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

The NEXUS JWST treasury program's Year-2 observations yielded the discovery of AT 2026abck, a young transient in a star-forming galaxy at z = 5.017 ± 0.001. It is the first robustly detected z≳5 transient found in a non-lensing field.

AT 2026abck was detected on 2026 Aug 7 in NIRCam F200W imaging, with a contemporaneous detection from parallel NIRCam F150W+F277W imaging. It brightened by >1.4 mag in F200W over ≲12 rest-frame days, as inferred from its last non-detection. The transient shows nearly identical magnitudes of ~27.4 mag in F150W, F200W, and F277W, and is undetected in F444W (>27.9 mag, 3σ).

A blackbody fit yields a hot photosphere of ~1.7×10^4 K, a radius of ~7×10^14 cm (~46 au), and a bolometric luminosity of ~3×10^43 erg/s. Its off-nucleus location and hot, UV-bright spectral energy distribution favor a core-collapse supernova origin, while its temperature and luminosity exceed those of normal SNe II at comparable early phases. The team estimates AT 2026abck was detected ~4–10 rest-frame days after explosion.

The host is an intermediate-mass (M_* ≈ 10^9.8 M_⊙), star-forming (SFR ≈ 20 M_⊙/yr) galaxy on the z≈5 main sequence, about 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 core-collapse supernovae 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 supernovae during their early, UV-bright phase.