A Multi-Wavelength View of the First Type Ic-BL Supernova with an Einstein Probe X-ray Shock Breakout

Abstract

In March 2026, the Einstein Probe (EP) discovered its most nearby (z = 0.0343) Fast X-ray Transient (FXT), EP260321a, the first EP FXT to provide a strong match to expectations for X-ray "shock breakout'" (SBO) emission. Here, we present our multi-wavelength follow-up campaign of EP260321a and its broad-line Type Ic (Ic-BL) supernova (SN) counterpart, SN2026gzf. 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 $E_K\gtrsim 10^{49}$ erg for circumburst densities $n>10^{-2}{\mathrm{cm}}^{-3}$ and assuming microphysical parameters $\hat{I}{\mu }_e=\hat{I}{\mu }_B=0.1$. Our radio data also constrains a median mass-loss rate of $\dot{M}\lesssim 1.2\times 10^{-5}{M}_{\odot }{\mathrm{yr}}^{-1}$ for a Wolf-Rayet progenitor. In addition, we derive SN2026gzf's properties, including ${}^{56}$Ni mass, diffusion timescale, and expansion velocities, from our $\sim$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 SN2026gzf'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 ZTF Bright Transient Survey and assuming all Type Ic-BL SNe produce EP260321a-like FXTs, we infer an expected rate of EP-detected SBOs of 4.4 - 16 year${}^{-1}$. 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 EP260321a.

Version accepted to ApJL with minor changes

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