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# EP260321a/SN 2026gzf: The Faintest Shock Breakout Associated with a Broad-lined Supernova

## Details

**Authors** O'Connor et al.

**Year** 2026

**Publisher** The Astrophysical Journal Letters

**Discipline** Astronomy

[Read it at the publisher](https://doi.org/10.3847/2041-8213/ae84ba) 
10.3847/2041-8213/ae84ba

## In authors' words

### Abstract

The explosion of a star is first marked by the shock wave breaking out of the stellar surface, producing a burst of ultraviolet and X-ray radiation. These events are observationally rare, despite likely accompanying the majority of supernovae (SNe). Here, we report on our multiwavelength observing campaign of the closest Einstein Probe fast X-ray transient (FXT) EP260321a at z = 0.0344. The thermal ( kT = 130 eV) X-ray emission with peak luminosity of 1.0 × 10 45 erg s −1 points to a shock breakout origin. We demonstrate that EP260321a is accompanied by a broad-lined Type Ic supernova, SN 2026gzf. The supernova (SN) properties, including its spectral evolution, lightcurve evolution, and expansion velocities, are all typical of the energetic stripped-envelope SNe associated with gamma-ray bursts (GRBs). However, deep X-ray upper limits obtained with the Chandra X-ray Observatory do not detect an X-ray afterglow, and instead exclude the afterglow of known GRBs or FXTs. If the stellar explosion launched a successful relativistic jet, we require that it had both a low Lorentz factor of Γ 0 < 30 and a kinetic energy of E kin < 10 49 erg for a stellar wind density of A * ≳ 1. We propose that EP260321a originated from a mildly relativistic, weak outflow that was choked by the progenitor star. This scenario is capable of naturally explaining its low X-ray luminosity and lack of prompt gamma-ray emission. EP260321a bridges the gap between SN 2008D and low-luminosity GRBs, suggesting a greater diversity in the physical parameters of stripped stars as they undergo terminal collapse.

### What they found (results)

X-ray and optical follow-up of supernova SN 2026gzf detected the faintest X-ray shock breakout ever recorded for a broad-lined Type Ic supernova, with no accompanying gamma-ray burst or high-velocity jet, unlike prior members of this class.

## Commentary

### In short

The finding turns on a category boundary: the event is classified within or outside the established broad-lined Type Ic supernova class based on the presence or absence of a relativistic jet signature, revealing a previously unrecognized subtype.

**Patterns it shows** D

**Added** 2026-08-14

**How to cite this** O'Connor et al. (2026). EP260321a/SN 2026gzf: The Faintest Shock Breakout Associated with a Broad-lined Supernova. The Astrophysical Journal Letters.