TITLE: GCN CIRCULAR NUMBER: 44076 SUBJECT: EP260321a: COLIBRÍ optical upper limit on any supernova shock breakout emission DATE: 26/03/22 07:01:20 GMT FROM: Rosa L. Becerra at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx> Edilberto Aguilar-Ruiz (UNAM), Antonio de Ugarte Postigo (LAM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), 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), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Benjamin Schneider (LAM) report: We imaged the field of the Einstein Probe X-ray transient EP260321a (Huang et al., GCN Circ. 44068) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-22 03:48 to 05:50 UTC (from 15.3 to 17.4 hours after the trigger) and obtained 24, 65, and 89 minutes of exposure, respectively, in the g, r, and z filters. The data were reduced and coadded with the COLIBRÍ ASU pipeline and analysed with STDWeb/STDPipe (Karpov 2025). 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 image, and after subtracting template images from the Legacy Survey DR10 (Dey et al. 2019) using STDWeb, we do not detect any new source at the FXT refined position (Huang et al., GCN Circ. 44075) down to the following 5-sigma limit: g > 23.8 r > 23.8 z > 22.8 Our upper limit in r is consistent with the ones reported by Lipunov et al. (GCN Circ. 44069) and Lee et al. (GCN Circ. 44070). We do see additional flux from the superposed star, also in agreement with Lee et al. In order to test the shock breakout scenario proposed by Huang et al. (GCN Circ. 44075), we compared our results against the light curve of the AT2016gkg (Bersten et al. 2018), shifted at the redshift of the galaxy 2dFGRS TGN352Z077 at z=0.034 (DESI Legacy Survey, Dey et al. 2019). At the time of our observations, we expect an AB magnitude about 19.2 at 570 nm, which lies between our g and r filters. Our upper limits are about 4.6 magnitudes fainter than this, and so we are unable to confirm the presence of a shock breakout. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44076. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...