COLIBRI discovery of a new optical transient

ATel #17589; Alan M. Watson (UNAM), Diego Lopez-Camara (UNAM), Ny Avo Rakotondrainibe (LAM), Noemie Globus (UNAM), Stephane Basa (UAR Pytheas), William H. Lee (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-Gregoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo Garcia Garcia (UNAM), Ramandeep Gill (UNAM), Nikos Mandarakas (LAM), Kin Ocelotl Lopez (UNAM), Francesco Magnani (CPPM), Enrique Moreno Mendez (UNAM), Margarita Pereyra (UNAM), Fredd Sanchez Alvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM)
on 10 Jan 2026; 09:57 UT
Credential Certification: Noemie Globus (globus@astro.unam.mx)

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We report the discovery of a new optical transient, 2025alij, at RA = 06:57:45.53, Dec = +23:22:18.2 (J2000; uncertainty ~0.2"). This source was discovered during the follow-up of the Einstein Probe alert EP251221a, but is located well outside the WXT error box, so we consider them unrelated. We initially observed from 2025-12-22 03:04 to 13:21 UTC and obtained 170 minutes of exposure in the r. The source was discovered at a preliminary magnitude of r = 21.38 +/- 0.01, using the DDRAGO two-channel wide-field imager on the COLIBRI telescope. The limiting magnitude of the image was r = 23.71 (10 sigma). We re-observed on 2026-01-08 from 04:57 to 08:43 UTC and obtained 30 minutes of exposure. The source is still present in images with a preliminary magnitude of r = 21.74 +/- 0.07. The data were reduced and coadded with the COLIBRI 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. At the position of the source, we note the presence of an underlying faint object in the public archive database of CFHT, which we identify as a potential host galaxy. No known minor planet is consistent with the position. Due to its brightness well beyond the archival images and its fading, we classify the object as an astronomical transient. We encourage follow-up at all wavelengths. We thank the staff of the Observatorio Astronomico Nacional on the Sierra de San Pedro Martir and the COLIBRI and DDRAGO engineering teams. COLIBRI 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 Astronomico Nacional on the Sierra de San Pedro Martir, Baja California, Mexico.

Discovery certificate for object 2025alij