EP260507a
GCN Circular 44501
Subject
EP260507a: REM optical/NIR observations
Date
2026-05-08T15:30:56Z (14 hours ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of EP260507a, detected by EP (Fu et al., GCN 44488) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile).
The observations were carried out in the g, r, i, z, and H bands, started on 2026-05-07 at 22:56:01 UT (i.e. 9.1 hr after the burst) and lasted for about 1 hour.
From preliminary photometry, we do not detect the optical/NIR counterpart (Corcoran et al., GCN Circ. 44489; Sankar et al., GCN Circ. 44490; Jiang et al., GCN Circ. 44491; Kumar et al., GCN Circ. 44492; Eyles-Ferris et al., GCN Circ. 44495; Jiang et al., GCN Circ. 44496; Li et al., GCN 44497; Globus et al., GCN 44498) down to the following 3sigma limits:
r > 19.8 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 10.0 hr after the trigger;
H > 14.5 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 9.2 hr after the trigger.
GCN Circular 44500
Subject
EP260507a: refined analysis of the EP-WXT and EP-FXT observations
Date
2026-05-08T13:33:09Z (16 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S.Y.Fu (HUST), Y. L. Wang (NAO, CAS; ICE, CSIC), C.L.Guo (NAO, CAS), X. Tian (GXU), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260507a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Fu et al., GCN 44488). The refined analysis of the WXT data shows that the event started at T0=2026-05-07T13:47:44.0 (UTC) and lasted for 15 s before the observation was interrupted by the autonomous follow-up observation (i.e. T90 > 15 s). The average 0.5–4 keV spectrum of WXT can be modeled with an absorbed power law, adopting a fixed Galactic hydrogen column density of 5.55 x 10^20 cm^-2 and an additional column density of 5.77 x 10^21 cm^-2. This additional absorption significantly improves the fit quality, yielding a photon index of 1.47 (+-0.79). The derived average unabsorbed 0.5-4 keV flux is 1.98 (-0.70/+2.20) x 10^(-8) erg/s/cm^2.
The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-05-07T13:51:52 (UTC), about 4 minutes after T0. The exposure time of this observation is 5046 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 206.899, -22.2364 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law adopting a fixed Galactic equivalent hydrogen column density of 5.55 x 10^20 cm^-2 and an additional column density of 8.33 x 10^20 cm^-2，yielding a photon index of 2.10 (+-0.06). The derived average unabsorbed 0.3-10 keV flux is 1.82 (-0.07/+0.08) x 10^(-11) 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 44498
Subject
EP260507a: COLIBRÍ optical observations
Date
2026-05-08T06:07:02Z (a day ago)
From
globus@astro.unam.mx
Via
Web form
Noémie Globus (UNAM), Francis Fortin (IRAP), 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), Leonardo García García (UNAM), Ramandeep Gill (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), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the EP260507a (Fu et al., GCN Circ. 44488) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-08 04:59 to 05:50 UTC (from 15.19 to 16.06 hours after the trigger) and obtained 39 minutes of exposure in the r 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 the optical candidate reported previously (Corcoran et al., GCN Circ. 44489; Sankar et al., GCN Circ. 44490; Jiang et al., GCN Circ. 44491; Kumar et al., GCN Circ. 44492; Bochenek et al., GCN Circ. 44494; Eyles-Ferris et al., GCN Circ. 44495; Jiang et al., GCN Circ. 44496) with preliminary magnitudes of:
r = 23.37 +/- 0.14,
z = 22.44 +/- 0.21.
From a combined power-law fit including our data and previously reported GCN Circular measurements, we derive a temporal decay index of α ≈ 0.6.
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 44497
Subject
EP260507a: SVOM/VT optical observations
Date
2026-05-08T03:08:14Z (a day 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, Y. N. Ma, Z. H. Yao, J. R. Xu, 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), J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.
