SN 2026gzf: Ic-BL Supernova & X-ray Breakout

• SN 2026gzf is a nearby broad-lined Type Ic supernova characterized by its thermal X-ray shock breakout and serves as a benchmark between classic Ic-BL explosions and low-luminosity GRB events.
• Extensive multi-wavelength follow-up, including rapid optical imaging and spectroscopy, provided precise constraints on explosion timing, ejecta velocities, and circumstellar material interactions.
• Spectropolarimetric studies reveal nearly spherical outer ejecta with pronounced Ca II line asphericity, supporting complex geometries and the possibility of a failed or choked jet.

SN 2026gzf is the optical supernova associated with the Einstein Probe fast X-ray transient EP260321a, and is identified across the 2026 follow-up literature as a nearby broad-lined Type Ic supernova (Ic-BL) linked to a thermal X-ray shock breakout at $z \approx 0.0343$–0.0345, corresponding to a luminosity distance of about $156$–$158$ Mpc under the adopted cosmologies (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026, Chen et al., 8 Jun 2026). The event is notable because the prompt high-energy signal is soft and thermal, the supernova itself is otherwise a fairly typical energetic Ic-BL in its optical properties, and late-time X-ray and radio observations found no detectable afterglow. For that reason, SN 2026gzf has been treated as a bridge object between the thermal shock-breakout class exemplified by SN 2008D and the broader population of low-luminosity GRB/Ic-BL explosions (O'Connor et al., 8 Jun 2026).

1. Discovery, identification, and observing record

The transient sequence began with EP260321a, the prompt Einstein Probe X-ray event, followed by the emergence of SN 2026gzf at the same location over the next hours to days. The papers quote several closely related X-ray times, including WXT detection at 2026-03-21 12:23:07 UTC, trigger at 2026-03-21 12:30:18 UTC, and a refined onset time $T_0=$ 2026-03-21 12:16:08 UTC; these correspond to different operational definitions of onset, detection, and trigger in the X-ray analyses (Rastinejad et al., 8 Jun 2026, Martin-Carrillo et al., 8 Jun 2026, Wen et al., 17 Jun 2026, Chen et al., 8 Jun 2026). Optical follow-up began unusually rapidly: Lulin observations started 1.25 hr after the X-ray trigger, the optical counterpart was detected by ZTF at $\delta t=0.78$ d, and FTW photometry began at 0.41 d after trigger (Chen et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026, O'Connor et al., 8 Jun 2026).

The association between the X-ray flash and the supernova was established through positional coincidence, early blue optical emission, re-brightening into a supernova, and subsequent spectroscopy. Follow-up campaigns were extensive and multi-instrument. Optical and near-IR photometry came from facilities including FTW, DECam/Blanco, ZTF, Rubin/LSST commissioning, IO:O/Liverpool Telescope, WINTER, Lulin Observatory, Swope, Pan-STARRS2, ATLAS, and Citra. Spectroscopy was obtained with SALT/RSS, HET/LRS2, DESI, Gemini/GMOS-N and GMOS-S, Palomar/NGPS, SOAR/Goodman, Magellan/FIRE, VLT/MUSE, NOT/ALFOSC, ANU/WiFeS, LOT/UVEX, Keck/NIRES, and IRTF/SpeX. High-energy and radio follow-up included Einstein Probe WXT/FXT, \textit{Chandra}/ACIS-S, the VLA, and additional uGMRT and ATCA reporting (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026, Martin-Carrillo et al., 8 Jun 2026, Chen et al., 8 Jun 2026).

This unusually dense temporal coverage is central to the literature on SN 2026gzf. Unlike most Ic-BL supernovae, the explosion time is constrained directly by the X-ray transient, and the first optical data probe the transition from breakout and cooling into the ordinary supernova rise. A plausible implication is that SN 2026gzf has become a benchmark case for separating prompt breakout emission, early interaction-powered light, and the later radioactive supernova component (Martin-Carrillo et al., 8 Jun 2026, Chen et al., 8 Jun 2026).

