Gamma-Ray Burst Phenomenology, Shock Dynamo, and the First Magnetic Fields

Abstract

A relativistic collisionless shock propagating into an unmagnetized medium leaves behind a strong large-scale magnetic field. This seems to follow from two assumptions: (i) GRB afterglows are explained by synchrotron emission of a relativistic shock, (ii) magnetic field can't exist on microscopic scales only, it would decay by phase space mixing. Assumption (i) is generally accepted because of an apparent success of the shock synchrotron phenomenological model of GRB afterglow. Assumption (ii) is confirmed in this work by a low-dimensional numerical simulation. One may hypothesize that relativistic shock velocities are not essential for the magnetic field generation, and that all collisionless shocks propagating into an unmagnetized medium generate strong largescale magnetic fields. If this hypothesis is true, the first cosmical magnetic fields could have been generated in shocks of the first virialized objects.Subject headings: magnetic fields -shock waves 1. GRB Afterglow Phenomenology

Are Gamma‐Ray Burst Shocks Mediated by the Weibel Instability?

“…The generated smallscale field should decay (Gruzinov 2001;Milosavljević & Nakar 2006), so B could be even lower. According to Panaitescu & Kumar (2002) and Yost et al (2003), the observed spectra and light curves of the GRB afterglows imply B $ 10 À3 Y10 À1 in most cases.…”

“…A long-standing difficulty with this assumption has been that the inferred magnetic field needed to fit the afterglow data typically requires that the magnetic energy density exceeds by many orders of magnitude the magnetic energy density that would be expected from the shock compression of the interstellar magnetic field of the host galaxy (Gruzinov & Waxman 1999;Gruzinov 2001). Many authors have therefore assumed that the shock somehow manufactures field energy to meet this requirement, but no convincing mechanism has been proposed to date.…”

“…However, simulations of colliding electron-proton flows show that while the electrons are readily isotropized, the protons acquire only small scattering in angles after passing the simulation box . How long the field persists after the shock is also an important question (Gruzinov 2001), but here we discuss whether the Weibel instability can even cause the shock in the first place. Moiseev & Sagdeev (1963;see also Sagdeev 1966) analyzed the structure of the nonrelativistic Weibel-driven shock and found that the width of the shock transition should be very large because electrons easily screen proton currents, thus suppressing development of the instability.…”

Magnetic Field Generation in Relativistic Shocks

“…2) despite the larger kinetic energy of the protons. Gruzinov (2001) anticipated this when he excluded the case where a small parameter in the theory might be important in his analysis of the Weibel instability: our analysis shows that the relevant small parameter isω 2 pp /Ω 2 ∼ 4m e /3m p . The result is a small equipartition parameter (44).…”

Generation of magnetic field fluctuations in relativistic electron–positron magnetoplasmas

“…On the other hand, the Weibel instability, which generates purely growing magnetic fields, is triggered by electron temperature anisotropy or by beams of contrastreaming electron streams in plasmas. The importance of the Weibel instability has been recently recognized in the context of the magnetic field generation in cosmological [8][9][10] and laboratory [11] plasmas. Computer simulations [12] conclusively demonstrate the generation of magnetic fields due to colliding electron clouds in an unmagnetized electron-ion plasma.…”