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The cosmic-ray precursor of relativistic collisionless shocks: A missing link in gamma-ray burst afterglows

Milosavljević, Miloš and Nakar, Ehud (2006) The cosmic-ray precursor of relativistic collisionless shocks: A missing link in gamma-ray burst afterglows. Astrophysical Journal, 651 (2). pp. 979-984. ISSN 0004-637X. http://resolver.caltech.edu/CaltechAUTHORS:MILapj06

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Abstract

Collisionless shocks are commonly argued to be the sites of cosmic-ray (CR) acceleration. We study the influence of CRs on weakly magnetized relativistic collisionless shocks and apply our results to external shocks in gamma-ray burst (GRB) afterglows. The common view is that the transverse Weibel instability (TWI) generates a small-scale magnetic field that facilitates collisional coupling and thermalization in the shock transition. The TWI field is expected to decay rapidly, over a finite number of proton plasma skin depths from the transition. However, the synchrotron emission in GRB afterglows suggests that a strong and persistent magnetic field is present in the plasma that crosses the shock; the origin of this field is a key open question. Here we suggest that the common picture involving TWI demands revision. Namely, the CRs drive turbulence in the shock upstream on scales much larger than the skin depth. This turbulence generates a large-scale magnetic field that quenches TWI and produces a magnetized shock. The new field efficiently confines CRs and enhances the acceleration efficiency. The CRs modify the shocks in GRB afterglows at least while they remain relativistic. The origin of the magnetic field that gives rise to the synchrotron emission is plausibly in the CR-driven turbulence. We do not expect ultra–high-energy cosmic-ray production in external GRB shocks.


Item Type:Article
Additional Information:© 2006 The American Astronomical Society. Received 2005 December 13; accepted 2006 July 26. We thank Jonathan Arons and Andrei Gruzinov for inspiring us to explore the importance of upstream turbulence. We are also indebted to Tony Bell, Don Ellison, and an anonymous referee for sending us detailed comments and to Steven Cowley and Marc Kamionkowski for illuminating conversations. M. M. thanks the Center for Cosmology and Particle Physics at New York University for hospitality and acknowledges support by NASA through Hubble Fellowship grant HST-HF-01188.01-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS5-26555. E. N. was supported at Caltech by a senior research fellowship from the Sherman Fairchild Foundation.
Subject Keywords:acceleration of particles—cosmic rays—gamma rays: bursts—plasmas—shock waves
Record Number:CaltechAUTHORS:MILapj06
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:MILapj06
Alternative URL:http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v651n2/20269/brief/20269.abstract.html
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7114
Collection:CaltechAUTHORS
Deposited By: Lindsay Cleary
Deposited On:09 Jan 2007
Last Modified:26 Dec 2012 09:29

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