EP260602a
GCN Circular 44816
Subject
EP260602a: EP-FXT follow-up observation
Date
2026-06-04T07:09:04Z (5 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Wang (PMO, CAS; UCB), G. J. Yang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The X-ray transient EP260602a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 44770) at 2026-06-02T08:11:20 (UTC).
Follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-03T14:02:47(UTC), ~30 hours after the WXT detection. The exposure time of the observation is around 6 ks. The on-ground analysis of the FXT B data found an uncatalogued source at R.A. = 240.8343 deg, DEC = 50.4636 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is approximately consistent with the position of the optical counterpart (Xu et al., GCN 44781; Watson et al., GCN 44790; Malesani et al., GCN 44807). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.03×10^20 cm^-2, and a photon index of 1.6 (-0.9, +1.2). The derived average unabsorbed 0.5-10 keV flux is 7.1 (-4.3, +13.0) ×10^(-14) 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 44810
Subject
EP260602a: Liverpool Telescope optical upper limits
Date
2026-06-03T19:58:47Z (5 months ago)
Edited On
2026-06-04T14:17:19Z (5 months ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
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A. Bochenek, D. A. Perley, J. L. Wise (LJMU), report:
We observed the field of EP260602a (Li et al., GCN 44770) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained multiple 120s exposures in SDSS gri filters, starting at 2026-06-02 21:15:26 UT, approximately 13.07 hours after trigger. The observations were interrupted and consequently resumed due to weather conditions and telescope tracking issues; some exposures needed to be discarded.
We do not detect any sources at the position of the counterpart (Xu et al., GCN 44781, Li et al., GCN 44794; also Watson et al., GCN 44790; He et al., GCN 44798, Malesani et al., GCN 44807) down to the 3 sigma limiting magnitudes of:
| MJD (mid) | T_mid-T_0 | Exp. Time | Filter | Mag limit. (AB) |
| ------------ | ------------ | ------------ | ------------ | --------------- |
| 61193.93925 | 14.34 h | 600 s | g | > 21.8 |
| 61193.91413 | 13.75 h | 960 s | r | > 22.1 |
| 61194.11373 | 18.54 h | 720 s | i | > 21.9 |
The stacked exposures were subtracted from PanSTARRS reference imaging using custom PSF-matching image subtraction code utilising Swarp, SourceExtractor, and PSFex. The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 44807
Subject
EP260602a: GTC and Liverpool Telescope optical observations
Date
2026-06-03T16:41:18Z (5 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), R. A. J. Eyles-Ferris (Leicester), J. N. D. van Dalen (Radboud), P. G. Jonker (Radboud), R. L. C. Starling (Leicester), A. J. Levan (Radboud & Warwick), J. Chacón (PUC), I. Yanes (GTC), A. Tejero (GTC), S. Geier (GTC) report on behalf of a larger collaboration:
We observed the field of EP260602a (Li et al., GCN 44770) with the Gran Telescopio Canarias (GTC) and the Liverpool Telescope (LT), both on the island of La Palma, equipped with the OSIRIS+ and IO:O instruments, respectively. Observations were performed in the Sloan r and z filters. We detect the optical/NIR counterpart (Xu et al., GCN 44781; Watson et al., GCN 44790; Li, GCN 44794; He et al. GCN 44798) in the GTC r and z stacked images, and marginally in the LT r image.
We obtain the following photometry for the counterpart, calibrated to Pan-STARRS and not corrected for Galactic extinction:
| t_mid - t_0 (hours) | Telescope/instrument | Filter | Exposure (s) | AB magnitude |
| --- | --- | --- | --- | --- |
| 16.7 | LT/IO:O | r | 7x150 | 22.62 +/- 0.29 |
| 17.0 | GTC/OSIRIS+ | r | 10x60 | 23.20 +/- 0.10 |
| 17.1 | GTC/OSIRIS+ | z | 10x45 | 22.70 +/- 0.10 |
| 17.4 | LT/IO:O | z | 4x150 | >22.15 |
GCN Circular 44798
Subject
EP260602a: NOT NIR J-band observations
Date
2026-06-03T11:14:23Z (5 months ago)
From
L. B. He at NAOC <helb@bao.ac.cn>
Via
Web form
L.B. He, X. Liu, Z.P. Zhu, J. An, S.Q. Jiang, D. Xu (NAOC), S.Y. Fu (HUST), J.P.U. Fynbo (DAWN/NBI), A.M. Kadela (NOT) report:
We observed the field of EP260602a detected by EP (Li et al., GCN 44770), using the 2.56m Nordic Optical Telescope (NOT) equipped with the NOTCam camera. A series of J-band frames were obtained.
The previously reported optical counterpart candidate (Xu et al., GCN 44781; Watson et al., GCN 44790) was detected in our stacked image with J = 21.47 +/- 0.18 (Vega), at a median time of ~ 16.44 hrs post-burst, calibrated with nearby 2MASS catalog and not corrected for Galactic extinction.