2. EP260321a and the shock-breakout interpretation

The prompt X-ray transient is consistently described as soft and thermal. One analysis reports a blackbody temperature $kT=164^{+40}_{-29}\,\mathrm{eV}$ and a time-averaged unabsorbed luminosity $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$, while another gives $E_p=0.164^{+0.040}_{-0.029}$ keV for WXT and $E_p=0.121^{+0.03}_{-0.03}$ keV for FXT, with average unabsorbed $0.5\!-\!4$ keV and $156$0 keV fluxes of $156$1 and $156$2, respectively (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026). The WXT event lasted about 432 s in the Einstein Probe analyses, and the literature repeatedly emphasizes that the emission is much softer than classical GRB prompt emission and lacks the additional nonthermal or power-law component seen in low-luminosity GRBs such as GRB 060218 and GRB 100316D (Rastinejad et al., 8 Jun 2026, Martin-Carrillo et al., 8 Jun 2026).

These properties are the main basis for the shock-breakout interpretation. The high-energy signal is combined with very early optical cooling behavior and the subsequent appearance of an Ic-BL supernova. One paper further notes that the duration is longer than the $156$3 s light-crossing time of a Wolf-Rayet star, suggesting either explosion asymmetry or breakout in extended dense material outside the star; another presents dense-CSM breakout scalings with $156$4 s, $156$5, and $156$6, yielding $156$7 cm and $156$8 (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026). By contrast, other papers summarize the X-ray and early optical data as favoring compact circumstellar material extending to roughly $156$9, or $158$0 cm (Wen et al., 17 Jun 2026, Chen et al., 8 Jun 2026).

The published breakout scale is therefore model dependent. What is not model dependent is the observational classification of EP260321a as an unusually soft, intrinsically faint extragalactic fast X-ray transient associated with a stripped-envelope explosion. In the 2026 literature it is repeatedly described as among the least luminous Einstein Probe FXTs, and in one paper as “by far the softest and least luminous event among all extragalactic fast X-ray transients detected by EP” (Rastinejad et al., 8 Jun 2026, Wen et al., 17 Jun 2026).

3. Optical and spectroscopic properties of the supernova

SN 2026gzf is classified as a Type Ic-BL because its spectra show broad, high-velocity absorption features characteristic of stripped-envelope broad-lined explosions. Early spectra display a blue continuum with broad troughs near $158$1 and $158$2 Å, later strengthening into features around $158$3 and $158$4 Å identified with blueshifted Fe II $158$5 and Si II $158$6. Later DESI and other spectra reveal a broad trough near $158$7 Å interpreted as the Ca II triplet, with possible contributions from O I $158$8 and Mg II $158$9 in the $T_0=$0–$T_0=$1 Å region (O'Connor et al., 8 Jun 2026). Across the optical sequence, the spectra show no obvious H or He absorption in one analysis, supporting a clean stripped-envelope classification, although other work discusses a possible blended He I/Na I D absorption from $T_0=$2 d and TARDIS models containing a small He mass of $T_0=$3; the event remains classified as Ic-BL rather than Type Ib in all studies (Rastinejad et al., 8 Jun 2026, Chen et al., 8 Jun 2026, Martin-Carrillo et al., 8 Jun 2026).

The expansion velocities are high by the standards of stripped-envelope supernovae. From Fe II $T_0=$4, one paper infers $T_0=$5 at 3.6 d, declining to $T_0=$6 by 11.3 d and remaining near that level to $T_0=$7 d; from Si II $T_0=$8, the velocity declines from $T_0=$9 at 3.6 d to $\delta t=0.78$0 by 32.6 d (O'Connor et al., 8 Jun 2026). Another study measures $\delta t=0.78$1 from Fe II $\delta t=0.78$2 at $\delta t=0.78$3 d, while a third summarizes an initial expansion velocity of $\delta t=0.78$4 declining as a power law with index $\delta t=0.78$5 over the first 60 d (Rastinejad et al., 8 Jun 2026, Martin-Carrillo et al., 8 Jun 2026). These values place the object squarely in the Ic-BL regime and, at early epochs, near the upper envelope of the GRB-SN velocity distribution (O'Connor et al., 8 Jun 2026).

