Double-Episode Jet Scenario

• The double-episode jet scenario is defined by two discrete jet events separated by a quiescent phase, each exhibiting unique timing, spectral, and environmental characteristics.
• Models utilize multi-wavelength analyses and hydrodynamic simulations to disentangle the emission and propagation properties of the two jets in systems like GRBs and radio galaxies.
• Applications range from gamma-ray bursts to recurrent AGN outbursts and solar reconnection events, providing key diagnostics for jet dynamics and energy evolution.

Searching arXiv for the cited paper and closely related “double-episode jet” works to ground the article in current literature. The double-episode jet scenario denotes a class of models in which an apparently single transient or outflow is decomposed into two distinct episodes of jet activity, jet-driven emission, or jet-related dynamical evolution, usually separated by a quiescent interval and often occurring in different ambient media. In long gamma-ray bursts, the formulation separates an early proto-black-hole or first-shell episode from a later canonical or second-shell outflow [(Izzo et al., 2012); (Wang et al., 5 Feb 2026)]. In radio galaxies, it describes recurrent FRII activity that produces outer and inner doubles [(Konar et al., 2013); (Walg et al., 2013)]. In the Galactic center, it refers to two successive active galactic nucleus jet episodes that drive the eROSITA and Fermi bubbles (Zhang et al., 18 Jul 2025). In solar physics, it designates two successive reconnection-driven jet phases with distinct initiation sites or reconnection regimes [(Chen et al., 2017); (Liu et al., 2011)]. Across these usages, the central idea is not merely temporal repetition, but a physically meaningful partition into two episodes with different spectral, kinematic, or environmental diagnostics.

1. Core structure of the scenario

A recurring feature of double-episode jet models is that the two episodes are distinguished simultaneously by timing, emission physics, and interaction with different surroundings. In GRB 970828, the prompt emission is divided into a first episode observed in the first $40$ s and a second episode observed after $t_0+50$ s, with the transition identified with black-hole formation (Izzo et al., 2012). In double-double radio galaxies, the first episode propagates into dense thermal gas, while the second propagates into the tenuous, magnetized nonthermal plasma of the old cocoon (Konar et al., 2013). In the 2012 coronal jet west of NOAA AR 11513, the two phases peak at $22{:}28\pm0.5$ UT and $22{:}32\pm0.5$ UT and originate from different magnetic patches separated by $2.8$ Mm (Chen et al., 2017). In the Galactic-center bubble model, the first jet pair was launched $15$ Myr ago and the second $5$ Myr ago, generating two detached shock-bounded structures (Zhang et al., 18 Jul 2025).

The scenario therefore relies on more than a multi-peaked light curve. The literature repeatedly requires a quiescent gap, a change in spectral decomposition, a change in launch site or ambient medium, or all of these together. A plausible implication is that the double-episode designation is reserved for cases in which a two-stage interpretation has dynamical content, rather than simply describing variability within one uninterrupted flow.

2. Gamma-ray burst realizations

In GRB 970828, the first episode from $t_0$ to $t_0+40$ s is interpreted as proto-black-hole emission, while the second episode from $t_0+50$ s onward is interpreted as a canonical, fireshell-type GRB (Izzo et al., 2012). For the first episode, spectral decomposition prefers either a Band model or two blackbodies plus a power-law, and in a single-BB+PL fit the blackbody temperature follows two sequential power-law decays tied to the two main spikes in the light curve. The radius of the thermal emitter is inferred from

$t_0+50$0

with $t_0+50$1 and $t_0+50$2 Gpc, and the fitted evolution is

$t_0+50$3

The expansion is described as non-relativistic, from $t_0+50$4 km up to $t_0+50$5 km. The second episode begins with a soft $t_0+50$6 s spike identified as the P-GRB. A Comptonization$t_0+50$7BB fit gives $t_0+50$8 keV and $t_0+50$9 erg cm$22{:}28\pm0.5$0; the corresponding energy is $22{:}28\pm0.5$1 erg. Setting $22{:}28\pm0.5$2 erg $22{:}28\pm0.5$3, and using

$22{:}28\pm0.5$4

with $22{:}28\pm0.5$5, yields $22{:}28\pm0.5$6 and $22{:}28\pm0.5$7. The circumburst medium required by the extended-afterglow simulation has $22{:}28\pm0.5$8 cm$22{:}28\pm0.5$9, cloud radii $22{:}32\pm0.5$0 cm, and $22{:}32\pm0.5$1. The paper argues that such an environment is in line with the observed large column density absorption and may have darkened both the supernova emission and the GRB optical afterglow.

