ON PARTICLE ACCELERATION RATE IN GAMMA-RAY BURST AFTERGLOWS

Journal Article · Tue Apr 10 04:00:00 EDT 2012 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/749/1/80· OSTI ID:22020446

Sagi, Eran; Nakar, Ehud ^[1]

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1. Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978 (Israel)

It is well known that collisionless shocks are major sites of particle acceleration in the universe, but the details of the acceleration process are still not well understood. The particle acceleration rate, which can shed light on the acceleration process, is rarely measured in astrophysical environments. Here, we use observations of gamma-ray burst (GRB) afterglows, which are weakly magnetized relativistic collisionless shocks in ion-electron plasma, to constrain the rate of particle acceleration in such shocks. We find, based on X-ray and GeV afterglows, an acceleration rate that is most likely very fast, approaching the Bohm limit, when the shock Lorentz factor is in the range of {Gamma} {approx} 10-100. In that case X-ray observations may be consistent with no amplification of the magnetic field in the shock upstream region. We examine the X-ray afterglow of GRB 060729, which is observed for 642 days showing a sharp decay in the flux starting about 400 days after the burst, when the shock Lorentz factor is {approx}5. We find that inability to accelerate X-ray-emitting electrons at late time provides a natural explanation for the sharp decay, and that also in that case acceleration must be rather fast, and cannot be more than a 100 times slower than the Bohm limit. We conclude that particle acceleration is most likely fast in GRB afterglows, at least as long as the blast wave is ultrarelativistic.

• OSTI ID: 22020446
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 749; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCELERATION
AMPLIFICATION
ASTROPHYSICS
COSMIC GAMMA BURSTS
COSMOLOGICAL MODELS
COSMOLOGY
EXPLOSIONS
MAGNETIC FIELDS
PLASMA
RELATIVISTIC RANGE
SHOCK WAVES
UNIVERSE
X RADIATION