Jittering Jets Explosion Mechanism (JJEM)

• JJEM is a model for core-collapse supernovae that utilizes intermittent, stochastic jets formed by fluctuating angular momentum in the accreting material.
• It predicts distinct point-symmetric remnant morphologies, such as multiple pairs of 'ears' and filaments, observed in well-resolved supernova remnants.
• High-resolution simulations reveal that jet-induced mixing shapes nucleosynthetic distributions and influences the neutron star and black hole mass outcomes.

The jittering jets explosion mechanism (JJEM) is a model of core-collapse supernovae (CCSNe) in which the explosion is powered primarily by multiple pairs of jets, each with dynamically changing axes dictated by stochastic angular momentum fluctuations in the accreting gas around a newly formed neutron star or black hole. This model stands in contrast to the traditional delayed neutrino mechanism, positing that jets both unbind the stellar envelope and leave distinctive, point-symmetric morphological signatures in supernova remnants. The following sections describe the physical principles underlying the JJEM, its predictive morphology, its implications for remnant properties and the neutron star–black hole mass gap, comparative aspects with alternative mechanisms, and its computational and observational foundations.

1. Fundamental Physical Principles and Jet-Launching Episodes

The JJEM is predicated on the accretion of gas with stochastic specific angular momentum during the immediate aftermath of core collapse. Hydrodynamic instabilities—particularly convection in the pre-collapse core and post-bounce spiral standing accretion shock instability (spiral-SASI)—amplify these angular momentum fluctuations. When a mass shell with local specific angular momentum $j_{\mathrm{conv}}$ exceeding a critical threshold ($j_{\mathrm{jje}}$) is accreted, an intermittent accretion disk or thick belt forms around the nascent neutron star. Each episode is short-lived (typically $0.01-0.1$ s), with the disk unable to fully relax its initial, intrinsically asymmetric structure.

The criterion for jet launching is succinctly expressed as: $$\bar{j}_{\mathrm{conv,01}} \gtrsim j_{\mathrm{jje}}$$ where $\bar{j}_{\mathrm{conv,01}}$ is the average specific angular momentum over a $0.01\,M_\odot$ shell near the iron core boundary. Typical threshold values used are $j_{\mathrm{jje}} = 2.5\times10^{15}$ or $5\times10^{15}\,\mathrm{cm}^2\,\mathrm{s}^{-1}$ (Shishkin et al., 2021, Wang et al., 12 Jan 2024). When this is met, a disk forms and launches a narrowly collimated ($\sim1$–$10^\circ$ half-opening angle) bipolar jet.

During the short duration, the disk remains thick and asymmetric. Opposite jets in each pair may differ in power due to the disk's inability to equilibrate the initial asymmetry in either mass or magnetic field configuration (Soker, 2023). Magnetic field amplification proceeds rapidly in this environment, but magnetic reconnection zones are extremely compact—of order $D_{\mathrm{rec}} \sim 0.005r$—placing stringent requirements on the resolution of simulations aiming to capture this process (Soker, 20 Sep 2024).

2. Morphological and Dynamical Predictions: Point-Symmetry and Wind-Rose Structures

The JJEM predicts that each jet-launching episode deposits energy and momentum along a distinct, often rapidly changing axis, leading to remnants with unmistakable point-symmetric ("wind-rose" [Editor's term]) morphologies. Observed features—across X-ray, infrared, optical, ultraviolet, and radio—include multiple pairs of "ears," filaments, clumps, cavities, or bubbles arrayed in a nearly 180° rotational symmetry about the remnant center. The centroid of symmetry for all axes nearly coincides with the supernova explosion site.

For instance, the Cassiopeia A remnant displays at least seven pairs of opposite features whose axes intersect at a central location, forming a wind-rose pattern (Bear et al., 12 Mar 2024). The Cygnus Loop and the Vela SNR exhibit S-shaped main jet axes attributed to the action of precessing jets late in the explosion, with other jet pairs forming additional symmetry axes (Shishkin et al., 20 Aug 2024, Soker et al., 4 Sep 2024). The Crab Nebula shows seven pairs of opposite "bays," each fitted quantitatively via parabolic edge analysis, with all symmetry axes converging at the explosion site (Shishkin et al., 12 Nov 2024).

3. Neutron Star and Black Hole Outcomes: Masses and the NS–BH Gap

The mass of the compact remnant formed in the JJEM is set by the mass coordinate at which the first convective zone meets the threshold $j_{\mathrm{jje}}$. For the $2.5\times10^{15}$ threshold, simulations of non-rotating progenitors in the $11-48\,M_\odot$ range yield neutron star gravitational masses of $1.3-1.8\,M_\odot$. For the more stringent $5\times10^{15}$ threshold, predicted masses extend up to $2.8\,M_\odot$, with the most massive remnants likely forming black holes (Shishkin et al., 2021).

In progenitors with appreciable pre-collapse rotation, the bulk specific angular momentum ($j_\mathrm{p}$) adds a constant vector component. The interplay between stochastic ($j_\mathrm{f}$) and constant angular momentum components determines the angular spread of the jets and thus the explosion's efficiency. The toy model of JJEM expresses the jet axis deviation as: $$\tan\alpha = \frac{j_\mathrm{f} \sin\theta}{j_\mathrm{f} \cos\theta + j_\mathrm{p}} = \frac{\sin\theta}{\cos\theta + \beta_\mathrm{p}}$$ with $\beta_\mathrm{p} = j_\mathrm{p}/j_\mathrm{f}$ (Soker, 2023).