Photometrically, the supernova is luminous but not exceptional within the Ic-BL population. Peak optical brightness is reported as roughly $\delta t=0.78$6 to $\delta t=0.78$7 mag, with a rest-frame rise time of about 14.2–15 d (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026). One study fits the multiband light curves with the Taddia et al. empirical function and obtains $\delta t=0.78$8 d, $\delta t=0.78$9 d, $kT=164^{+40}_{-29}\,\mathrm{eV}$0 d, and $kT=164^{+40}_{-29}\,\mathrm{eV}$1 d, together with $kT=164^{+40}_{-29}\,\mathrm{eV}$2 mag (O'Connor et al., 8 Jun 2026). Another finds a low-order polynomial $kT=164^{+40}_{-29}\,\mathrm{eV}$3-band peak of $kT=164^{+40}_{-29}\,\mathrm{eV}$4 mag at $kT=164^{+40}_{-29}\,\mathrm{eV}$5 d and concludes that SN 2026gzf lies well within the $kT=164^{+40}_{-29}\,\mathrm{eV}$6 distribution of optically discovered Type Ic-BL supernovae (Rastinejad et al., 8 Jun 2026).

The first-day optical behavior is atypical even though the later supernova resembles an ordinary Ic-BL. Lulin data beginning 1.25 hr after the trigger show a rapid first-night brightening of $kT=164^{+40}_{-29}\,\mathrm{eV}$7–0.66 mag in $kT=164^{+40}_{-29}\,\mathrm{eV}$8 over $kT=164^{+40}_{-29}\,\mathrm{eV}$9–5 hr, with an early color $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$0 mag (Chen et al., 8 Jun 2026). The first-hours emission is more than 1 mag brighter than expected from a hydrodynamic model powered only by $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$1Ni, and radioactive models fit the main peak only after excluding the first $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$2 days (Chen et al., 8 Jun 2026). This early luminous blue excess is one of the major reasons why circumstellar interaction features prominently in the published interpretations.

4. Explosion models and circumstellar material

The physical parameters inferred for SN 2026gzf vary substantially across studies because the adopted models differ. One paper fits a semi-analytic Arnett model to the bolometric light curve using a photospheric velocity of $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$3 and obtains $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$4, $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$5, $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$6, and $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$7, while cautioning that shock heating or circumstellar interaction could make the quoted nickel mass an upper limit (O'Connor et al., 8 Jun 2026). Another study fits five Redback light-curve models and finds that the physically constrained combined CSM+$$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$8Ni prescription is preferred, yielding $$L_{\rm X}\approx2.2\times10^{44}\,\mathrm{erg\,s^{-1}}$$9, $E_p=0.164^{+0.040}_{-0.029}$0, $E_p=0.164^{+0.040}_{-0.029}$1, $E_p=0.164^{+0.040}_{-0.029}$2, $E_p=0.164^{+0.040}_{-0.029}$3, $E_p=0.164^{+0.040}_{-0.029}$4, $E_p=0.164^{+0.040}_{-0.029}$5, and $E_p=0.164^{+0.040}_{-0.029}$6 d (Rastinejad et al., 8 Jun 2026).

A third paper advances a more massive failed-jet breakout scenario and prefers a CSM+SN model with $E_p=0.164^{+0.040}_{-0.029}$7, $E_p=0.164^{+0.040}_{-0.029}$8, $E_p=0.164^{+0.040}_{-0.029}$9, $E_p=0.121^{+0.03}_{-0.03}$0, $E_p=0.121^{+0.03}_{-0.03}$1, and $E_p=0.121^{+0.03}_{-0.03}$2, with a CSM density exponent $E_p=0.121^{+0.03}_{-0.03}$3–1.9 (Martin-Carrillo et al., 8 Jun 2026). A fourth uses rapid optical data and hydrodynamic STELLA calculations to argue for ejecta interaction with $E_p=0.121^{+0.03}_{-0.03}$4 of CSM at $E_p=0.121^{+0.03}_{-0.03}$5 cm, with a wind-like density profile

$E_p=0.121^{+0.03}_{-0.03}$6

and an inferred mass-loss rate of $E_p=0.121^{+0.03}_{-0.03}$7 during the final 3.5 days before explosion for an assumed wind speed of $E_p=0.121^{+0.03}_{-0.03}$8 (Chen et al., 8 Jun 2026).

