EP260605a

Rosa L. Becerra (UNAM), Massimiliano Lincetto (CPPM), Asuka Kuwata (UNAM), Sarah Antier (IJCLAB), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), 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), 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), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report:

We imaged the field of the EP260605a (Li et al., GCN Circ. 44847
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 ) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-06 07:27 to 08:05 UTC (from 21.93 to 22.59 hours after the trigger) and obtained 29 minutes of simultaneous exposure in the r and z filters.

Given the proximity of the localization region to the Galactic bulge, we obtained additional observations to serve as templates. The field was reimaged on 2026-06-12 from 07:06 to 08:06 UTC (from 6.90 to 6.94 days after the trigger), yielding 42 minutes of simultaneous exposure in the r and z filters.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the SkyMapper catalog, is in the AB system, and is not corrected for Galactic extinction.

In the image subtraction, we do not detect any new source within the FXT error region (Li et al., GCN Circ. 44867
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 ) down to the following 5-sigma limit:

r > 21.4
z > 20.6

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.

A.Li (BNU), C. Y. Wang (THU), Z.-C. Zou (NJU), X. H. Tang (THU), H. Sun (NAO, CAS) on behalf of the Einstein Probe (EP) team:
 
The fast X-ray transient EP260605a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 44847
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 ) at 2026-06-05T09:31:00 (UTC). 
 
Follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2026-06-06T10:39:36(UTC), ~25 hours after the WXT detection. The exposure time of the observation is around 6 ks. The on-ground analysis of the FXT data found an uncatalogued source within the error circle of WXT at R.A. = 273.298 deg, DEC = -40.3945 deg (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic) . The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.03 x 10^21 cm^-2, and a photon index of 3.3 (-0.6, +0.5). The derived average unabsorbed 0.5-10 keV flux is 1.1 (-0.4, +0.9) ×10^(-13) erg/s/cm^2. The uncertainties are at the 90% 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).

A.Li (BNU), C. Y. Wang (THU), Z.-C. Zou (NJU), X. H. Tang (THU), H. Sun (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 EP260605a. The transient was identified during ground-based telemetry analysis. The WXT light curve started at 2026-06-05T09:31:00 (UTC). The WXT position of the source is R.A. = 273.360 deg, DEC = -40.392 deg (J2000) with an uncertainty of 3.2 arcmin in radius (90% C.L. statistical and systematic).

The duration of the event is around 500 s. The WXT spectrum can be fitted with an absorbed power-law, with the absorption fixed at the Galactic value of 1.03 x 10^21 cm^-2, and a photon index of 1.76 (-0.58, +0.61). The unabsorbed 0.5-4 keV flux is 4.3 (-1.3, +1.6) x 10^-11 erg/s/cm2. All uncertainties are reported in 90% C.L.

We have arranged follow-up observation with the Follow-up X-ray Telescope onboard EP and will update the information upon receiving the data.

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).