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 $z=5.017\pm 0.001$, 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 $\lesssim 12$ rest-frame days, as inferred from the last non-detection. AT 2026abck has nearly identical magnitudes of $\sim 27.4$ mag in F150W, F200W, and F277W, and is undetected in F444W ($>27.9$ mag, 3$\sigma$). It is the first robustly detected $z\gtrsim 5$ transient in a non-lensing field. A blackbody fit yields a hot photosphere ($\sim 1.7\times {10}^4\mathrm{K}$) with a radius of $\sim 7\times {10}^{14}$ cm ($\sim 46$ au) and a bolometric luminosity of $\sim 3\times {10}^{43}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$. 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 $\sim 4$-10 rest-frame days after explosion. The host is an intermediate-mass ($M_{\ast }\approx {10}^{9.8}{M}_{\odot }$), star-forming ($\mathrm{S}\mathrm{F}\mathrm{R}\approx 20M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$) galaxy on the $z\approx 5$ main sequence, $\sim 3.3$ 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.