GCN 45257: EP260728a: COLIBRÍ optical upper limit
2026-07-30T10:16:03.540Z | rev 0
Jean-Grégoire Ducoin (CPPM), Fredd Sánchez Álvarez (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), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (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), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report:
We imaged the field of the EP260728a (Yang et al., GCN Circ. 45248) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. 2026-07-30T05:58:16 to 2026-07-30T07:24:22 UTC (from 37.2 to 38.6 hours after the trigger) and obtained 64 minutes of exposure in the z filters.
The data were reduced, coadded and analysed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the SkyMapper DR4 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the FXT source position (Yang
et al., GCN Circ. 42250) down to the following 3-sigma limit:
z > 21.5
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 45250: EP260728a: EP-FXT follow-up observation
2026-07-29T07:40:29.148Z | rev 0
G. J. Yang, J. W. Hu (NAO, CAS), H. Zhou (PMO), C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the fast X-ray transient EP260728a (detected by EP-WXT, GCN 45248). The observation started at 2026-07-28 16:50:26 UTC, about 4 minutes after the WXT trigger, with an exposure time of 3.6 ks. The FXT telemetry data show that an uncatalogued source was detected within the WXT error circle at R.A. = 302.8576, DEC = -31.7138 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic).
The 0.5-10 keV FXT spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.2 x 10^21 cm^-2. The best-fit photon index is 2.23 (-0.18/+0.18). The unabsorbed 0.5-10 keV flux is 1.22 (-0.12/+0.14) x 10^-12 erg cm^-2 s^-1. The uncertainties are at the 1 sigma 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 45248: EP260728a: Einstein Probe detection of a fast X-ray transient
2026-07-29T02:47:29.228Z | rev 0
G. J. Yang, J. W. Hu (NAO, CAS), H. Zhou (PMO), C. C. Jin (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 EP260728a. The transient triggered EP-WXT (ID: 01709276975) at 2026-07-28T16:46:27 (UTC). The on-ground analysis shows the WXT position of the source is R.A. = 302.859 deg, DEC = -31.693 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic). The refined analysis of the WXT data shows that the event started at T0=2026-07-28T16:45:19 (UTC) and lasted for about 70 s before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 3.1 (+2.0/-1.8)×10^21 cm^-2 and a photon index of 1.63 (+0.77/-0.72). The derived average unabsorbed 0.5-4 keV flux is 4.2 (+1.6/-0.8) x 10^-9 erg/s/cm^2. The uncertainties are at the 1 sigma confidence level for the above parameters.
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. = 302.8558 deg, DEC = -31.7161 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).