EP260610a
GCN Circular 44908
Subject
EP260610a: refined analysis of the EP-WXT and follow-up EP-FXT observations
Date
2026-06-11T08:52:18Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Y. Ren, C. L. Guo, Z. X. Ling (NAO, CAS), H.Z. Wu (HUST, CAS), on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260610a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-06-10T04:41:46 (UTC). We note that the trigger time reported in Wu et al., GCN 44889 was incorrect. The correct trigger time is 2026-06-10T04:49:21 (UTC). The WXT light curve lasted for around 450 seconds before the interruption of the autonomous follow-up. 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 0.46 × 10^21 cm^−2 and a photon index of 0.89 (+0.60/−0.57). The derived average unabsorbed 0.5–4 keV flux is 2.71 (+1.21/−0.70) × 10^−10 erg s^−1 cm^−2.
The autonomous follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-10T04:52:19 (UTC). The exposure time was approximately 3.5 ks. The on-ground analysis of the FXT data detected an uncatalogued X-ray source at R.A. = 306.1022 deg, DEC = −25.9045 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 0.46 × 10^21 cm^−2 and a photon index of 1.95 (+0.07/−0.07). The derived average unabsorbed 0.5–10 keV flux is 0.82 (+0.06/−0.05) × 10^−11 erg s^−1 cm^−2. The uncertainties quoted above are at the 90% confidence level.
We note that follow-up observations were carried out with COLIBRÍ (de Ugarte Postigo et al., GCN 44890) and SVOM/VT (Li et al., GCN 44896), but no optical counterpart was detected in these observations.
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 44896
Subject
EP260610a: SVOM/VT optical upper limits
Date
2026-06-10T14:40:01Z (a month 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 a ToO observation of the field of EP260610a triggered by Einstein Probe (Wu et al., GCN 44889). The observation started at 2026-06-10T05:47:51 UTC, i.e., about 1.18 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
No uncatalogued optical source was detected at the FXT source position (Wu et al., GCN 44889) in our stacked images, compared to the Legacy Survey. The 3-sigma upper limits are:
Mid_time Band Exposure Time Magnitude (AB)
2.27 h VT_B 83*50 s > 23.8 mag
2.27 h VT_R 84*50 s > 23.4 mag
The magnitudes are in the AB system and are not corrected for Galactic extinction.
Our non-detection is consistent with the observation from COLIBRÍ (Postigo et al., GCN 44890).
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 44890
Subject
EP260610a: COLIBRÍ optical upper limits
Date
2026-06-10T10:12:50Z (a month ago)
Edited On
2026-06-10T12:45:21Z (a month ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
Antonio de Ugarte Postigo (LAM), Diego López-Cámara (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (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), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) report:
We imaged the field of EP260610a (Wu et al., GCN Circ. 44889) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-10 08:23:40 to 09:56:22 UTC (from 3.78 to 5.32 hours after the trigger) and obtained 69 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded and analysed 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.
In the stacked image, we do not detect any new source at the FXT source position (Wu et al., GCN Circ. 44889) down to the following 5-sigma limits:
r > 24.0 mag
z > 22.6 mag
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 44889
Subject
EP260610a: Einstein Probe detection of a fast X-ray transient
Date
2026-06-10T06:22:59Z (a month 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 EP260610a. The transient triggered EP-WXT (ID: 01709268833) at 2026-06-10 04:37:09(UTC). The WXT position of the source is R.A. = 306.104 deg, DEC = -25.926 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 at 2026-06-10T04:49:21(UTC). Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 306.1055 deg, DEC = -25.9057 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
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).