Maximum synchrotron frequency for shock-accelerated particles

Kumar, Pawan; Hernández, Roberto A.; Bošnjak, Željka; Duran, R. Barniol

doi:10.1111/j.1745-3933.2012.01341.x

Cited by 58 publications

(52 citation statements)

References 43 publications

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“…Bearing in mind the assumptions of this model, the VERITAS non-detection can be associated with a cutoff in the synchrotron photon spectrum at ∼100 GeV. The theoretical limit on the synchrotron cutoff energy can be expressed as (Kumar et al 2012). Here, B w is the magnetic field immediately behind the shock front and it carries a fraction ( B ) of the shocked gas energy density.…”

Section: Discussionmentioning

confidence: 99%

“…However, Kouveliotou et al (2013) find that both spectral and temporal extrapolations, from optical to multiGeV energies, are consistent with the synchrotron mechanism, though such an interpretation requires significant modifications to current models of particle acceleration in GRB afterglow shocks. In the context of the synchrotron model, we interpret the VERITAS upper limit in a scenario where the uniform magnetic field assumption in the shocked interstellar medium (ISM) is relaxed (Kumar et al 2012), and the magnetic field decays (Sari & Esin 2001) with an electron spectrum (dN/dE) ∝ E −2.45 and breaks at 100, 140, and 180 GeV (solid, dot-dashed, and dashed lines). The electron energy distribution is determined from the LAT-measured spectrum, as described in the text.…”

Section: Discussionmentioning

confidence: 99%

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Constraints on Very High Energy Emission From GRB 130427a

Aliu

Aune

Barnacka

et al. 2014

ApJ

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Prompt emission from the very fluent and nearby (z = 0.34) gamma-ray burst GRB 130427A was detected by several orbiting telescopes and by ground-based, wide-field-of-view optical transient monitors. Apart from the intensity and proximity of this GRB, it is exceptional due to the extremely long-lived high-energy (100 MeV to 100 GeV) gamma-ray emission, which was detected by the Large Area Telescope on the Fermi Gamma-Ray Space Telescope for ∼70 ks after the initial burst. The persistent, hard-spectrum, high-energy emission suggests that the highest-energy gamma rays may have been produced via synchrotron self-Compton processes though there is also evidence that the high-energy emission may instead be an extension of the synchrotron spectrum. VERITAS, a ground-based imaging atmospheric Cherenkov telescope array, began follow-up observations of GRB 130427A ∼71 ks (∼20 hr) after the onset of the burst. The GRB was not detected with VERITAS; however, the high elevation of the observations, coupled with the low redshift of the GRB, make VERITAS a very sensitive probe of the emission from GRB 130427A for E > 100 GeV. The non-detection and consequent upper limit derived place constraints on the synchrotron self-Compton model of high-energy gamma-ray emission from this burst.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Constraints on Very High Energy Emission From GRB 130427a

Aliu

Aune

Barnacka

et al. 2014

ApJ

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“…Radiative energy losses are likely to limit the maximum electron Lorentz factor to values smaller than γ sat,e . The electrons reaching the most extreme energies will have to remain in the acceleration region throughout their life, so that they will preferentially cool in the Weibel-generated fields, rather than in the background field B 0 (at odds with the assumption by Kumar et al (2012)). By comparing Equations (4) and (14), we can constrain the combination λ c/ω pi B 0.03.…”

Section: Ismmentioning

confidence: 99%

“…As we have described in Section 5.1, we find that the acceleration of protons by the external shocks of GRB afterglows is too slow (given that t ∝ ε 2 ) to explain the extreme Lorentz factors of UHECRs. In the early phases of GRB afterglows, electrons can be accelerated up to Lorentz factors γ sync,e ∼ 10 7 before suffering catastrophic synchrotron losses in the Weibel-generated fields (as opposed to the background pre-shock field, as argued by Kumar et al (2012)). Their synchrotron radiation can produce the ∼ GeV photons detected by the Fermi telescope (e.g., Ackermann et al 2010;De Pasquale et al 2010;Ghisellini et al 2010) in early GRB afterglows.…”

