:44Z (4 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
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A. P. C. van Hoof (Radboud), D. Mata Sanchez (IAC and ULL), G. Corcoran (UCD), A. J. Levan (Radboud and Warwick), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), David Garcia Alvarez (GTC), report on behalf of a larger collaboration:
We observed the optical counterpart (Levan et al., GCN 44897; Xin et al., GCN 44900; Zhu et al., GCN 44902; Burkhonov et al., GCN 44904; Moreno Mendez et al., GCN 44906) of the fast X-ray transient EP260610b (Wu et al., GCN 44895; Guo et al., GCN 44912) using the Gran Telescopio Canarias (GTC) located in the island of La Palma (Canary Islands, Spain), equipped with the OSIRIS+ spectrograph. A sequence of 3x1200 s observations was secured using grism R1000B, which covers the wavelength range 3600-7790 AA. The observation mid time was 2026 Jun 11.183 UT (15.58 hr after the EP/WXT trigger).
In the acquisition image (taken 14.96 hr after trigger), we measure an AB magnitude r = 23.38 +- 0.11, calibrated against the Pan-STARRS catalog, and not corrected for Galactic extinction. The delivered seeing was 0.8".
Weak continuum is detected across the entire wavelength range. From the lack of hydrogen absorption, we can set a firm upper limit to the redshift z < 2.1. Weak absorption features can also be identified matching Mg II and Fe II at z = 1.10. Line-stacking analysis confirms a significant absorption system at this redshift, which we suggest to be the likely redshift of EP260610b.
GCN Circular 44912
Subject
EP260610b: refined analysis of the EP-WXT and follow-up EP-FXT observations
Date
2026-06-11T12:50:17Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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C. L. Guo, H. Y. Ren, Z. X. Ling (NAO, CAS), H.Z. Wu (HUST, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260610b was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Wu et al., GCN 44895), and followed by several telescopes (Levan et al. GCN 44897, Xin et al. GCN 44900, Zhu et al. GCN 44902, Burkhonov et al. GCN 44904, Méndez et al. GCN 44906). The WXT observation started at T0=2026-06-10 12:44:25 (UTC), and lasted for around 230 s before being interrupted by autonomous follow-up. The WXT light curve shows an increase in flux during the last 30 s. The average WXT 0.5–4 keV spectrum can be fitted with an absorbed power-law model with a fixed Galactic hydrogen column density of 2.43 × 10^20 cm^−2 and a photon index of 1.48 (+0.81/−0.70). The derived average unabsorbed 0.5–4 keV flux is 2.39 (+1.35/−0.90) × 10^−10 erg s^−1 cm^−2.
The autonomous follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP started at 2026-06-10 12:50:34 (UTC), about 370s after T0. The exposure time was approximately 3726s. The on-ground analysis of the FXT data detected an uncatalogued X-ray source at R.A. = 267.6268 deg, DEC = 46.5388 deg (J2000), with an uncertainty of 10 arcsec in radius (90% C.L., including statistical and systematic uncertainties). The average FXT 0.5–10 keV spectrum can be fitted with an absorbed power-law model with a fixed Galactic hydrogen column density of 2.43 × 10^20 cm^−2 and a photon index of 2.19 (+0.1/−0.1). The derived average unabsorbed 0.5–10 keV flux is 7.36 (+0.4/−0.4) × 10^−12 erg s^−1 cm^−2. The uncertainties quoted above are at the 90% confidence level.
Launched on January 9, 2024, EP is a space X-ray observatory designed 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 44906
Subject
EP260610b: COLIBRÍ optical observations
Date
2026-06-11T06:56:12Z (4 months ago)
From
enriquemm@astro.unam.mx
Via
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Enrique Moreno Méndez (UNAM), Dalya Akl (NYUAD), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), 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), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the EP260610b (Wu et al., GCN Circ. 44895) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-11 05:16 to 06:32 UTC (from 16.48 to 17.73 hours after the trigger) and obtained 54 minutes of simultaneous exposure in the r/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.
We marginally detect the optical counterpart reported by Levan et al., GCN Circ. 44897, in the r band, at a preliminary magnitude of:
r ≈ 23.7,
In z, we do not detect the source down to the following 3-sigma limit:
z > 22.0.
These values are consistent with those reported by Gemini (Levan et al., GCN Circ. 44897), SVOM/VT (Xin et al., GCN Circ. 44900), NOT (Zhu et al., GCN Circ. 44902) and MAO/AZT-22 (Burkhonov et al., GCN Circ. 44904).
