EP260126a
GCN Circular 43551
Subject
EP260126a: SVOM/VT optical upperlimit
Date
2026-01-29T07:03:18Z (13 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
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H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO osbervation on EP260126a (Fu et al., GCN 43520). The observation started on 2026-01-26T16:14:40 UT (i.e. 11.837 hour after trigger) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.
No uncatalogued sources are detected in single or stacked images within error box of EP-FXT (Fu et al., GCN 43520). The 3 sigma limit magnitudes are derived as below:
Mid time | Band | Exposure Time | 3 sigma limit magnitude
13.907 hour VT_B 29*70 sec >23.4 mag
13.907 hour VT_R 29*70 sec >22.9 mag
Our non-detection is consistent with earlier reports from other telescopes (Zhu et al., GCN 43521; Globus et al., GCN 43522; Eyles-Ferris et al., GCN 43523; Eyles-Ferris & Habeeb, GCN 43524; Lee et al., GCN 43525; Taguchi et al., GCN 43541).
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 43541
Subject
EP260126a: Seimei/TriCCS optical upper limit
Date
2026-01-28T14:52:16Z (14 days ago)
From
Kenta Taguchi <kentagch@kusastro.kyoto-u.ac.jp>
Via
Web form
Kenta Taguchi (Kyoto University) reports on behalf of a larger collaboration:
We observed the field of EP260126a detected by the Einstein Probe (Fu et al., GCN 43520) using the Tricolor CMOS Camera and Spectrograph (TriCCS) on the 3.8-m Seimei Telescope at the Okayama Observatory of Kyoto University.
Our observation started at 13:30:04 UT on 2026-01-26 (MJD = 61066.5625).
For each band, we took 30-second exposures 62 times (i.e., the total exposure time is 1860 s = 31 min).
From our preliminary analysis, the 5-sigma limiting magnitudes are 21.74, 21.36, and 21.33 AB mag for g, r, and i-band, respectively.
We detected no transient sources in the FXT error circle.
GCN Circular 43526
Subject
EP260126a: EP-FXT follow-up observation
Date
2026-01-27T13:40:40Z (15 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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S.Y. Fu (HUST), Z.M. Wang (BNU), J.H. Wu (GZHU), H.Y. Liu, H.W. Pan (NAOC, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of EP260126a (detected by EP-WXT, GCN 43520) about 19.1 hours after the WXT detection, with an exposure time of 5.8 ks. The FXT telemetry data show that an uncatalogued source was detected within the WXT error circle at R.A. = 103.7551, DEC = 37.7053 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT X-ray spectrum of this uncatalogued source can be fitted with an absorbed powerlaw with a hydrogen column density fixed at the Galactic value of 1.21 x 10^21 cm^-2 and a photon index of 1.76 (+/- 0.20). The derived unabsorbed 0.5-10 keV flux is approximately 5.65 (+1.0/-0.85) x 10^-13 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Multi-band follow-up observations are encouraged.
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 43525
Subject
EP260126a: Optical upper limit with Kinder observations
Date
2026-01-27T12:48:50Z (15 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
M.-H. Lee, A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), J. H. Gillanders, S. J. Smartt (both Oxford), Y. J. Yang (NYUAD), Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, W.-J. Hou, H.-Y. Hsiao, H.-C. Lin, J.-K. Guo (all NCU), S. Yang, Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the fast X-ray transient EP260126a (Fu et al., GCN 43520) using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 13:53 UTC on the 26 January 2026 (MJD 61066.578), 9.46 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. Moreover, we used AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry. We do not detect any new or uncataloged sources within the 20" EP-FXT localization circle. The details of the observations and the measured 3-sigma upper limits (in the AB system) are as follows:
Telescope | Filter | MJD (start)| t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
SLT | r | 61066.578 | 9.46 | 300 * 6 | >21.0 | 2".8 | 1.04
SLT | i | 61066.606 | 10.14 | 300 * 3 | >20.3 | 2".5 | 1.03
The non-detections in our observations are consistent with those reported by Zhu et al. (GCN 43521), Globus et al. (GCN 43522), Eyles-Ferris et al. (GCNs 43523 and 43524).
The presented upper limits are calibrated using the field stars from the Pan-STARRS catalog and are not corrected for the expected Galactic foreground extinction of A_r = 0.24 and A_i = 0.18 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 43524
Subject
EP260126a: Liverpool Telescope optical upper limits
Date
2026-01-27T09:58:21Z (15 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris and N. Habeeb (Leicester) report on behalf of a larger collaboration:
We observed the field of EP260126a (Fu et al., GCN 43520) with the 2m Liverpool Telescope on La Palma using the IO:O instrument. We obtained 6x150s exposures in each of the SDSS i’ and SDSS g’ filters starting at 2026-01-26 23:48:56 UT, approximately 19.4 hours after the X-ray detection.
