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We present the results of a series of 3D special relativistic hydrodynamic simulations of a gamma-ray burst (GRB) jet in a massive circumstellar medium (CSM) surrounding the progenitor star. Our simulations reproduce the jet morphology transitioning from a well-collimated state to a thermal pressure-driven state for a range of CSM masses and outer radii. The jet–CSM interaction redistributes the jet energy to materials expanding into a wide solid angle and results in a quasi-spherical ejecta with four-velocities from |$\Gamma \beta \simeq 0.1$| to 10. The mass and kinetic energy of the ejecta with velocities faster than |$0.1c$| are typically of the order of |$0.1\, M_{\odot }$| and |$10^{51}\:\mbox{erg}$| with only a weak dependence on the CSM mass and radius for the explored CSM parameter ranges. We find that the numerically obtained density structure of the mildly relativistic ejecta is remarkably universal. The radial density profile is well approximated as a power-law function of the radial velocity with an index of |$-5$|⁠ , |$\rho \propto v^{-5}$|⁠ , in agreement with our previous simulations and other studies, as well as those suggested from recent studies on early-phase spectra of supernovae associated with GRBs. Such fast ejecta rapidly becomes transparent following its expansion. Gradually releasing the trapped thermal photons, the ejecta gives rise to bright UV–optical emission within |${\sim} 1\:$| d. We discuss the potential link of the relativistic ejecta resulting from jet–CSM interaction to GRB-associated supernovae as well as fast and blue optical transients. [ABSTRACT FROM AUTHOR]

Observations of both gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) point to the idea that some relativistic jets are suffocated by their environment before we observe them. In these 'choked' jets, all the jet's kinetic energy is transferred into a hot and narrow cocoon of near-uniform pressure. We consider the evolution of an elongated, axisymmetric cocoon formed by a choked jet as it expands into a cold power-law ambient medium ρ ∝  R −α, in the case where the shock is decelerating (α < 3). The evolution proceeds in three stages, with two breaks in behaviour: the first occurs once the outflow has doubled its initial width, and the second once it has doubled its initial height. Using the Kompaneets approximation, we derive analytical formulae for the shape of the cocoon shock, and obtain approximate expressions for the height and width of the outflow versus time in each of the three dynamical regimes. The asymptotic behaviour is different for shallow (⁠|$\alpha \leq 2$|⁠) and steep (2 < α < 3) density profiles. Comparing the analytical model to numerical simulations, we find agreement to within ∼15 per cent out to 45 deg from the axis, but discrepancies of a factor of 2–3 near the equator. The shape of the cocoon shock can be measured directly in AGNs, and is also expected to affect the early light from failed GRB jets. Observational constraints on the shock geometry provide a useful diagnostic of the jet properties, even long after jet activity ceases. [ABSTRACT FROM AUTHOR]

Observations of several Fanaroff–Riley (FR) type-I sources reveal outflowing bipolar bubbles of hot gas surrounded by a weak forward shock. We consider the possibility that these bubbles were driven by choked relativistic jets that failed to penetrate the ambient intracluster medium. Using new results on choked jets linking the geometry of the forward shock to the jet properties, we infer robust limits on the radius R ch at which the jet was quenched in five well-studied FR type-I sources, finding typically R ch ∼ 10 kpc. We further show that, in order to reach this radius in less than the current age of the system, the jet must have been tightly collimated, with the jet head subtending an angle of |$\theta _{\rm h}\lesssim 2^{\circ }$|⁠. The ambient pressure is not high enough to explain this collimation, suggesting that the jet was collimated by interaction with its own cocoon. Although the choking radius is well-constrained, we find a degeneracy between the initial jet opening angle before collimation, θ0, and the duration of jet activity, t b, with |$(t_{\rm b}/1\rm Myr)(\theta _0/5^{\circ })^{-2}\sim 0.1$|⁠. We speculate that the working time and/or opening angle of the jet may be important factors contributing to the FR type-I/type- II morphology in galaxy clusters, with short-lived or wide jets being choked to form bipolar bubbles filled with diffuse radio emission, and longer-lived or narrow jets successfully escaping the cluster core to produce cocoons with radio hotspots. [ABSTRACT FROM AUTHOR]

We consider a model for the low-luminosity gamma-ray burst GRB 060218 that plausibly accounts for multiwavelength observations to day 20. The model components are: (1) a long-lived (tj ~ 3000 s) central engine and accompanying low-luminosity (Lj ~ 1047 erg s-1), semirelativistic (γ ~ 10) jet; (2) a low-mass (~4 × 10-3M☉) envelope surrounding the progenitor star; and (3) a modest amount of dust (AV ~ 0.1 mag) in the interstellar environment. Blackbody emission from the transparency radius in a low-power jet outflow can fit the prompt thermal X-ray emission, and the nonthermal X-rays and gamma-rays may be produced via Compton scattering of thermal photons from hot leptons in the jet interior or the external shocks. The later mildly relativistic phase of this outflow can produce the radio emission via synchrotron radiation from the forward shock. Meanwhile, interaction of the associated SN 2006aj with a circumstellar envelope extending to ~1013 cm can explain the early optical emission. The X-ray afterglow can be interpreted as a light echo of the prompt emission from dust at ~30 pc. Our model is a plausible alternative to that of Nakar, who recently proposed shock breakout of a jet smothered by an extended envelope as the source of prompt emission. Both our results and Nakar's suggest that bursts such as GRB 060218 may originate from unusual progenitors with extended circumstellar envelopes, and that a jet is necessary to decouple the prompt emission from the supernova. [ABSTRACT FROM AUTHOR]