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confidence: 99%

The Maximum Energy of Accelerated Particles in Relativistic Collisionless Shocks

Sironi

Spitkovsky

Arons

2013

ApJ

381

494

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The afterglow emission from gamma-ray bursts (GRBs) is usually interpreted as synchrotron radiation from electrons accelerated at the GRB external shock that propagates with relativistic velocities into the magnetized interstellar medium. By means of multi-dimensional particle-in-cell simulations, we investigate the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0 σ 10 −1 . The pre-shock magnetic field is orthogonal to the flow, as generically expected for relativistic shocks. We find that relativistic perpendicular shocks propagating in electron-positron plasmas are efficient particle accelerators if the magnetization is σ 10 −3 . For electron-ion plasmas, the transition to efficient acceleration occurs for σ 3 × 10 −5 . Here, the acceleration process proceeds similarly for the two species, since the electrons enter the shock nearly in equipartition with the ions, as a result of strong pre-heating in the self-generated upstream turbulence. In both electron-positron and electron-ion shocks, we find that the maximum energy of the accelerated particles scales in time as ε max ∝ t 1/2 . This scaling is shallower than the so-called (and commonly assumed) Bohm limit ε max ∝ t, and it naturally results from the small-scale nature of the Weibel turbulence generated in the shock layer. In magnetized plasmas, the energy of the accelerated particles increases until it reaches a saturation value ε sat /γ 0 m i c 2 ∼ σ −1/4 , where γ 0 m i c 2 is the mean energy per particle in the upstream bulk flow. Further energization is prevented by the fact that the self-generated turbulence is confined within a finite region of thickness ∝ σ −1/2 around the shock. Our results can provide physically grounded inputs for models of non-thermal emission from a variety of astrophysical sources, with particular relevance to GRB afterglows.

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“…The maximum synchrotron photon energy 50 MeV is obtained in the assumption of the fastest acceleration-the Bohm acceleration, 5 Fellow of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD). 6 Kumar et al (2012) suggest that particles can be accelerated in the low background magnetic field region and radiate in the high magnetic field region (i.e., in the amplified turbulence magnetic field region), so the maximum synchrotron emission can exceed 50 MeV. However, once the particles enter into the low background magnetic field region, the particles cannot be scattered back to the upstream due to the lack of microturbulence, and the acceleration will not continue.…”

Section: Introductionmentioning

confidence: 99%

ON THE ORIGIN OF > 10 GeV PHOTONS IN GAMMA-RAY BURST AFTERGLOWS

Wang

Liu

Lemoine

2013

ApJ

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Fermi/LAT has detected long-lasting high-energy photons (>100 MeV) from gamma-ray bursts (GRBs), with the highest energy photons reaching about 100 GeV. One proposed scenario is that they are produced by high-energy electrons accelerated in GRB forward shocks via synchrotron radiation. We study the maximum synchrotron photon energy in this scenario, considering the properties of the microturbulence magnetic fields behind the shock, as revealed by recent particle-in-cell simulations and theoretical analyses of relativistic collisionless shocks. Due to the small-scale nature of the microturbulent magnetic field, the Bohm acceleration approximation, in which the scattering mean free path is equal to the particle Larmor radius, breaks down at such high energies. This effect leads to a typical maximum synchrotron photon of a few GeV at 100 s after the burst and this maximum synchrotron photon energy decreases quickly with time. We show that the fast decrease of the maximum synchrotron photon energy leads to a fast decay of the synchrotron flux. The 10-100 GeV photons detected after the prompt phase cannot be produced by the synchrotron mechanism. They could originate from the synchrotron self-Compton emission of the early afterglow if the circumburst density is sufficiently large, or from the external inverse Compton process in the presence of central X-ray emission, such as X-ray flares and prompt high-latitude X-ray emission.

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Maximum synchrotron frequency for shock-accelerated particles

Cited by 58 publications

References 43 publications

Constraints on Very High Energy Emission From GRB 130427a

Constraints on Very High Energy Emission From GRB 130427a

The Maximum Energy of Accelerated Particles in Relativistic Collisionless Shocks

ON THE ORIGIN OF > 10 GeV PHOTONS IN GAMMA-RAY BURST AFTERGLOWS

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