Faintest Shock Breakout in Broad-Lined Ic Supernova Hints at Choked Jets and Wider Diversity in Stellar Collapse

The arXiv preprint (v1, 8 Jun 2026) details EP260321a at z=0.0344, identified via Einstein Probe as a thermal X-ray transient (kT=160 eV, L_peak=2.2e44 erg/s) interpreted as shock breakout, paired with SN 2026gzf showing typical broad-lined Type Ic traits: expansion velocities ~20,000 km/s, light-curve shape, and spectral evolution matching GRB-linked events. Multi-wavelength data from optical, X-ray (Chandra non-detection), and radio exclude standard GRB afterglows, requiring any jet to have Gamma_0<30 and E_kin<1e49 erg for A_*>=1, favoring a mildly relativistic choked outflow. This single-event study (no statistical sample) bridges SN 2008D's faint breakout and low-luminosity GRBs but relies on model assumptions for wind density and jet parameters; as a preprint it awaits peer review and could shift with refined progenitor modeling. Related analyses in Nature 2009 (Soderberg et al. on SN 2008D X-ray breakout, n=1 event) and ApJ 2020 (Margutti et al. on llGRB diversity) reveal the pattern missed here: EP260321a implies stripped-envelope progenitors span a continuum of outflow energies rather than discrete jet success/failure, potentially explaining absent gamma-rays in many Ic-BL events via envelope choking. Limitations include reliance on one nearby transient and lack of direct jet imaging, underscoring need for future Einstein Probe statistics.