GCN Circular 44794
Subject
EP260602a: SVOM/VT optical magnitudes correction
Date
2026-06-03T08:16:36Z (5 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
The prior reporting of the magnitudes of EP260602a (Xu et al., GCN 44781) was affected by light blooming near the target. The corrected measurements are as below:
Mid time | Band | Exposure Time | Brightness
9.49 h VT_B 93*50 s 22.7 +/- 0.2 mag
9.51 h VT_R 84*50 s 21.5 +/- 0.1 mag
The measurements are in the AB magnitude without correction for Galactic extinction:
We apologize for the errors, and gratefully acknowledge the critical support from the COLIBRÍ team in identifying this discrepancy (Watson et al., GCN 44790).
GCN Circular 44790
Subject
EP260602a: COLIBRÍ optical observations
Date
2026-06-03T06:45:07Z (5 months ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Alan M. Watson (UNAM), Marion Guelfand (CPPM), 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), Enrique Moreno Méndez (UNAM), 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 EP260602a (Li et al., GCN Circ. 44770) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-03 03:38 to 05:17 UTC (from 19.45 to 21.10 hours after the trigger) and obtained 15, 30, 29, and 74 minutes of exposure in the g, r, i, and z filters, respectively.
The data were reduced and coadded with the COLIBRÍ 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.
We detect a faint source at a position consistent with that of the counterpart reported by Xu et al. (GCN Circ. 44781). This source is not seen in Pan-STARRS DR2 or Legacy Survey DR10 (Dey et al. 2019) images. The preliminary magnitudes of the source are, after subtracting the PS images to remove the 20-mag source 3.5 arcsec to the south, are:
g = 23.27 +/- 0.40
r = 23.26 +/- 0.21
i = 23.28 +/- 0.24
z = 22.98 +/- 0.39
The colors in our photometry are moderately red and do not suggest a high-redshift counterpart. Xu et al. report a very red color of VT_B - VT_R ≈ 2.0 +/- 0.3. Comparing griz photometry to VT photometry is not straightforward, but approximating VT_B ≈ g + r and VT_R ≈ i + z, we estimate much less extreme colors of VT_B - VT_R ≈ 0.13 +/- 0.32.
One possible explanation for this difference is that Xu et al. observed a high-redshift or highly-reddened afterglow which has now faded, and we are observing the host or an intervening galaxy. However, the lack of detection of a corresponding source in both Pan-STARRS and Legacy Survey images argues against this. We currently do not have an explanation for this difference.
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 44781
Subject
EP260602a: SVOM/VT optical candidate
Date
2026-06-03T02:55:12Z (5 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
J. R. Xu, H. L. Li, C. Wu, Y. L. Qiu, L. P. Xin, 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, L. Lan, J. S. Deng, J. Y. Wei (NAOC) and J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observations to the field of EP260602a triggered by Einstein Probe (Li et al., GCN 44770). The observation started at 2026-06-02T15:37:33 UTC, 7.44 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
An uncatalogued optical source was detected within WXT's error box (Li et al., GCN 44770) in both VT channels, compared to the Legacy Survey. The position is at R.A., Dec. = 240.862173, 50.563348 degrees, equivalent to:
R.A. (J2000) = +16:03:26.92
Dec. (J2000) = +50:33:48.05
with an uncertainty of 0.5 arcsec.
The following measurements are in the AB magnitude without correction for Galactic extinction:
Mid time | Band | Exposure Time | Brightness
9.49 h VT_B 93*50 s 23.2 +/- 0.3 mag
9.51 h VT_R 84*50 s 21.17 +/- 0.15 mag
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 44772
Subject
EP260602a: Optical upper limits with Kinder observations
Date
2026-06-02T17:23:06Z (5 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
A. Aryan, T.-W. Chen, W.-J. Hou, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang (HNAS), Y.-H. Lee, A. Sankar.K, M.-H. Lee, A. Dutta, Y.-C. Pan, C.-C. Ngeow, K. N.-T. Ho, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, K. W. Smith, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the FXT EP260602a (Li et al., GCN 44770) using the 40cm Seisi Lulin Telescope (SLT) at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 15:31 UTC on the 2nd of June 2026 (MJD 61140.5068), 7.34 hours after the EP-WXT detection.
We utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. Neither in the stacked nor in the difference image did we detect any signature of any new or uncatalogued source.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude| avg. Seeing | Med. Airmass SLT | r | 61140.5068 | 7.34 | 300 * 6 | >20.0 | 1".77 | 1.13
The presented upper limit is calibrated using the field stars from the PanSTARRS catalog and is not corrected for an expected galactic extinction of A_r = 0.06 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN Circular 44770
Subject
EP260602a: Einstein Probe detection of a fast X-ray transient
Date
2026-06-02T16:35:29Z (5 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Wang (PMO, CAS; UCB), G. J. Yang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260602a. The transient triggered EP-WXT (ID: 01709266359) at 2026-06-02T08:11:20 (UTC). The WXT position of the source is R.A. = 240.886 deg, DEC = 50.563 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The WXT spectrum can be fitted with an absorbed power-law, with the absorption fixed at the Galactic value of 2.03 x 10^20 cm^-2, and a photon index of 1.7 (-0.5, +0.5). The unabsorbed 0.5-4 keV flux is 9.0 (-2.4, +3.2) x 10^-11 erg/s/cm2.
A follow-up observation with the Follow-up X-ray Telescope (FXT) onboard EP is scheduled.
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).