The physics of gamma-ray bursts & relativistic jets

We provide a comprehensive review of major developments in our understanding of gamma-ray bursts, with particular focus on the discoveries made within the last fifteen years when their true nature was uncovered. We describe the observational properties of photons from the radio to multi-GeV bands, both in the prompt emission and the afterglow phases. Mechanisms for the generation of these photons in GRBs are discussed and confronted with observations to shed light on the physical properties of these explosions, their progenitor stars and the surrounding medium. After presenting observational evidence that a powerful, collimated, jet moving at close to the speed of light is produced in these explosions, we describe our current understanding regarding the generation, acceleration, and dissipation of the jet and compare these properties with jets associated with AGNs and pulsars. We discuss mounting observational evidence that long duration GRBs are produced when massive stars die, and that at least some short duration bursts are associated with old, roughly solar mass, compact stars. The question of whether a black-hole or a strongly magnetized, rapidly rotating neutron star is produced in these explosions is also discussed. We provide a brief summary of what we have learned about relativistic collisionless shocks and particle acceleration from GRB afterglow studies, and discuss the current understanding of radiation mechanism during the prompt emission phase. We discuss theoretical predictions of possible high-energy neutrino emission from GRBs and the current observational constraints. Finally, we discuss how these explosions may be used to study cosmology, e.g. star formation, metal enrichment, reionization history, as well as the formation of first stars and galaxies in the universe.

/pdf/the-physics-of-gamma-ray-bursts-and-relativistic-jets-571z4a98ye.pdf

The Physics of Gamma-Ray Bursts and Relativistic Jets

Magnetospheres of many astrophysical objects can be accurately described by the low-inertia (or ''force-free'') limit of MHD. We present a new numerical method for solution of equations of force-free relativistic MHD based on the finite-difference time-domain (FDTD) approach with a prescription for handling spontaneous formation of current sheets. We use this method to study the time-dependent evolution of pulsar magnetospheres in both aligned and oblique magnetic geometries. For the aligned rotator we confirm the general properties of the time-independent solution of Contopoulos et al. (1999). For the oblique rotator we present the 3D structure of the magnetosphere and compute, for the first time, the spindown power of pulsars as a function of inclination of the magnetic axis. We find the pulsar spindown luminosity to be L {approx} ({mu}{sup 2}{Omega}{sub *}{sup 4}/c{sup 3})(1 + sin{sup 2}{alpha}) for a star with the dipole moment {mu}, rotation frequency {Omega}{sub *}, and magnetic inclination angle {alpha}. We also discuss the effects of current sheet resistivity and reconnection on the structure and evolution of the magnetosphere.

/pdf/time-dependent-force-free-pulsar-magnetospheres-axisymmetric-18kuhelgzx.pdf

Time-dependent Force-free Pulsar Magnetospheres: Axisymmetric and Oblique Rotators

We describe a conservative, shock-capturing scheme for evolving the equations of general relativistic magnetohydrodynamics. The fluxes are calculated using the Harten, Lax, & van Leer scheme. A variant of constrained transport, proposed earlier by Toth, is used to maintain a divergence-free magnetic field. Only the covariant form of the metric in a coordinate basis is required to specify the geometry. We describe code performance on a full suite of test problems in both special and general relativity. On smooth flows we show that it converges at second order. We conclude by showing some results from the evolution of a magnetized torus near a rotating black hole.

https://iopscience.iop.org/article/10.1086/374594/pdf

Harm: a numerical scheme for general relativistic magnetohydrodynamics

We present a new numerical code, ECHO, based on an Eulerian Conservative High Order scheme for time dependent three-dimensional general relativistic magnetohydrodynamics (GRMHD) and magnetodynamics (GRMD). ECHO is aimed at providing a shock-capturing conservative method able to work at an arbitrary level of formal accuracy (for smooth flows), where the other existing GRMHD and GRMD schemes yield an overall second order at most. Moreover, our goal is to present a general framework, based on the 3+1 Eulerian formalism, allowing for different sets of equations, different algorithms, and working in a generic space-time metric, so that ECHO may be easily coupled to any solver for Einstein's equations. Various high order reconstruction methods are implemented and a two-wave approximate Riemann solver is used. The induction equation is treated by adopting the Upwind Constrained Transport (UCT) procedures, appropriate to preserve the divergence-free condition of the magnetic field in shock-capturing methods. The limiting case of magnetodynamics (also known as force-free degenerate electrodynamics) is implemented by simply replacing the fluid velocity with the electromagnetic drift velocity and by neglecting the matter contribution to the stress tensor. ECHO is particularly accurate, efficient, versatile, and robust. It has been tested against several astrophysical applications, including a novel test on the propagation of large amplitude circularly polarized Alfven waves. In particular, we show that reconstruction based on a Monotonicity Preserving filter applied to a fixed 5-point stencil gives highly accurate results for smooth solutions, both in flat and curved metric (up to the nominal fifth order), while at the same time providing sharp profiles in tests involving discontinuities.

/pdf/echo-an-eulerian-conservative-high-order-scheme-for-general-6x9x8026d5.pdf

ECHO: an Eulerian Conservative High Order scheme for general relativistic magnetohydrodynamics and magnetodynamics

This paper formulates time-dependent, force-free, degenerate electrodynamics as a hyperbolic system of conservation laws. It is shown that this system has four characteristic modes, a pair of fast waves propagating with the speed of light and a pair of Alfven waves. All these modes are linearly degenerate. The results of this analytic study can be used in developing upwind numerical schemes for the electrodynamics of black holes and pulsar magnetospheres. As an example, this paper describes a simple one-dimensional numerical scheme based on linear and exact Riemann solvers.

S.S. Komissarov

Papers

On the Properties of Time-Dependent, Force-Free, Degenerate Electrodynamics