The high-energy tail of GRB 941017: Comptonization of synchrotron self-absorbed photons

Asaf Pe'er, Eli Waxman

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Abstract

The recent detection of an unusually hard spectral component in gamma-ray burst GRB 941017 extending to ≥200 MeV is hard to explain as a synchrotron emission from shock-accelerated electrons. It was argued to imply acceleration of protons to ultra-high energy. We show here that the "high-energy tail" can be explained as emission from shock-accelerated electrons in the early afterglow epoch, taking into account the effect of synchrotron self-absorption. High-energy observations set in this case stringent constraints on model parameters: a lower limit to the total explosion energy E ≳ 5 × 1053 ergs (assuming spherical symmetry); an upper limit to the density of gas surrounding the explosion, n ≲ 10-2(E/10 54 ergs) cm-3; a lower limit to the expansion Lorentz factor Γi ≳ 200; and an upper limit to the fraction of thermal energy carried by the magnetic field behind the shock driven into the surrounding medium, εB, f ≤ 10-4. Such constraints cannot be inferred from keV-MeV data alone. The unusually low value of εB, f and the unusually high ratio E/n may account for the rareness of GRB 941017-type high-energy tails. Tighter constraints on model parameters may be obtained in the future from optical and sub-TeV observations.

Original languageEnglish
Pages (from-to)L1-L4
JournalAstrophysical Journal
Volume603
Issue number1 II
DOIs
StatePublished - 1 Mar 2004
Externally publishedYes

Bibliographical note

Funding Information:
We thank J. Granot and D. Guetta for helpful discussions. This work was supported in part by a Minerva grant and by a grant from the Rosa and Emilio Segré fund.

Funding

We thank J. Granot and D. Guetta for helpful discussions. This work was supported in part by a Minerva grant and by a grant from the Rosa and Emilio Segré fund.

FundersFunder number
Rosa and Emilio Segré fund

    Keywords

    • Gamma rays: bursts
    • Gamma rays: theory
    • Radiation mechanisms: nonthermal

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