We performed image subtraction on the stacked images using reference images from Pan-STARRS and also compared the stacked and reference images manually. Within the EP/FXT localisation, and across the field, we detect no new sources in either the subtractions or stacked images consistent with the results of Zhu et al. (GCN 43521), Globus et al. (GCN 43522) and Eyles-Ferris et al. (GCN 43523).
We derive 3-sigma upper limits of i’ > 23.3 and g’ > 23.0 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction.
GCN Circular 43523
Subject
EP260126a: Further NOT optical upper limits
Date
2026-01-27T09:56:26Z (15 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), J. A. Quirola-Vasquez (Radboud), J. N. D. van Dalen (Radboud), G. Corcoran (UCD), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD) and A. J. Levan (Radboud) report on behalf of a larger collaboration:
We observed the field of the fast X-ray transient EP260126a (Fu et al., GCN 43520) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were obtained in the SDSS r and z bands, consisting of 3x300 s and 5x200 s exposures, respectively, starting on 2026-01-26 at 23:08:13 UTC (~18.7 hr after the WXT trigger).
No new source is detected within or at the border of the EP/FXT uncertainty region (Fu et al., GCN 43520) compared to Legacy DR10. From the stacked images, we obtain the following 3-sigma upper limits:
r > 24.3
z > 23.2
These upper limits are in AB magnitudes, calibrated using nearby stars from the Pan-STARRS catalog, and are not corrected for Galactic extinction. Our non-detection is consistent with the results of Zhu et al. (GCN 43521) and Globus et al. (GCN 43522).
GCN Circular 43522
Subject
EP260126a: COLIBRÍ optical upper limits
Date
2026-01-27T04:44:01Z (16 days ago)
From
globus@astro.unam.mx
Via
Web form
Noémie Globus (UNAM), Jean-Luc Atteia (IRAP), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), 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), Asuka Kuwata (UNAM), 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 EP 260126a (Fu et al., GCN Circ. 43520) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-27 02:21 to 04:06 UTC (from 21.9 to 23.7 hours after the trigger) and obtained 79 minutes of exposure in the r and z filters.
The data were reduced, coadded, calibrated and analyzed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and was not corrected for Galactic extinction.
In the stacked image, comparing to LS (Dey et al. 2019), we do not detect any new source at the FXT uncertainty region (Fu et al., GCN Circ. 43520) down to the following preliminary 5-sigma limits:
r > 24.0,
z > 23.1.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
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 43521
Subject
EP260126a: NOT optical observations
Date
2026-01-27T04:35:07Z (16 days ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
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Z.P. Zhu, J. An, X. Liu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J.P.U. Fynbo (DAWN/NBI), S. Bijavara (NOT) report on behalf of a large collaboration:
We observed the field of EP260126a (Fu et al., GCN 43520) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 22:13:05 UT on 2026-01-26, i.e., 17.8 hr after the EP trigger, and we obtained 5 x 180 s frames in the Sloan r-band and 5 x 200 s frames in the Sloan z- band.
No new optical source is detected within the EP/FXT error circle (Fu et al., GCN 43520), down to 5-sigma upper limits of r ~ 23.6 and z ~ 22.5, calibrated with nearby Pan-STARRS stars and not corrected for Galactic extinction.
GCN Circular 43520
Subject
EP260126a: Einstein Probe detection of a fast X-ray transient
Date
2026-01-27T02:36:26Z (16 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S.Y. Fu (HUST), Z.M. Wang (BNU), J.H. Wu (GZHU), H.Y. Liu, H.W. Pan (NAOC, 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 EP260126a. The event started at 2026-01-26T04:25:01 (UTC) and lasted for about 1400 seconds. The WXT position of the source is R.A. = 103.737 deg, DEC = 37.699 deg (J2000) with an uncertainty of 2.1 arcmin in radius (90% C.L. statistical and systematic).
The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.16(-/+0.17) and with a fixed galactic hydrogen column density of 1.21 x 10^21 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 2.77 (-0.26/+0.29) x 10^-10 erg/s/cm^2. The peak flux in the 0.5-4 keV band reaches 1.49(-0.14/+0.16) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed at 2026-01-26T23:08:05 (UTC), about 19 hours after the trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 103.7537 deg, DEC = 37.7067 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).