A critical implication is the natural emergence of the $2.5$–$5\,M_\odot$ neutron star–black hole remnant mass gap: for $\beta_\mathrm{p} < 1$, jets are isotropic and accretion halts quickly (producing NSs), but as $\beta_\mathrm{p}$ increases through unity, accretion continues in the equatorial plane, driving up remnant masses (and forming BHs), with the transition sharply populating the NS and BH ends and leaving the mass gap sparsely filled.

4. Hydrodynamical, Instability, and Nucleosynthetic Consequences

The jet–core interaction in the JJEM is inherently multidimensional. As jets drill through the core, they inflate lobes and cavities, generating pressure inversion zones that are unstable to Rayleigh–Taylor instabilities. The acceleration of low-density material into high-density regions produces fingers and vortices that break up shells and clumps, further fragmenting the ejecta (Braudo et al., 13 Mar 2025).

Three-dimensional simulations employing adaptive mesh refinement and advanced hydrodynamical solvers (e.g., FLASH v4.8) show that a sequence of jet pairs—each with independently set duration, direction, and opening angle—can account for observed clumps, elongated bubbles, and even H-shaped features as in SNR W49B. These complex morphologies are manifest even with modest numbers (3–5) of jet pairs, due to the secondary generation of clump pairs by hydrodynamical instabilities.

Moreover, the mixing induced by jets and the resultant vortical flows profoundly affect the distribution of nucleosynthetic products, including the ejection of deep-seated elements (O, Ne, Mg) along axes observed in Vela and N132D (Soker et al., 4 Sep 2024, Soker, 1 Jul 2025).

5. Distinctions from the Delayed Neutrino Mechanism

The delayed neutrino explosion paradigm relies on the revival of the stalled post-bounce shock via neutrino heating. While three-dimensional simulations implementing this mechanism often show some explosion, their energies typically fall below observed values (≲$10^{51}$ erg), and many models fail for massive progenitors. Furthermore, the neutrino mechanism naturally produces stochastic, non-ordered morphologies; instabilities such as Rayleigh–Taylor and convection do not yield multiple, point-symmetric axis systems (Shishkin et al., 2021, Soker, 13 Nov 2024).

The JJEM, in contrast, produces prominent point-symmetric morphologies, efficiently removes material via jet feedback even in massive stars (avoiding "failed" supernovae), and can explain the multi-pair axis patterns, bent filaments, and chemical overturns seen in well-resolved remnants (e.g., Cassiopeia A, Crab, SN 1987A) (Bear et al., 12 Mar 2024, Shishkin et al., 12 Nov 2024, Soker, 2023, Soker, 23 Apr 2024).

Neutrino heating remains present in JJEM as a boosting process—neutrino-driven convection may increase the efficacy of the spiral-SASI and thus the formation of intermittent disks, amplifying the explosion energy but not replacing jets as the primary agent.

6. Gravitational Wave Predictions and Observational Diagnostics

The JJEM predicts gravitational waves sourced by the large-scale, turbulent bubbles (cocoons) that jets inflate as they interact with the extended stellar core. This leads to GW signatures at frequencies of $5$–$30$ Hz, with strains

$$h \approx 4 \times 10^{-22}\left(\frac{D}{10\,\mathrm{kpc}}\right)^{-1} \left(\frac{E_{2j}}{10^{50}\,\mathrm{erg}}\right)$$

for Galactic supernovae (Soker, 2023). These frequencies are distinct from the higher-frequency ($100$–$2000$ Hz) GWs produced by inner-core instabilities in the neutrino-driven model. Current and planned GW detectors can probe this lower frequency band, enabling discrimination between the two mechanisms.

Point-symmetric morphologies, wind-rose axis arrangements, S-shaped main jet axes, and the distribution of nucleosynthetic products along jet axes serve as additional critical observational diagnostics for the JJEM (Bear et al., 12 Mar 2024, Wang et al., 12 Jan 2024, Shishkin et al., 20 Aug 2024, Soker, 15 Apr 2025, Soker, 1 Jul 2025, Soker et al., 14 Aug 2025).

7. Computational and Numerical Challenges

Simulating the JJEM at high fidelity is numerically demanding. Accurately capturing the formation and evolution of intermittent, thick, highly asymmetric accretion disks, along with rapid, dynamically important magnetic reconnection, requires at least $\Delta \lesssim 0.002~r$ spatial resolution near the neutron star—well beyond the capabilities of most current global CCSN codes (Soker, 20 Sep 2024). The width of reconnection layers, $D_{\rm rec} \sim 0.005r$, sets a critical scale for both jet launching and the rate of energy release.

Three-dimensional hydrodynamical simulations with multiple jet-injection episodes (e.g., using FLASH), and physically motivated models for pre-collapse angular momentum fluctuations (derived from mixing-length theory or multidimensional core convection studies), are necessary to realize the full dynamics of the JJEM in silico (Braudo et al., 13 Mar 2025). Advances in adaptive mesh refinement, improved microphysics, and dynamic jet launching algorithms are essential priorities for the field.

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The jittering jets explosion mechanism provides a physically consistent, numerically validated, and observationally testable paradigm for core-collapse supernova explosions. It directly links pre-collapse stellar structure to remnant properties, predicts the broad variety of morphologies and compact object masses observed, and naturally explains the multi-axis, point-symmetric patterns ubiquitous in supernova remnants. Continued development of high-resolution simulations and detailed multiwavelength observations will further refine the model and sharpen its distinction from neutrino-dominated alternatives.