GCN 44766: EP260601a: Einstein Probe detection of an X-ray transient
2026-06-02T11:52:06.788Z | rev 0
G. J. Yang (NAO, CAS), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Wang (PMO, CAS; UCB), W. D. Zhang (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 EP260601a. The transient triggered EP-WXT (ID: 01709266307) at 2026-06-01T19:13:22 UTC. The WXT position of the source is R.A. = 257.649 deg, Dec = -1.647 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The preliminary analysis of the WXT data shows that the burst started at T_0 = 2026-06-01T19:12:41 UTC. The light curve shows an initial flare lasting approximately 100 s, followed by tail emission. During the flare, the unabsorbed 0.5-4 keV flux was 5.2 (-1.7/+3.6) x 10^-08 erg cm^-2 s^-1.
The time-averaged WXT 0.5-4 keV spectrum over the entire detected emission can be described by an absorbed power law, with the hydrogen column density left as a free parameter. The best-fit hydrogen column density is 6.7 (-1.5/+1.6) x 10^21 cm^-2. The best-fit photon index is 1.2 (-0.3/+0.3). The unabsorbed 0.5-4 keV flux is 2.4 (-0.2/+0.3) x 10^-10 erg cm^-2 s^-1.
A follow-up observation with the Follow-up X-ray Telescope (FXT) onboard EP is scheduled.
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).
GCN 44765: EP-WXT trigger 01709266307 (EP260601a): COLIBRÍ optical observations
2026-06-02T11:05:28.981Z | rev 0
Marion Guelfand (CPPM), Camila Angulo (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), 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), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We observed the field of the EP-WXT Trigger 01709266307 (EP260601a) with the DDRAGO two-channel wide-field imager mounted on the COLIBRÍ telescope. We observed from 2026-06-02 08:36 UTC to 09:00 UTC (from 13.38 to 13.39 hours after the trigger) and obtained 20 minutes of simultaneous exposure in the r and z filters, respectively.
The data were reduced and coadded with the COLIBRI pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the Pan-STARRS-DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.
We detected the optical counterpart reported by Liu et al., GCN Circ. 44764.
The preliminary magnitude derived for that source is:
r = 21.93 +/- 0.15
z = 21.07 +/- 0.12
The source is fading between the two observations.
Further observations are ongoing.
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 44764: EP-WXT trigger 01709266307 (EP260601a): JinShan optical counterpart detection
2026-06-02T10:23:04.250Z | rev 0
X. Liu, Z.P. Zhu, D. Xu, J. An, S.Q. Jiang, L.B. He (NAOC), S.Y. Fu, A.D. Zhu, L. Lei (HUST), J.Z. Liu (XAO) report:
We observed the field of EP-WXT Trigger 01709266307 (EP260601a) detected by Einstein Probe (EP), using the JinShan 100B telescope located at Altay, Xinjiang, China. A series of r-band frames were obtained.
An uncatalogued and decaying source is detected within the EP/WXT error circle at coordinates
R.A. (J2000) = 17:10:38.24
Dec. (J2000) = -01:38:42.4
with an uncertainty of ~ 1.0 arcsec. The source had r ~ 18.4 mag at a median time of ~ 0.73 hr post-trigger, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction of A_V ~ 1.15 mag. We thus think the source is the optical counterpart of EP260601a.
We also note the presence of a faint source with g ~ 25.2 mag in the Legacy Survey DR10 image at the above position, which could be the host of EP260601a.
We acknowledge the excellent support from T.Q. Chen, and J.F. Zhang for enabling these observations.