GRB 110801A presents a different but structurally related case (Wang et al., 5 Feb 2026). Episode I extends from $22{:}32\pm0.5$2 s to $22{:}32\pm0.5$3 s, followed by a quiescent gap from roughly $22{:}32\pm0.5$4 s to $22{:}32\pm0.5$5 s, and Episode II from $22{:}32\pm0.5$6 s to $22{:}32\pm0.5$7 s. The optical light curve begins rising at $22{:}32\pm0.5$8 s, well before the second X-ray/$22{:}32\pm0.5$9-ray episode at $2.8$0 s. Joint broadband fitting during Episode II favors a two-component model,

$2.8$1

over single-component fits. The power-law component is interpreted as the afterglow of the first burst, dominating the optical band, while the Band component is attributed to the prompt emission of the second burst, dominating the high-energy bands. The early optical rise is modeled as a reverse-shock plus forward-shock transition,

$2.8$2

with summed flux

$2.8$3

The inferred parameters are $2.8$4, $2.8$5 rad, $2.8$6 erg, $2.8$7, and $2.8$8. The paper also notes caveats and alternatives: a PL+blackbody model can mimic the X-ray excess but yields unphysical host-extinction evolution; a physical synchrotron model is a viable candidate for Episode II; and refreshed-shock energy injection was considered.

Taken together, these GRB applications treat the quiescent interval as physically diagnostic. In GRB 970828, the $2.8$9 s gap is associated with the actual collapse of the core through its Schwarzschild radius, while in GRB 110801A the $15$0 s interval is argued to rule out a single continuous outflow and to favor two separate launches [(Izzo et al., 2012); (Wang et al., 5 Feb 2026)].

3. Episodic radio galaxies and recurrent FRII jets

In double-double radio galaxies, the double-episode scenario is formulated in terms of two separate epochs of FRII jet activity whose outer and inner doubles evolve in very different environments (Konar et al., 2013). The outer double propagates into dense thermal gas with $15$1 and $15$2, while the inner double propagates into the old cocoon, where $15$3 and $15$4 a few $15$5G. The jet-head speed is written in relativistic form as

$15$6

As $15$7, $15$8 and $15$9. Despite the different environments, the observed injection indices satisfy $5$0 to within $5$1. The proposed explanation is that the jet spine Lorentz factor must be large enough, $5$2, so that the upstream proper speed in the shock frame remains above the $5$3 threshold for the asymptotic strong-shock value $5$4, i.e.

$5$5

The same work argues for pair-plasma jets and a fast-spine/slow-sheath structure, with $5$6 for the spine and $5$7 for the sheath.

Numerical simulations of episodic AGN outbursts develop the same scenario dynamically (Walg et al., 2013). The outburst cycle is divided into four phases; the distinctive phase is the restarted jet propagating completely inside the hot and inflated cocoon left behind by the initial jet. The jet-head advance speed is estimated as

$5$8

with $5$9 and $t_0$0. In the simulations, the initial jet in the undisturbed intergalactic medium has $t_0$1, whereas the restarted jet inside the remnant cocoon has $t_0$2. The cocoon environment is characterized by $t_0$3, $t_0$4, and $t_0$5. The Mach disc and bow shock are much weaker in the restarted phase, and the total mass injected into the cocoon up to a given length is reduced to

$t_0$6

This is the basis for the claim that the restarted jet propagates almost unimpeded, with strong radial integrity and only a very small fraction of shocked jet material flowing back through the cocoon.

Observational evidence for recurrent activity predates these unified models. In 4C23.56, Chandra data show smooth ICCMB X-rays in the old lobes, compact shocks in the current jets, and faint symmetric radio emission beyond the current hotspots (Blundell et al., 2010). The X-ray shock lies $t_0$7 kpc upstream of the radio shock on both sides. The cooling times are written as

$t_0$8

and at $t_0$9 the paper gives $t_0+40$0. The interpretation is a relic Episode I followed by a current Episode II, with leakage of freshly accelerated electrons into pre-existing weakly magnetized relic lobes.

A later theoretical development relates the two episodes to continuous accretion through a black-hole spin reversal (Garofalo et al., 26 Dec 2025). With $t_0+40$1, the first-order spin evolution is

$t_0+40$2

and the quiescent time between $t_0+40$3 and $t_0+40$4 is

$t_0+40$5

If the inner jet lasts

$t_0+40$6

then

$t_0+40$7

The paper reports a correlation between the quiescent time and the inner-jet lifetime with Pearson $t_0+40$8, and interprets the persistent FRII morphology of both episodes as a consequence of the absence of a disk tilt during the zero-spin crossing.