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 44904
Subject
EP260610b: MAO/AZT-22 optical observations
Date
2026-06-11T04:37:49Z (4 months ago)
From
Yodgor Rajabov at UBAI <rajabov@astrin.uz>
Via
Web form
O. Burkhonov, Y. Rajabov, B.Abidkhanov, S. Ehgamberdiev, Y. Tillayev, T. Boyqobilov, A. Shaymanov report on behalf of UBAI team.
We observed the field of EP transient EP260610b (Wu et al., GCN 44895), with the AZT-22 1.5m telescope of the Maidanak Observatory (MAO) starting on 2026-06-10 at 20:09:52 UT. In total we obtained 6x300 s exposures in the R-band using 4kx4k CCD SNUCAM camera (Im et al., 2010).
The optical counterpart (Levan et al., GCN 44897; Xin et al., GCN 44900; Zhu et al., GCN 44902) is detected in the stacked frame.
Preliminary photometry is the following:
Date MidTime Exptime t-T0 Filter OT Err. UL SNR Site/Telescope
(nxs) (mid, days)
2026-06-10T20:24:52 6x300 0,317072 R 22.85 0.15 23.26 6.4 MAO/AZT-22
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022) web interface called STDweb (Karpov et al., 2025).
Images obtained in Johnson Cousins filters were calibrated using the Gaia DR3 Synphot catalog.
The data has not corrected for the Galactic extinction.
Maidanak astronomical observatory (MAO) is an observational facility of the Ulugh Beg Astronomical
Institute (UBAI), Uzbekistan Academy of Sciences (http://maidanak.uz/).
GCN Circular 44902
Subject
EP260610b: NOT optical observation
Date
2026-06-11T02:42:23Z (4 months ago)
Edited On
2026-06-11T17:29:22Z (4 months ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
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Z.-P. Zhu, X. Liu, L.-B. He, J. An, S.-Q. Jiang, D. Xu (NAOC), S.-Y. Fu (HUST),J.PU. Fynbo (DAWN/NBI) reports on behalf of a large collaboration:
We observed the field of EP260610b detected by EP (Wu et al., GCN 44895), using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observation started at 2026-06-10 22:20:33 UT, i.e., about 0.4 days post-burst, and a series of r-band frames were obtained.
The previously reported optical counterpart (Levan et al., GCN 44897; Xin et al., GCN 44900) is detected in our stacked images with a preliminary brightness of r ~ 22.5, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.
GCN Circular 44900
Subject
EP260610b: SVOM/VT optical confirmation with blue color
Date
2026-06-11T01:25:37Z (4 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
L. P. Xin, H. L. Li, 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 EP260610b triggered by Einstein Probe (Wu et al., GCN 44895). The observation started at 2026-06-10T13:47:19 UTC, 59.5 minutes post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical counterpart (Levan et al., GCN 44897) was 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
2.13 h VT_B 91*50 s 22.6 +/- 0.2 mag
2.13 h VT_R 80*50 s 23.0 +/- 0.3 mag
We note the source has a blue color in VT observations.
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 44897
Subject
EP260610b: Gemini North optical counterpart candidate
Date
2026-06-10T16:54:45Z (4 months ago)
From
Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>
Via
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A. J. Levan (Radboud & Warwick), J. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD), A. P. C. van Hoof (Radboud), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), J. Chacón (PUC), H. Sun (NAO, CAS), F. E. Bauer (SSI and UTA) report on behalf of a larger collaboration:
We observed the localisation region of EP260610b (Wu et al. 2026, GCN 44895) with the Gemini North Telescope and the GMOS-N instrument on Mauna Kea, Hawaii, USA. Observations began at 13:36:51 UT on 10 June 2026 and consisted of 7 × 100 s exposures in the SDSS r band (~0.81 hr after T0) and 3 × 100 s exposures in the SDSS z band (~1.08 hr after T0).
By comparison with the Legacy Survey images, we identify a candidate optical counterpart at the following coordinates (J2000):
RA = 17:50:29.72
Dec = +46:32:10.6
Photometry performed on the stacked images yields r = 22.23 ± 0.03 and z = 22.19 ± 0.05 AB mag, calibrated against nearby stars from the Pan-STARRS catalog and not corrected for Galactic extinction.
We acknowledge the excellent support from the Gemini North staff in the execution of these challenging observations.
GCN Circular 44895
Subject
EP260610b: Einstein Probe detection of an X-ray transient
Date
2026-06-10T13:28:05Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H.Z. Wu, (HUST, CAS),C. L. Guo, H. Y. Ren, 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 EP260610b. The transient triggered EP-WXT (ID: 01709268876) at 2026-06-10T12:48:17 (UTC). The WXT position of the source is R.A. = 267.601 deg, DEC = 46.534 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. = 267.6268 deg, DEC = 46.5388 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).