The structure of weakly magnetized γ-ray burst jets

Abstract

The interaction of gamma-ray burst (GRB) jets with the dense media into which they are launched promote the growth of local hydrodynamic instabilities along the jet boundary. In a companion paper we study the evolution of hydrodynamic (unmagnetized) jets, finding that mixing of jet-cocoon material gives rise to an interface layer, termed jet-cocoon interface (JCI), which contains a significant fraction of the system energy. We find that the angular structure of the jet + JCI, when they reach the homologous phase, can be approximated by a flat core (the jet) + a power-law function (the JCI) with indices that depend on the degree of mixing. In this paper we examine the effect of subdominant toroidal magnetic fields on the jet evolution and morphology. We find that weak fields can stabilize the jet against local instabilities. The suppression of the mixing diminishes the JCI and thus reshapes the jet’s post-breakout structure. Nevertheless, the overall shape of the outflow can still be approximated by a flat core + a power-law function, although the JCI power-law decay is steeper. The effect of weak fields is more prominent in long GRB jets, where the mixing in hydrodynamic jets is stronger. In short GRB jets there is small mixing in both weakly magnetized and unmagnetized jets. This result influences the expected jet emission which is governed by the jet’s morphology. Therefore, prompt and afterglow observations in long GRBs may be used as probes for the magnetic nature at the base of the jets.

Magnetic inhibition of the recollimation instability in relativistic jets

“…Hence, the flow Lorentz factor may significantly increase compared to its value at the nozzle. Although Gottlieb et al (2020a) do not show the variation of jet Lorentz factor, this cannot be large as the jet radius prior to the reconfinement point increases only slightly in models for long GRB and only by a factor of few in their setup for short GRBs. At the nozzle, they set Γ 0 θ 0 = 0.7, which is only slightly below of Γθ 0 = 1 in our simulations.…”

“…When we were working on this paper, the results of a related numerical study of relativistic jets was published by Gottlieb et al (2020a). They also investigated the impact of magnetic field on the recollimation instability of relativistic jets, but in the context of gamma ray bursts (GRB), and concluded that σ 10 −2 leads to a suppression of the instability.…”

Black hole to breakout: 3D GRMHD simulations of collapsar jets reveal a wide range of transients

“…Such dissipation was not seen in the RMHD simulations of Bromberg & Tchekhovskoy (2016), possibly since the simulation box was too small and the jets were not sufficiently evolved. If magnetic dissipation is sufficient to reduce the magnetization to a level of σ 10 −2 , the jet continues to evolve as a hydrodynamic jet (Levinson & Begelman 2013;Gottlieb et al 2020). In this regime hydrodynamic instabilities growing on the jet boundary will cause strong mixing between jet and cocoon material and may disrupt the jet (Gottlieb et al 2021a).…”

“…In this regime hydrodynamic instabilities growing on the jet boundary will cause strong mixing between jet and cocoon material and may disrupt the jet (Gottlieb et al 2021a). In the intermediate regime of 10 −2 σ 1 jets are stable to both current-driven and boundary instabilities (Gottlieb et al 2020). It is therefore important to carefully analyze the jet magnetization above the nozzle and distinguish between the various cases.…”

Chemical stratification in a long gamma-ray burst cocoon and early-time spectral signatures of supernovae associated with gamma-ray bursts

“…The jet propagation in a massive star with different settings is important for understanding the dynamical properties of the GRB jet and has been investigated by phenomenological simulation studies in 2D (Aloy et al 2000;Lazzati & Begelman 2005;Morsony et al 2007;Nagakura et al 2011;De Colle et al 2012a,b;Mizuta & Ioka 2013;Duffell & MacFadyen 2015;Hamidani et al 2017;De Colle et al 2018b) and 3D (Wang et al 2008;López-Cámara et al 2013), and in the presence of magnetic fields (e.g., Tchekhovskoy et al 2010;Bromberg & Tchekhovskoy 2016;Gottlieb et al 2020Gottlieb et al , 2021. These numerical studies usually assume a long-lasting engine required for explaining the ultra-relativistic jet and its prompt gamma-ray emission.…”