4. Galactic-center bubbles and blazar double jets

In the Milky Way, the double-episode jet scenario has been used to explain the eROSITA and Fermi bubbles through two successive active galactic nucleus jet activities from the Galactic center (Zhang et al., 18 Jul 2025). The simulations solve the ideal, adiabatic hydrodynamic equations with $t_0+40$9, a static gravitational potential $t_0+50$0, and no magnetic fields. Jets are injected bi-symmetrically along the $t_0+50$1-axis with identical parameters except duration: $t_0+50$2 kpc $t_0+50$3 kpc, $t_0+50$4 kpc, $t_0+50$5 cm s$t_0+50$6, $t_0+50$7, kinetic-to-thermal power ratio $t_0+50$8, $t_0+50$9 erg s$t_0+50$00, and thermal luminosity $t_0+50$01 erg s$t_0+50$02. Episode 1 lasted $t_0+50$03 Myr and was launched $t_0+50$04 Myr ago; Episode 2 lasted $t_0+50$05 Myr and was launched $t_0+50$06 Myr ago. At $t_0+50$07 Myr the first shock sits at $t_0+50$08 kpc and the second at $t_0+50$09 kpc, with mild Mach numbers $t_0+50$10. The first jet pair created the eROSITA bubbles, now extending to $t_0+50$11 kpc with $t_0+50$12 keV; the second created the Fermi bubbles, now reaching $t_0+50$13 kpc with $t_0+50$14 keV. Total injected energies are $t_0+50$15 erg and $t_0+50$16 erg. The authors state that a single-burst model cannot produce two detached forward shocks.

In blazar OJ287, the double-jet model is instead geometric and periodic (Qian, 2018). The source is modeled as a supermassive black-hole binary, each hole launching its own relativistic jet, usually termed the northern and southern jets. Both jets precess with the same period $t_0+50$17 yr, which is also the optical period. The common parabolic trajectory is written as

$t_0+50$18

and the precession phase as

$t_0+50$19

Apparent speed and Doppler factor are given by

$t_0+50$20

Typical fitted values are $t_0+50$21 and viewing angles $t_0+50$22. Periodic double-peaked optical outbursts are attributed to two enhanced accretion episodes per binary orbit, each launching synchrotron-emitting knots whose observed flux is modeled as

$t_0+50$23

The double-jet framework is used here to unify VLBI kinematics, radio-optical timing, and periodic optical structure within a single synchrotron-jet mechanism.

These two applications share the language of two jets or two episodes, but they differ in ontology. In the Galactic-center case the emphasis is on two temporally separated AGN outbursts that leave two detached shock systems; in OJ287 the emphasis is on two persistent nozzles in a binary, with periodic double-peaked activity arising from repeated accretion modulation. This suggests that “double-episode jet” and “double-jet” are adjacent but not identical classifications.

5. Stellar interiors, common envelopes, and close-binary progenitors

A stellar-interior version of the scenario appears in the double common envelope jets supernova framework (Soker, 2021). In a triple-star configuration, a neutron star first merges with a main-sequence companion inside the envelope of a red supergiant, and only later does the surviving compact remnant spiral into the red-supergiant core. Episode I is the NS–MS merger; Episode II is the NS/BH–core merger. In both episodes, jet launching requires high accretion rates, sufficient specific angular momentum to form an accretion disk, and jet efficiency $t_0+50$24. The jet energy is parameterized as

$t_0+50$25

or, for nonrelativistic jets with $t_0+50$26 km s$t_0+50$27,

$t_0+50$28

For Episode I, $t_0+50$29, $t_0+50$30, $t_0+50$31 day–$t_0+50$32 week, $t_0+50$33 erg, and $t_0+50$34 erg s$t_0+50$35. For Episode II, $t_0+50$36, $t_0+50$37, $t_0+50$38 s, $t_0+50$39 erg, and $t_0+50$40 erg s$t_0+50$41. The requirement for unbinding the envelope is written as

$t_0+50$42

with $t_0+50$43. The predicted observables are two well-separated light-curve peaks, total energy $t_0+50$44 erg, very high expansion velocities $t_0+50$45 km s$t_0+50$46, strong asphericity and polarization, and possible r-process signatures, neutrinos, or gravitational waves.