SVOM/VT performed observations with automatic follow-up observations to the Xray transient EP260507a triggered by Einstein Probe (Fu et al., GCN 44488). The observation started at 2026-05-07T14:45:27 UTC, i.e. 57.5 minutes post trigger and lasted for 2 orbits (about 1.9 hours) in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
The optical counterpart (Corcoran et al., GCN 44489; Sankar et al., GCN 44490; Jiang et al., GCN 44491; Kumar et al., GCN 44492; Eyles-Ferris et al., GCN 44495; Jiang et al., GCN 44496) was clearly detected in both channels. The measurements in AB magnitude are as follows:
Mid time | Band | Exposure Time | Brightness
1.35 h VT_B 56*50 s 21.92 +/- 0.12 mag
1.35 h VT_R 55*50 s 20.91 +/- 0.08 mag
2.96 h VT_B 56*50 s 22.29 +/- 0.14 mag
2.96 h VT_R 55*50 s 21.25 +/- 0.10 mag
Our photometry was not corrected for Galactic extinction.
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 44496
Subject
EP260507a: NOT optical observations
Date
2026-05-08T01:42:18Z (a day ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang, L.B. He, Z.P. Zhu, J. An, X. Liu, D. Xu (NAOC), S.Y. Fu (HUST), J. P. U. Fynbo (DAWN/NBI), A. L. Bouquin (NOT) report:
We observed the field of EP260507a detected by EP (Fu et al., GCN 44488), using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We obtained 3 x 300 s frames in the Sloan g, r, i-band, respectively.
The optical afterglow (Corcoran et al., GCN 44489; Sankar et al., GCN 44490; Jiang et al., GCN 44491; Kumar et al., GCN 44492; Eyles-Ferris et al., GCN 44495) is detected in our stacked images. Preliminary photometry results are as follows:
Tmid-T0(hr) Mag MagErr Filter
9.54 23.04 0.12 r
9.82 23.61 0.17 g
10.10 22.77 0.17 i
calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.
GCN Circular 44495
Subject
EP260507a: further Liverpool Telescope observations
Date
2026-05-08T01:20:37Z (a day ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris (Leicester) reports on behalf of a larger collaboration:
We observed the field of EP260227a (Fu et al., GCN 44488) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 6x200s exposures in each of the SDSS r’ and z’ filters starting at 2026-05-07 22:51:45 UT, approximately 9.06 hours after the X-ray detection.
We detect the optical counterpart (Corcoran et al., GCN 44489; Sankar et al., GCN 44490; Jiang et al., GCN 44491; Kumar et al., GCN 44492; Bochenek et al., GCN 44494) in both the r’ z’ bands and measure the following magnitudes:
r’ = 22.76 +/- 0.21
z’ = 21.98 +/- 0.30
Our photometry is calibrated to Pan-STARRS and not corrected for Galactic extinction.
GCN Circular 44494
Subject
EP260507a: Liverpool Telescope optical upper limits
Date
2026-05-07T23:34:35Z (a day ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of EP260507a (Fu et al., GCN 44488) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 5x100s exposures in SDSS g, r, and i filters, starting at 2026-05-07 22:20:09 UT, approximately 8.54 hours after trigger.
We do not detect any sources at the position of the optical counterpart (Corcoran et al., GCN 44489; Sankar et al., GCN 44490; Jiang et al., GCN 44491, Kumar et al., GCN 44492) down to the 3-sigma limiting magnitude of g > 22.05, r > 21.92 and i > 22.07.
Further observations are planned. The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular 44492
Subject
EP260507a: GOTO optical counterpart detection
Date
2026-05-07T19:02:11Z (a day ago)
From
Amit Kumar at The Open University, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar, S. Belkin, D. O'Neill, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, J. Casares, B. Godson, T. Killestein and M. Pursiainen on behalf of GOTO collaboration
We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to EP-WXT triggered EP260507a (Fu et al., GCN 44488). The observations were conducted with GOTO-South at 2026-05-07 13:57:30 UT and 2026-05-07 15:04:48 UT (9.5 and 76.8 mins post-trigger, respectively), consisting of 4x90s exposures in the GOTO L-band (400-700 nm). Difference imaging was performed using deeper template observations of the same pointings.
