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.

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The Physics of Gamma-Ray Bursts and Relativistic Jets

We investigate radiatively inefficient accretion flow models for Sgr A*, the supermassive black hole in our Galactic center, in light of new observational constraints. Confirmation of linear polarization in the submillimeter emission argues for accretion rates much less than the canonical Bondi rate. We consider models with low accretion rates and calculate the spectra produced by a hybrid electron population consisting of both thermal and nonthermal particles. The thermal electrons produce the submillimeter emission and can account for its linear polarization properties. As noted in previous work, the observed low-frequency radio spectrum can be explained if a small fraction (≈1.5%) of the electron thermal energy resides in a soft power-law tail. In the innermost region of the accretion flow, turbulence and/or magnetic reconnection events may occasionally accelerate a fraction of the electrons into a harder power-law tail. We show that the synchrotron emission from these electrons, or the Compton upscattering of synchrotron photons by the same electrons, may account for the X-ray flares observed by Chandra.

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Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A

In magnetized astrophysical outflows, the dissipation of field energy into particle energy via magnetic reconnection is often invoked to explain the observed non-thermal signatures. By means of two- and three-dimensional particle-in-cell simulations, we investigate anti-parallel reconnection in magnetically dominated electron-positron plasmas. Our simulations extend to unprecedentedly long temporal and spatial scales, so we can capture the asymptotic state of the system beyond the initial transients, and without any artificial limitation by the boundary conditions. At late times, the reconnection layer is organized into a chain of large magnetic islands connected by thin X-lines. The plasmoid instability further fragments each X-line into a series of smaller islands, separated by X-points. At the X-points, the particles become unmagnetized and they get accelerated along the reconnection electric field. We provide definitive evidence that the late-time particle spectrum integrated over the whole reconnection region is a power law whose slope is harder than –2 for magnetizations σ 10. Efficient particle acceleration to non-thermal energies is a generic by-product of the long-term evolution of relativistic reconnection in both two and three dimensions. In three dimensions, the drift-kink mode corrugates the reconnection layer at early times, but the long-term evolution is controlled by the plasmoid instability which facilitates efficient particle acceleration, analogous to the two-dimensional physics. Our findings have important implications for the generation of hard photon spectra in pulsar winds and relativistic astrophysical jets.

http://iopscience.iop.org/article/10.1088/2041-8205/783/1/L21/pdf

Relativistic reconnection: an efficient source of non-thermal particles

Методы подавления шумов и помех в электронных системах. (Noise reduction techniques in electronic systems)

We have calculated self-consistent equilibria of a collisionless relativistic electron-positron gas in the vicinity of a magnetic X-point. For the considered conditions, pertinent to extragalactic jets, we find that leptons are accelerated up to Lorentz factors Γ0 = κeB0L2/mc2 1, where B0 is the typical magnetic field strength, ≡ E0/B0, with E0 the reconnection electric field, L is the length scale of the magnetic field, and κ ≈ 12. The acceleration is due to the dominance of the electric field over the magnetic field in a region around the X-point. The distribution function of the accelerated leptons is found to be approximately dn/dγ ∝ γ-1 for γ Γ0. The apparent distribution function may be steeper than γ-1 due to the distribution of Γ0 values and/or the radiative losses. Self-consistent equilibria are found only for plasma inflow rates to the X-point less than a critical value.

Lepton Acceleration by Relativistic Collisionless Magnetic Reconnection

The authors have derived a new approximation for the case of an electromagnetic wave interacting with a long (compared to a wavelength) transmission line. This treatment does not make additional assumptions made by previous authors, allowing for reradiation of electromagnetic radiation. They show that this approximation reduces to previous results in the appropriate limiting cases. They also introduce a new approximation that removes some of the previous assumptions.

Interaction of an electromagnetic wave with transmission lines, including reradiation

The theory of astron ion‐ring equilibria is considered from a new point of view. In contrast with previous equilibria, the new equilibria have a distribution of canonical momentum which is invariant to axisymmetric changes in the external magnetic field. As a function of the single particle constants of the motion, the energy H, and the canonical momentum Pϑ, the distribution function, f (H,Pϑ) is written as f=g (Pϑ) F (H,Pϑ)[A (H,Pϑ)]−1. Here, g (Pϑ) is the invariant distribution function for canonical momentum; A (H,Pϑ) is the area of the poloidal (r,z) plane accessible to a ring particle with constants H and Pϑ; and F (H,Pϑ) is a nonnegative function having the normalization F dH F=const. The possibility of a third constant of the single particle motion is not included. In contrast with some previous equilibria, f (H,Pϑ) is nonzero only in the regions of the Pϑ,H plane in which there are trapped particle orbits. With this representation of f, it becomes possible to study the adiabatic compression of io...

On the theory of astron equilibria

Previous studies of exponential rigid‐rotor astron equilibria include particles which are not trapped in the self‐field of the configuration. The modification of these studies required to exclude untrapped particles is derived.

A re‐analysis of exponential rigid‐rotor astron equilibria

A comprehensive study of exponential-rigid-rotor equilibria, for strong electron and ion rings indicates the presence of a sizeable percentage of untrapped particles in all equilibria with aspect-ratios R/a 4. Such aspect-ratios are required in fusion-relevant rings. Significant changes in the equilibria are observed when untrapped particles are excluded by the use of a truncated exponential-rigid-rotor distribution function.

D. A. Larrabee

Papers

Lepton Acceleration by Relativistic Collisionless Magnetic Reconnection

Interaction of an electromagnetic wave with transmission lines, including reradiation

On the theory of astron equilibria

A re‐analysis of exponential rigid‐rotor astron equilibria

Truncated exponential-rigid-rotor model for strong electron and ion rings