A Multiwavelength View of the First Type Ic-BL Supernova with an Einstein Probe X-Ray Shock Breakout

Abstract In 2026 March, the Einstein Probe (EP) discovered its nearest ( z = 0.0343) Fast X-ray Transient (FXT), EP 260321a, the first EP FXT to provide a strong match to expectations for X-ray “shock breakout” (SBO) emission. Here, we present our multiwavelength follow-up campaign of EP 260321a and its broad-line Type Ic (Ic-BL) supernova (SN) counterpart, SN 2026gzf. We show that our radio follow-up extending over 5.8–54.5 days post-FXT rules out an on-axis jet counterpart of isotropic-equivalent kinetic energy E K ≳ 10 49 erg for circumburst densities n > 10 −2 cm −3 and assuming microphysical parameters ϵ e = ϵ B = 0.1. Our radio data also constrain a median mass-loss rate of M ̇ ≲ 1.2 × 1 0 − 5 M ⊙ yr − 1 for a Wolf–Rayet progenitor. In addition, we derive SN 2026gzf’s properties, including 56 Ni mass, diffusion timescale, and expansion velocities, from our ∼nightly cadence optical data and compare them with those of optically discovered Type Ic-BL SNe, finding that SN 2026gzf is well within the 90% confidence interval across all properties. We further fit SN 2026gzf’s light curve and determine that combined emission from both interaction with CSM and 56 Ni radioactive decay provides the best fit with plausible model parameters. Finally, using the rate of Ic-BL SNe from the Zwicky Transient Facility Bright Transient Survey and assuming all Type Ic-BL SNe produce EP 260321a-like FXTs, we infer an expected rate of EP-detected SBOs of 4.4–16 yr −1 . This is inconsistent at the 90% confidence level with current EP detection rates, potentially indicating that most Type Ic-BL SNe produce less luminous X-ray SBO signals compared to EP 260321a.