Kavli Affiliate: Nathan P. Lourie
| Summary:
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 $n > 10^-2~rm cm^-3$ and assuming microphysical parameters $ε_e = ε_B = 0.1$. Our radio data also constrains a median mass-loss rate of $dotM lesssim 1.2 times 10^-5 M_odot~rm yr^-1$ 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.
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