EP260731a — All Circulars

Antonio de Ugarte Postigo (LAM),  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), 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), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), Alan M. Watson (UNAM) report:

We imaged the field of EP260731a (Ni et al., GCN Circ. 45261) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-07-31 05:52:50 to 07:10:03 UTC (from 4.9739 to 6.2776 hours after the trigger) and obtained 39x60 s of simultaneous exposure in the z-band filter. Some further images obtained during this time window were lost due to passing clouds.

The data were reduced, coadded and analysed with the COLIBRÍ ASU and CPPM pipelines. The photometry was calibrated using nearby stars from the PanSTARRS 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 (Ni et al., GCN Circ. 45261) down to the following 5-sigma limit:

z > 20.9

This upper limit is consistent with the one reported by Liu et al. (GCN Circ. 45263).

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.

X. Liu, J. An, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J. P. U. Fynbo (DAWN/NBI), A. L. Bouquin, A. Schenone-Zanuzzi (NOT) report:
 
We observed the field of EP260731a detected by EP (Ni et al., GCN 45261), using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observation started at 2026-07-31 03:16:25.90 UT, i.e., 2.37 hours post-trigger, and a series of z-band frames were obtained. The target was 16 deg from the moon at 98% illumination at the time of the observation.

No uncatalogued optical transient is detected in the stacked image within the EP-FXT error circle (Ni et al., GCN 45261) down to a 5-sigma limiting magnitude of z ~ 19.8 at a median time of 2.46 hr post-trigger, calibrated with Pan-STARRS DR2 stars in the field.

K.R. Ni (CCNU) , Y.F. Liang (PMO, CAS), H.Y. Liu, and 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 EP260731a. The transient triggered EP-WXT (ID: 01709277303) at 2026-07-31T00:54:24 (UTC). The WXT position of the source is R.A. = 344.424 deg, DEC = -11.764 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).

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. = 344.4279 deg, DEC = -11.7623 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).