Study	Preferred interpretation	Representative parameters
(O'Connor et al., 8 Jun 2026)	Arnett radioactive fit with caveat about interaction	$E_p=0.121^{+0.03}_{-0.03}$9, $0.5\!-\!4$0, $0.5\!-\!4$1 erg
(Rastinejad et al., 8 Jun 2026)	Physically constrained combined CSM+$0.5\!-\!4$2Ni	$0.5\!-\!4$3, $0.5\!-\!4$4, $0.5\!-\!4$5 erg, $0.5\!-\!4$6 AU
(Martin-Carrillo et al., 8 Jun 2026)	Failed jet breakout with compact circumstellar shell	$0.5\!-\!4$7, $0.5\!-\!4$8, $0.5\!-\!4$9 erg, $156$00
(Chen et al., 8 Jun 2026)	Early luminous blue excess from ejecta–CSM interaction	$156$01, $156$02 cm, $156$03

The common conclusion across these otherwise different models is that the first-day emission is difficult to explain with a centrally concentrated one-zone radioactive light curve alone. The main disagreement concerns how much circumstellar interaction is required, how much ejecta mass is present, and whether the broader event should be understood primarily as a weak shock breakout in structured material, a combined CSM+$156$04Ni supernova, or a failed-jet explosion with a compact shell (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026, Martin-Carrillo et al., 8 Jun 2026, Chen et al., 8 Jun 2026).

5. Relativistic-outflow constraints and three-dimensional geometry

Late-time X-ray and radio observations provide the strongest constraints on any relativistic outflow. \textit{Chandra} observed at 15.4 d for 19.81 ks and at 39.0 d for 59.1 ks, detecting 0 photons within $156$05 in either epoch. For a standard afterglow power law with photon index $156$06, the $156$07 upper limits on unabsorbed $156$08–$156$09 keV flux are $156$10 and $156$11, corresponding to $156$12 at 158 Mpc (O'Connor et al., 8 Jun 2026). In radio, one paper quotes a VLA $156$13 limit of $156$14 at 59.5 d, while another reports multiepoch $156$15–$156$16 GHz measurements between 5.7 and 54.5 d, including a faint $156$17 source in nearly all 6 GHz images with integrated flux $156$18 that is offset by 0.197″ from the SN position and interpreted as underlying star formation rather than a counterpart (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026).

Under these limits, a normal on-axis GRB-like afterglow is excluded in all published analyses, but the precise quantitative bound depends on the adopted outflow model. A wind-medium Gaussian structured-jet analysis with VegasAfterglow finds that for $156$19, $156$20, $156$21, $156$22, and $156$23, the allowed region is $156$24 and $156$25 for $156$26; that study states that essentially all previously known long and short GRBs, XRFs, and EP FXTs are excluded except the unusually faint off-axis afterglow of GRB 170817A (O'Connor et al., 8 Jun 2026). A constant-density ISM analysis with Redback/vegasafterglow rules out an on-axis jet counterpart of isotropic-equivalent kinetic energy $156$27 erg for $156$28 under standard microphysical assumptions, but allows an off-axis jet, a structured jet, or unusually low $156$29 and $156$30 (Rastinejad et al., 8 Jun 2026). A related study concludes that a hidden jet can survive only if the viewing angle is $156$31 for a weak EP250304a-like afterglow or $156$32 for a more typical GRB-like afterglow (Martin-Carrillo et al., 8 Jun 2026).

These constraints are the basis for the preferred dynamical interpretations. One paper argues for “a mildly relativistic, weak outflow that was choked by the progenitor star” (O'Connor et al., 8 Jun 2026). Another describes the event as a failed jet breakout in a $156$33 circumstellar shell (Martin-Carrillo et al., 8 Jun 2026). A third adopts a more cautious intermediate view: an on-axis standard-microphysics GRB afterglow is ruled out, but a weak, off-axis, or microphysically inefficient jet remains possible (Rastinejad et al., 8 Jun 2026).

Optical polarimetry adds an independent geometric constraint. Imaging polarimetry at day 4.6 gives broad-band Stokes values consistent with zero, and day-16.5 spectropolarimetry finds continuum values $156$34 and $156$35, implying continuum polarization at the $156$36 level or lower (Wen et al., 17 Jun 2026). In the Thomson-scattering interpretation with $156$37, this corresponds to a projected axial ratio of about $156$38, so the outer ejecta are nearly spherical. By contrast, the Ca II near-IR triplet shows a line-polarization peak of about $156$39, with polarization maxima near $156$40 and $156$41, a primary component spanning $156$42–$156$43, and a distinct secondary component extending above $156$44 (Wen et al., 17 Jun 2026).