A related but more explicitly relativistic engine-level model has been proposed for long GRBs from close binaries with compact companions (Gao, 29 Mar 2026). The first jet is the collapsar jet; the second may be launched by the companion after accreting supernova ejecta. The engine-frame delay is

$t_0+50$47

or more precisely

$t_0+50$48

The first jet has $t_0+50$49, while the second has $t_0+50$50; the ratio $t_0+50$51 is treated as a free parameter. The internal collision radius is

$t_0+50$52

For aligned jets, two regimes arise. If $t_0+50$53, one expects up to three high-energy episodes: J1 prompt, J2 prompt delayed by $t_0+50$54, and the J2–J1 collision. If $t_0+50$55, J2 internal dissipation appears as an X-ray flare at $t_0+50$56, followed by an afterglow plateau from energy injection. Misaligned cases produce de-boosted prompt signals, late-time radio rebrightening, or double-lobed radio structures. The paper further emphasizes time-resolved polarimetry, writing a toy angular dependence

$t_0+50$57

to describe characteristic changes in polarization degree and angle across the multiple emission episodes.

In both the double-CEJSN and close-binary GRB formulations, the double-episode structure is generated by binary or triple-star evolution rather than by a single engine that pauses and restarts. The common element is that two distinct launch episodes are tied to two distinct accretion configurations.

6. Solar two-phase jets and broader terminological extensions

Solar observations provide some of the clearest cases in which an apparently continuous jet resolves into two physically distinct episodes. In the complex coronal jet of 2012 July 2, high-resolution AIA and HMI data show two successive, partially overlapping phases with roots $t_0+50$58 Mm apart (Chen et al., 2017). Phase 1 peaks at $t_0+50$59 UT and Phase 2 at $t_0+50$60 UT. Each spire reaches $t_0+50$61 Mm length and $t_0+50$62 Mm width, with $t_0+50$63 cm s$t_0+50$64 and $t_0+50$65 cm s$t_0+50$66. Flux cancellation is quantified by

$t_0+50$67

with $t_0+50$68 Mx s$t_0+50$69 and $t_0+50$70 Mx s$t_0+50$71. The reconnection electric field is estimated as

$t_0+50$72

and with $t_0+50$73 km s$t_0+50$74 and $t_0+50$75 G the paper gives $t_0+50$76 V m$t_0+50$77. Each phase has an energy budget of approximately $t_0+50$78 erg. The authors explicitly note that moderate resolution would blur the two cancellation sites and merge both spires into one long event.

The 2010 July 20 jet presents a related two-stage pattern, but here the two phases are “standard” and “blowout” rather than spatially distinct footpoints (Liu et al., 2011). The standard stage lasts from $t_0+50$79 to $t_0+50$80 UT and is characterized by interchange reconnection, an inverted-Y spire, and outward blobs with $t_0+50$81 km s$t_0+50$82. The blowout stage lasts from $t_0+50$83 to $t_0+50$84 UT and is triggered by an A6-class microflare at the polarity inversion line. During this stage the spire broadens, swings at $t_0+50$85 km s$t_0+50$86, unwinds at $t_0+50$87 km s$t_0+50$88, and a loop rises at $t_0+50$89 km s$t_0+50$90. The magnetic-flux cancellation rate is $t_0+50$91 Mx s$t_0+50$92 along $t_0+50$93 cm, giving $t_0+50$94 Mx cm$t_0+50$95 s$t_0+50$96. The dimensionless reconnection rate is estimated as

$t_0+50$97

in the standard stage and $t_0+50$98 in the blowout stage. A rough thermal-energy estimate gives $t_0+50$99 erg and the blob kinetic energy $22{:}28\pm0.5$00 erg. The interpretation is a jet-scale magnetic breakout in which gradual interchange reconnection is followed by flare reconnection that removes the envelope field and releases the twisted core.

In a different literature, “double jet” refers not to repeated launching but to a split upper-tropospheric circulation state over Eurasia (Mascolo et al., 17 Jul 2025). The jet-separation index is defined by

$22{:}28\pm0.5$01

with $22{:}28\pm0.5$02 and $22{:}28\pm0.5$03. The $22{:}28\pm0.5$04 percentile thresholds are $22{:}28\pm0.5$05 m s$22{:}28\pm0.5$06 in CESM and $22{:}28\pm0.5$07 m s$22{:}28\pm0.5$08 in ERA5. A plausible implication is that the phrase “double jet” has become a broader descriptor for two-lobed or split jet structure across disciplines, even when the physics is unrelated to recurrent launching.

Across the astrophysical literature, however, the double-episode jet scenario has a more specific meaning: two discrete jet-related episodes, each with its own timing, spectral or morphological signature, and interaction history. Its importance lies in forcing a decomposition of apparently unitary phenomena into two dynamically distinct stages, whether those stages correspond to proto-black-hole emission and canonical GRB production, recurrent FRII activity, two AGN outbursts from the Galactic center, sequential mergers inside a common envelope, or two reconnection phases in the solar atmosphere.