Relativistic MHD simulations of core-collapse GRB jets: 3D instabilities and magnetic dissipation (1508.02721v1)

Abstract: Relativistic jets naturally occur in astrophysical systems that involve accretion onto compact objects, such as core collapse of massive stars in gamma-ray bursts (GRBs) and accretion onto supermassive black holes in active galactic nuclei (AGN). It is generally accepted that these jets are powered electromagnetically, by the magnetised rotation of a central compact object. However, how they produce the observed emission and survive the propagation for many orders of magnitude in distance without being disrupted by current-driven non-axisymmetric instabilities is the subject of active debate. We carry out time-dependent 3D relativistic magnetohydrodynamic simulations of relativistic, Poynting flux dominated jets. The jets are launched self-consistently by the rotation of a strongly magnetised central compact object. This determines the natural degree of azimuthal magnetic field winding, a crucial factor that controls jet stability. We find that the jets are susceptible to two types of instability: (i) a global, external kink mode that grows on long time scales and causes the jets to bodily bend sideways. Whereas this mode does not cause jet disruption over the simulated distances, it substantially reduces jet propagation speed. We show, via an analytic model, that the growth of the external kink mode depends on the slope of the ambient medium density profile. In flat density distributions characteristic of galactic cores, an AGN jet may stall, whereas in stellar envelopes the external kink weakens as the jet propagates outward; (ii) a local, internal kink mode that grows over short time scales and causes small-angle magnetic reconnection and conversion of about half of jet electromagnetic energy flux into heat. Based on the robustness and energetics of the internal kink mode, we suggest that this instability is the main dissipation mechanism responsible for powering GRB prompt emission.

Overview of "Relativistic MHD simulations of core-collapse GRB jets: 3D instabilities and magnetic dissipation"

This paper presents an in-depth paper of relativistic, magnetohydrodynamic (MHD) simulations focused on the propagation and stability of jets in gamma-ray bursts (GRBs) and active galactic nuclei (AGN). It tackles key challenges in understanding how these highly magnetized, Poynting flux-dominated jets maintain stability and dissipate magnetic energy as they travel vast distances from their origin at a central compact object like a black hole or a neutron star.

Main Findings

Firstly, the paper highlights the distinction between "headless" jets, which expand in evacuated funnels, and "headed" jets, which drill through dense ambient media typically found in the stellar envelopes associated with GRBs. The simulations reveal distinct stability characteristics and emission mechanisms between these two configurations.

Headless jets, when unobstructed, show minimal susceptibility to non-axisymmetric instabilities and accelerate efficiently to high Lorentz factors. Their internal structure, dominated by strong poloidal fields and supplemented by weaker toroidal components, provides a natural damping of instability growth.

Conversely, headed jets are subject to significant torque from accumulated toroidal magnetic fields, leading to the potential for both local internal and global external kink instabilities. The paper demonstrates that internal kink modes, which occur shortly after collimation of the jet, serve as a primary mechanism for magnetic energy dissipation. This reconfiguration aligns the jet more with a thermal pressure-supported structure, stabilizing it against further internal instability.

Significant attention is given to the effect of an external kink mode, a broadly helically-deforming instability arising primarily near the jet head. The paper elucidates how this mode decelerates jet propagation by increasing effective cross-section through which the jets must push against the ambient medium, potentially affecting the observable attributes of GRBs and FRI-type AGNs.

Numerical and Analytical Modelling

The authors employ both numerical and analytical models to simulate and predict jet behavior under varied initial conditions. Critical parameters are explored, such as jet luminosity, external density profiles, and the ratio of toroidal to poloidal field strengths, to illustrate their influence on the stability and dynamics of the jets. Theoretical models are used to track growth rates for kink instabilities, including the dynamical interplay of Lorentz factors and magnetic pitch angles, providing predictions that align well with numerical findings.

Implications and Applications

The implications of this paper are profound for both astrophysical theory and observational astronomy. For GRBs, understanding jet stability and energy dissipation mechanisms offers insight into their observed timescales, spectral properties, and often enigmatic emission profiles. The identification of critical luminosity figures below which jets cannot successfully break out of progenitor stars aligns with the observed lack of low-luminosity GRBs, possibly delineating a natural selection process driven by magnetic instabilities.

In AGNs, the analysis assists in explaining the morphological divisions between FRI and FRII galaxy jets, attributing them to disturbance patterns caused by kink instabilities under differing ambient density conditions. This points to a nuanced understanding of jet composition and evolution influenced by their environmental context, potentially serving as diagnostic tools for inferring conditions in galactic nuclei.

Conclusions and Future Directions

The paper provides a comprehensive framework for jet dynamics, combining the physical fundamentals of relativistic MHD with robust computational simulation. It suggests that kink instabilities, both internal and external, are pivotal in shaping the jetted emissions we attribute to some of the universe's most dramatic astronomical events. Future research can build on these findings by extending simulations to cover longer spatial and temporal scales or by exploring the interplay of additional physical processes, such as radiative feedback, in jet evolution.