The spectropolarimetric interpretation is therefore not of a globally ellipsoidal supernova, but of a nearly spherical photosphere containing a strongly aspherical Ca II line-forming region. A 3D Monte Carlo opacity model with two enhanced cone-like Ca II components, opening half-angle $156$45, enhancement factor $156$46, and misaligned symmetry axes $156$47 and $156$48 can plausibly reproduce the line profile and polarization for a viewing angle of $156$49 from the primary symmetry axis (Wen et al., 17 Jun 2026). This suggests that axisymmetric internal excitation structure and nearly spherical outer ejecta can coexist in an Ic-BL explosion associated with a soft X-ray breakout.

6. Host environment, precursor activity, and broader significance

SN 2026gzf occurred in an actively star-forming, low-metallicity environment. One study places the explosion $156$50, or 2.5 kpc, from the host center and finds a local metallicity $156$51–7.95, lower than the host center value of $156$52 (O'Connor et al., 8 Jun 2026). Another measures $156$53 at the SN site and $156$54 for the host-integrated value, resolving the star-forming complex into two knots and placing the SN $156$55 pc from the centroid of the unresolved blue knot (Martin-Carrillo et al., 8 Jun 2026). A third quotes host and local metallicities of $156$56 and $156$57, corresponding to $156$58 (Chen et al., 8 Jun 2026). Although the absolute abundance scale differs across methods, the published papers agree that the system is low mass, subsolar in metallicity, and strongly star-forming, with a local blue knot or H II-region-like environment (Rastinejad et al., 8 Jun 2026, Martin-Carrillo et al., 8 Jun 2026, Chen et al., 8 Jun 2026).

A distinctive result is the detection of pre-explosion activity. Using 60 epochs of Pan-STARRS1 $156$59-band imaging between 2015 and 2026, one paper finds variable excess flux at the SN position over the previous $156$60 yr, with average precursor brightness $156$61 mag ($156$62 mag) from $156$63 to $156$64 yr and $156$65 mag ($156$66 mag) during the final $156$67 yr, implying a brightening by a factor of about 1.5 (Chen et al., 8 Jun 2026). The same study identifies six $156$68-band episodes at MJDs 57779.5, 60021.3, 60315.5, 60696.5, 60761.3, and 61053.5, argues that the chance-alignment probability is very low, and interprets the final $156$69 yr brightening as broadly consistent with enhanced eruptive mass loss during late oxygen burning. It further suggests that an unobserved silicon-burning episode in the final $156$70 d may have produced the compact nearby material responsible for the X-ray breakout and first-day optical excess (Chen et al., 8 Jun 2026).

In the broader transient context, SN 2026gzf is important because its prompt and late-time observables are mismatched in a way not seen in classical GRB-SNe. The prompt X-ray luminosity is stated to be $156$71 fainter than GRB 060218/SN 2006aj and GRB 100316D/SN 2010bh, and $156$72–$156$73 fainter than recent Einstein Probe Ic-BL breakout candidates, while the optical supernova remains entirely typical of the energetic Ic-BL population (O'Connor et al., 8 Jun 2026). This suggests that prompt breakout luminosity does not map directly onto later supernova optical luminosity.

The event has also motivated rate arguments. If all Type Ic-BL supernovae produced EP260321a-like shock breakouts, one paper estimates an Einstein Probe detection rate of $156$74–$156$75, whereas the observed rate up to May 2026 is effectively one such detection in $156$76 yr, inconsistent at the 90% confidence level though still consistent at 95% confidence when Poisson uncertainties are included (Rastinejad et al., 8 Jun 2026). A plausible implication is that most Ic-BL supernovae produce less luminous X-ray shock-breakout signals than EP260321a, or that breakout luminosities vary substantially.

SN 2026gzf is therefore treated in the 2026 literature as a key test case for terminal collapse in stripped stars. It combines a definitive X-ray shock-breakout counterpart, a luminous but otherwise ordinary Ic-BL supernova, stringent non-detections of late afterglow emission, evidence for structured circumstellar material, low continuum polarization with strong Ca II line polarization, and possible decade-scale precursor activity. Taken together, these properties make it a central example in current debates over weak jets, failed jets, shock breakout in extended material, and the diversity of collapse outcomes among stripped-envelope progenitors (O'Connor et al., 8 Jun 2026, Rastinejad et al., 8 Jun 2026, Wen et al., 17 Jun 2026, Martin-Carrillo et al., 8 Jun 2026, Chen et al., 8 Jun 2026).