GCN 44976: EP260616a/GRB 260616B: EP-FXT follow-up observation
2026-06-18T10:37:23.987Z | rev 0
D. Y. Li, T. Y. Liu (NAO, CAS), J. H. WU(GZHU), Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe team
EP-FXT performed a follow-up observation of EP260616a/GRB 260616B (Li et al., GCN 44956; Preis & Greiner, GCN 44951; Malacaria et al., GCN 44953; Asaoka et al., GCN 44966) about 25.5 hours after the WXT detection, with an exposure time of 6.2 ks. An uncatalogued source was detected within the WXT error circle at R.A. = 256.0839, DEC = -32.3007 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT spectrum can be fitted with an absorbed powerlaw with a hydrogen column density fixed at the Galactic value of 3.4 x 10^21 cm^-2 and a photon index of 1.62 (+0.27, -0.26). The derived unabsorbed 0.5-10 keV flux is approximately 8.7 (+2.2, -1.7) x 10^-13 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
No optical counterpart have been detected (Mandarakas et al., GCN 44965). Further EP-FXT follow-up observations will be arranged.
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 44965: EP260616a/GRB260616B: COLIBRÍ optical upper limit
2026-06-17T14:44:26.110Z | rev 0
Nikos Mandarakas (LAM), Diego López-Cámara (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), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report:
We imaged the field of the EP 260616a (Li et al., GCN Circ. 44956), which is consistent in time and location with GRB 260616B (Malacaria et al., GCN Circ. 44953), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-17 05:56:39 to 07:16:28 UTC (from 22.86 to 24.20 hours after the trigger) and obtained 48 minutes of simultaneous exposure in the r and z filters.
The data were reduced, coadded and analysed with the ASU COLIBRÍ 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.
In the stacked image, we do not detect any new source at the WXT source position (Li et al., GCN Circ. 44956) down to the following 5-sigma limit:
r > 22.76 mag
z > 22.20 mag
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 44956: EP260616a: EP-WXT detection of a fast X-ray transient
2026-06-16T16:27:09.814Z | rev 0
D. Y. Li (NAO, CAS), J. H. Wu (GZHU), T. Y. Liu, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient, designated EP260616a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2026-06-16T07:05:16 (UTC). The position of the source is R.A. = 256.059 deg, DEC = -32.305 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic). No onboard alert or automated X-ray follow-up observation was triggered at this position due to the low Galactic latitude. The start time and the position of the transient is consistent with that of GRB 260616B detected by Fermi/GBM (GCN 44953).
The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.8 (+0.3/-0.3). The column density is fixed at the Galactic value of 3.4 x 10^21 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 4.1 (+/-0.5) x 10^-10 erg/s/cm^2. The peak flux is around 3.4 x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Follow-up observations by EP-FXT will be arranged.
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).