NEXUS: Discovery of a Luminous Young Transient at z=5

We report the discovery of AT 2026abck, a young transient in a star-forming galaxy at z=5.017 plus or minus0.001, identified from Year-2 observations of the NEXUS JWST treasury program. AT 2026abck was detected on 2026 Aug 7 in NIRCam F200W imaging, with contemporaneous detection from parallel NIRCam F150W+F277W imaging. It brightened by >1.4 mag in F200W over 12 rest-frame days, as inferred from the last non-detection. AT 2026abck has nearly identical magnitudes of about27.4 mag in F150W, F200W, and F277W, and is undetected in F444W (>27.9 mag, 3 σ). It is the first robustly detected z 5 transient in a non-lensing field. A blackbody fit yields a hot photosphere (about1.7 times10 4, K) with a radius of about7 times10 14 cm (about46 au) and a bolometric luminosity of about3 times10 43, erg, s-1. Its off-nucleus location and hot, UV-bright SED favor a core-collapse supernova origin, while its temperature and luminosity exceed those of normal SNe II at comparable early phases. We estimate that AT 2026abck was detected about4-10 rest-frame days after explosion. The host is an intermediate-mass (M* about10 9.8,M), star-forming (SFR about20,M, yr-1) galaxy on the z about5 main sequence, about3.3 dex more massive than the ultra-faint host of the lensed SN Eos at z=5.13. Together, the two events show that CCSNe at z>5 occur across a wide range of host environments, and AT 2026abck demonstrates that high-cadence blank-field JWST surveys can capture high-redshift SNe during their early, UV-bright phase.