We detect the optical counterpart of EP260507a (Corcoran et al., GCN 44489; Sankar et al., GCN 44490; Jiang et al., GCN 44491) in the GOTO L-band at 2026-05-07 13:57:30 UT (9.5 mins post-trigger), with a measured magnitude of 19.56 ± 0.11 AB. The source is no longer detected at the next epoch obtained at 2026-05-07 15:04:48 UT (76.8 mins post-trigger), with a 3-sigma upper limit of 20.4 AB mag.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular 44491
Subject
EP260507a: TRT optical counterpart detection
Date
2026-05-07T18:11:13Z (a day ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), X. Liu, Z.P. Zhu, J. An, L.B. He, D. Xu (NAOC) report:
We observed the field of EP260507a detected by EP (Fu et. al, GCN 44488), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observation started at 2026-05-07T14:27:55.509 UT, i.e., 39.98 mins post-burst, and a series of R-band frames were obtained.
The optical afterglow (Corcoran et. al, GCN 44489; Sankar et. al, GCN 44490) is marginally detected in our stacked images with a brightness of R = 20.8 +/- 0.36 at a median time of 45.88 mins since trigger, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.
GCN Circular 44490
Subject
EP260507a: Kinder optical counterpart candidate
Date
2026-05-07T15:19:55Z (2 days ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
A. Sankar.K, A. Aryan, Y.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), M.-H. Lee, A. Dutta, K. N.-T. Ho, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, 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, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP260507a (Fu et al., GCN 44488) using the 40cm SLT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86). The first SLT epoch of observations started at 14:06 UTC on 7th May 2026 (MJD 61167.5837), 0.30 hr after the EP-WXT trigger.
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. In the stacked frame as well as in the difference image, we detect an uncatalogued optical counterpart candidate at RA, DEC = 13:47:35.567, -22:14:08.38 with an uncertainty of 1". The optical counterpart candidate is well inside the EP-FXT error circle of 20".
The details of the observations and measured PSF magnitude with template subtraction (in the AB system) of the possible counterpart of EP260507a are as follows:
--------------------------------------------------------------------------------------------------
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | r | 61167.5837 | 0.30 | 300 * 6 | 20.64 +/- 0.07 | 2".07 | 1.44
The presented magnitudes are calibrated using the field stars from ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitudes/upper-limits are not corrected for an expected galactic extinction of A_r = 0.16 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 44489
Subject
EP260507a: LCO optical counterpart
Date
2026-05-07T15:16:19Z (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), J. A. Chácon (PUC), J. N. D. van Dalen (Radboud), F. E. Bauer (SSI and UTA), P. G. Jonker (Radboud), A. J. Levan (Radboud & Warwick) and D. B. Malesani (DAWN/NBI and Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260507a (Fu et al., GCN 44488) using an LCO 1m telescope located at the Siding Spring Observatory (Australia) equipped with the SINISTRO instrument. We obtained 6x300s exposures in the SDSS-r filter starting at 2026-05-07 14:34:48 (~47 minutes after the trigger).
Within the EP/FXT error region (Fu et al., GCN 44488), we detect a likely optical counterpart at a position of (J2000):
RA 13:47:35.57 (206.89822)
Dec -22:14:08.40 (-22.23566)
We measure a magnitude of r = 21.21 +/- 0.07 (AB) calibrated to PanSTARRS and not corrected for Galactic extinction.
GCN Circular 44488
Subject
EP260507a: Einstein Probe detection of an X-ray transient
Date
2026-05-07T14:40:02Z (2 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S. Y. Fu (HUST), Y. L. Wang (NAO, CAS; ICE, CSIC), C. L. Guo (NAO, CAS), X. Tian (GXU), W. D. Zhang (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 EP260507a. The transient triggered EP-WXT (ID: 01709261593) at 2026-05-07T13:47:57 (UTC). The WXT position of the source is R.A. = 206.895 deg, DEC = -22.243 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. = 206.8963 deg, DEC = -22.2361 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).