EP261023a
GCN Circular 46027
Subject
EP261023a: J-band upper limit with WINTER
Date
2026-10-25T17:22:22Z (2 days ago)
From
tanishk.mohan@iitb.ac.in
Via
Web form
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP261023a (Yang et al., GCN 46014) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began on 2026-10-24 at 10:20:58.132 UTC in the J band (30.29 hr after the WXT trigger), consisting of 30*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We do not detect the optical counterpart (Lei et al., GCN 46015) in our difference image, and obtain the following 5-sigma upper limit: J = 19.14 mag (AB). Optical and near infrared upper limits are also reported by Pankov et al., GCN 46018; Zhang et al., GCN 46023.
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 46023
Subject
EP261023a: J band upper limit by SYSU 80cm infrared telescope
Date
2026-10-25T09:05:37Z (2 days ago)
From
zhanghr33@mail2.sysu.edu.cn
Via
Web form
Hao-Ran Zhang, Shu-Quan Bai, Zhong-Nan Dong, Rui-Chen Gao, Si-Rui Huang, Jin-Ji Li, Jia-Qi Lin, Pu Lin, Yun Shi, Jing-Kai Sun, Zhi-Xiang Tan, Lu-Yao Wen, Hao-Nan Yang, P. H. Thomas Tam, Rong-Feng Shen, and Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of EP261023a detected by the Einstein Probe (Yang et al., GCN 46014), with the Sun Yat-sen University 80cm infrared telescope in the J band. The observations were carried out on 2026-10-24 from 11:55:12 to 12:58:45 UT, beginning 31.87 hr after the EP-WXT trigger. A total of 157 frames of 20s each were stacked, giving 3140s on source.
We do not detect any new source within the optical afterglow localization (RA, Dec = 3.984, +51.5986 deg, J2000; 0.4 arcsec uncertainty; reported by NOT, Lei et al., GCN 46015) in our stacked image, and obtain a 5-sigma upper limit of J = 19.1 mag (Vega). Observation by Mondy started on ~9.8 hr after the trigger also yield a 3-sigma upper limit of R = 22.2 mag (Pankov et al., GCN46018).
The SYSU 80cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 46018
Subject
EP261023a: Mondy optical upper limit
Date
2026-10-24T09:07:49Z (3 days ago)
From
Nicolai Pankov at Independent Researcher <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), A. Moskvitin (SAO RAS), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:
We observed the field of the Einstein Probe fast X-ray transient EP261023a (Yang et al., GCN 46014) with the 1.5-m AZT-33IK telescope of Sayan Solar Observatory (Mondy). Observations started on 2026-10-23 at 13:30:00 UT, i.e. ~0.41 days after the EP/WXT trigger. A series of 30x120 s frames was obtained in the R filter.
The optical counterpart (Lei et al., GCN 46015 and 45972) is not detected in the co-add frame from Mondy. Preliminary upper limit and observation details are given below.
Date UTstart t-T0 Exp. Filter OT Err. UL
(mid, days) (s) (3sigma)
2026-10-23 13:30:00 0.41437 30x120 R n/d n/d 22.2
The photometry was calibrated against nearby USNO-B1.0 stars (R2 magnitudes) and is not corrected for Galactic extinction.
GCN Circular 46015
Subject
EP261023a: NOT optical counterpart candidate
Date
2026-10-23T15:42:14Z (4 days ago)
From
syfu@nao.cas.cn
Via
Web form
L. Lei, S.Y. Fu, A.D. Zhu (HUST), D. Xu, J. An, L.B. He, S.Q. Jiang, X. Liu, Z.P. Zhu (NAOC), J.P.U. Fynbo (DAWN/NBI), S. Oliver (NOT) report:
We observed the field of EP261023a detected by EP (Yang et al., GCN 46014), using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We obtained 5 x 200 s Sloan r-band frames.
An optical source is detected within the EP-FXT error circle (Yang et al., GCN 46014) in the stacked image at coordinates
R.A. (J2000) = 0:15:56.16
Dec. (J2000) = +51:35:54.81
with an uncertainty of ~ 0.4 arcsec. The source had r = 22.71+/-0.13 mag at a median time of ~ 63.7 min post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.
An object is weakly detected at the above position in the Pan-STARRS survey images, and it has r = 23.3 +/- 0.2, i = 21.3 +/- 0.2, and z = 20.4 +/- 0.1 mag, calibrated with Pan-STARRS field stars. If the object is interpreted as the host galaxy, it is consistent with an early-type galaxy.
We think the NOT source is the optical counterpart candidate of EP261023a.
GCN Circular 46014
Subject
EP261023a: Einstein Probe detection of an X-ray transient
Date
2026-10-23T14:56:22Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. N. Yang (NAO,CAS), Z. X. Li (IHEP), M. Y. Wang (XMU), Q. Y. Wu, H. W. Pan (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 EP261023a. The transient triggered EP-WXT (ID 01709321471) at 2026-10-23 04:03:19 (UTC). The WXT position of the source is R.A. = 3.994 deg, DEC = 51.589 deg (J2000) with an uncertainty of 2.749 arcmin in radius (90% C.L. statistical and systematic). The transient starts from T0 = 2026-10-23 04:02:14 (UTC), with the source remaining bright for ~64 s until the end of this observation. Analysis of this earliest WXT observation shows that its spectrum can be fitted with a power-law model, with the absorption column density fixed at the Galactic value of 2e21 cm^-2. The best-fit photon index is 1.3(+/-0.5), and the unabsorbed flux in the 0.5-4 keV band is 2.0(-0.4/+0.6) e-9 erg/s/cm^2.
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. = 3.9824 deg, DEC = 51.5984 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). we jointly fitted the FXT-A and FXT-B spectra in the 0.5–10 keV band with an absorbed power-law model. The hydrogen column density was fixed at the Galactic value. The best-fit photon index is 2.6(+/-0.5). The average unabsorbed 0.5-10 keV flux is 8.5(+2.2/-1.8) e-13 erg/s/cm^2. Within the position error of this source, there is only one star, Gaia DR3 395023397938295040, located at a distance of 1.5 kpc. The association between the FXT source and the WXT trigger remains uncertain.
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).