EP260924a
GCN Circular 45757
Rui-Chen Gao, Jin-Ji Li, Zhong-Nan Dong, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Hao-Ran Zhang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:
We observed the field of the fast X-ray transient EP260924a (Jiang et al., GCN 45737) using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our observations were carried out on 2026 Sep 25 13:47 UT, 16 hours after the trigger, using stacked images in the J band.
We do not detect any counterpart at the position of EP260924a , down to a 5-sigma depth of J ~ 19.22 mag (Vega).
The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.
GCN Circular 45754
Y.H. Jiang (NJU), J. P. Chen (SYSU), Y. Q. Zhao (PRIC,USTC), and Y. Liu (NAO,CAS) on behalf of the Einstein Probe (EP) team:
We performed a follow-up observation of EP260924a (Jiang et al., GCN 45737). The observation started at 2026-09-25T15:24:01 (UTC, T0+17.5 hours). The effective exposure time of the observation is around 4.9 ks. On-ground analysis of the FXT data found a weak uncatalogued source at R.A. = 339.0218, DEC = 47.0322 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The source is faint, and its association with the WXT detection (Jiang et al., GCN 45737) remains uncertain. The average unabsorbed flux is around 7.5e-14 erg/s/cm^2 in the 0.5-10 keV energy band.
No counterparts were detected by WINTER (Mohan et al., GCN 45742).
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 45742
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 EP260924a (Jiang et al., GCN 45737) 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 2026-09-25 at 06:20:22.162 UTC in the J band (8.5 hr after the EP trigger), consisting of 15*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 any new source in the WXT localization in our stacked image. We obtain the following 5-sigma upper limit: J = 19.19 mag (AB).
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 45737
Y.H. Jiang (NJU), J. P. Chen (SYSU), Y. Q. Zhao (PRIC,USTC), and Y. Liu (NAO,CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient, designated EP260924a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient was detected by CMOS30 during an observation starting from 2026-09-24T21:49:27 (UTC), and identified by ground-based telemetry analysis. The transient started around the beginning of the observation and lasted for about 300s. The WXT position of the source is R.A. = 339.0046 deg, DEC = 47.0245 deg (J2000) with an uncertainty of 2.83 arcmin in radius (90% C.L. statistical and systematic). No cataloged X-ray sources have been found within the WXT error circle.
The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a Galactic hydrogen column density of 4.3e21 cm^-2 and a photon index of 4.7 (-/+3.5). The derived average unabsorbed 0.5-4 keV flux is 1.7(-1.6/+25.0) e-10 erg cm^-2 s^-1. All uncertainties are reported in 90% C.L.
We have arranged a follow-up observation with the Follow-up X-ray Telescope onboard EP and will update the information upon receiving the data. Multi-wavelength observations are strongly encouraged.
Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).