EP260527a — All Circulars

Edilberto Aguilar-Ruiz (UNAM), Francesco Magnani (CPPM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), 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), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Benjamin Schneider (LAM) and report:

We imaged the field of the EP260527a  (J. Yang et al, GCN Circ. 44718) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-28 04:13 to 06:05 UTC (from 21.90 to 23.76 hours after the trigger) and obtained 18, 62, and 82 minutes of exposure in the g, r, and z filters.

The data were reduced and coadded with the COLIBRÍ ASU 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 images, we detect an uncatalogued source within the  FXT uncertainty region at:

RA (J2000) = 12:52:13.13 = 193.05473 degrees
Dec (J2000) = +01:28:56.8 = 1.48244 degrees

The preliminary magnitudes derived for that source is:

g = 22.25 +/- 0.32,
r = 22.19 +/- 0.17,
z = 21.52 +/- 0.15.

The detection in g suggests that it is not a high-redshift event. 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.

Edilberto Aguilar-Ruiz (UNAM), Francesco Magnani (CPPM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), 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), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Benjamin Schneider (LAM) and report:

We imaged the field of the EP260527a  (J. Yang et al, GCN Circ. 44718) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-28 04:13 to 06:05 UTC (from 21.90 to 23.76 hours after the trigger) and obtained 18, 62, and 82 minutes of exposure in the g, r, and z filters.

The data were reduced and coadded with the COLIBRÍ ASU 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 images, we detect an uncatalogued source within the  FXT uncertainty region at:

RA (J2000) = 12:52:13.13 = 193.05473 degrees
Dec (J2000) = +01:28:56.8 = 1.48244 degrees

The preliminary magnitudes derived for that source is:

g = 22.25 +/- 0.32,
r = 22.19 +/- 0.17,
z = 21.52 +/- 0.15.

The detection in g suggests that it is not a high-redshift event. 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.

J. Yang (ZZU), B. T. Wang (YNAO, CAS), D. F. Hu (PMO, CAS), J. P. Feng, B. Zhang (USTC), D. Zhu, K. J. Zhang (YNU) and W. D. Zhang (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 EP260527a. The transient triggered EP-WXT (ID: 01709266119) at 2026-05-27T06:19:54 (UTC). The WXT position of the source is R.A. = 193.057 deg, DEC = 1.473 deg (J2000) with an uncertainty of 2.3 arcmin in radius (90% C.L. statistical and systematic). The preliminary analysis of the WXT data shows that the event started at 2026-05-27T06:16:18 (UTC) and lasted for about 200 seconds. The average WXT 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with a fixed Galactic hydrogen column density of 1.57 x 10^20 cm^-2 and a photon index of 0.73(+0.44/-0.42). The unabsorbed 0.5-4.0 keV flux is 1.12(+0.33/-0.28) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

This event did not trigger the Follow-up X-ray Telescope (FXT) autonomous follow-up. A follow-up observation with the FXT was performed about 20.5 hours after the WXT detection. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 193.0519 deg, DEC = 1.4832 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 are 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).