Komissarov 2009 磁场的稀释波可以加速喷流的速度

主要内容:
当磁化喷流穿出恒星包层之后失去外部压力支撑，可以产生稀释波，它能加速喷流（不是加热）

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Title:		Rarefaction acceleration of ultrarelativistic magnetized jets in gamma-ray burst sources
Authors:		Komissarov, Serguei S.; Vlahakis, Nektarios; Königl, Arieh
Affiliation:		AA(Department of Applied Mathematics, The University of Leeds, Leeds LS2 9GT), AB(Section of Astrophysics, Astronomy and Mechanics, Physics Department, University of Athens, 15784 Zografos, Athens, Greece), AC(Department of Astronomy and Astrophysics and Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA)
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 407, Issue 1, pp. 17-28. (MNRAS Homepage)
Publication Date:		09/2010
Origin:		WILEY
MNRAS Keywords:		MHD, relativistic processes, methods: numerical, gamma-ray: burst: general
Abstract Copyright:		(c) Journal compilation © 2010 RAS
DOI:		10.1111/j.1365-2966.2010.16779.x
Bibliographic Code:		2010MNRAS.407...17K

Abstract

When a magnetically dominated superfast-magnetosonic long/soft gamma-ray burst (GRB) jet leaves the progenitor star, the external pressure support will drop and the jet may enter the regime of ballistic expansion, during which additional magnetic acceleration becomes ineffective. However, recent numerical simulations by Tchekhovskoy et al. have suggested that the transition to this regime is accompanied by a spurt of acceleration. We confirm this finding numerically and attribute the acceleration to a sideways expansion of the jet, associated with a strong magnetosonic rarefaction wave that is driven into the jet when it loses pressure support, which induces a conversion of magnetic energy into kinetic energy of bulk motion. This mechanism, which we dub rarefaction acceleration, can only operate in a relativistic outflow because in this case the total energy can still be dominated by the magnetic component even in the superfast-magnetosonic regime. We analyse this process using the equations of relativistic magnetohydrodynamics and demonstrate that it is more efficient at converting internal energy into kinetic energy when the flow is magnetized than in a purely hydrodynamic outflow, as was found numerically by Mizuno et al. We show that, just as in the case of the magnetic acceleration of a collimating jet that is confined by an external pressure distribution - the collimation-acceleration mechanism - the rarefaction-acceleration process in a magnetized jet is a consequence of the fact that the separation between neighbouring magnetic flux surfaces increases faster than their cylindrical radius. However, whereas in the case of effective collimation-acceleration the product of the jet opening angle and its Lorentz factor does not exceed ~1, the addition of the rarefaction-acceleration mechanism makes it possible for this product to become >>1, in agreement with the inference from late-time panchromatic breaks in the afterglow light curves of long/soft GRBs.

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