Theory of “Jitter” Radiation from Small-Scale Random Magnetic Fields and Prompt Emission from Gamma-Ray Burst Shocks

By M. Medvedev. 2000

208 citations recorded by the source corpus.

We demonstrate that the radiation emitted by ultrarelativistic electrons in highly nonuniform, small-scale magnetic fields is different from synchrotron radiation if the electron's transverse deflections in these fields are much smaller than the beaming angle. A quantitative analytical theory of this radiation, which we refer to as jitter radiation, is developed. It is shown that the emergent spectrum is determined by statistical properties of the magnetic field. The jitter radiation theory is then applied to internal shocks of γ-ray bursts (GRBs). The model of a magnetic field in GRBs proposed by Medvedev & Loeb in 1999 is used. The spectral power distribution of radiation produced by the power-law-distributed electrons with a low-energy cutoff is well described by a sharply broken power law: P(ω) ∝ ω1 for ω ≲ ωjm and P(ω) ∝ ω-(p-1)/2 for ω ≳ ωjm, where p is the electron power-law index and ωjm is the jitter break frequency, which is independent of the field strength but depends on the electron density in the ejecta, ωjm ∝ n1/2, as well as on the shock energetics and kinematics. The total emitted power of jitter radiation is, however, equal to that of synchrotron radiation. Since large-scale fields may also be present in the ejecta, we construct a two-component, jitter + synchrotron spectral model of the prompt γ-ray emission. Quite surprisingly, this model seems to be readily capable of explaining several properties of time-resolved spectra of some GRBs, such as (1) the violation of the constraint on the low-energy spectral index called the synchrotron "line of death," (2) the sharp spectral break at the peak frequency, inconsistent with the broad synchrotron bump, (3) the evidence for two spectral subcomponents, and (4) possible existence of emission features called "GRB lines." We believe these facts strongly support both the existence of small-scale magnetic fields and the proposed radiation mechanism from GRB shocks. As an example, we use the composite model to analyze GRB 910503, which has two spectral peaks. At last, we emphasize that accurate GRB spectra may allow precise determination of fireball properties as early as several minutes after the explosion.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/a1d5cca275f3814f2ccf14fc6bc110560735d97a

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In Astronomy and Astrophysics

1. Bing Zhang
2. A. Spitkovsky
3. E. Liang
4. Binbin Zhang
5. Pawan Kumar
6. Xuefeng Wu
7. E. Nakar
8. He Gao

In Nuclear and High Energy Physics

1. Bing Zhang
2. A. Spitkovsky
3. L. Sironi
4. B. Cerutti
5. D. Giannios
6. A. Pe’er
7. U. Keshet
8. M. Lemoine

Citations in the corpus

• Gamma-Ray Bursts in the Swift Era (2007, unknown): “…& Mochkovitch (1998), Pilla & Loeb (1998), Medvedev & Loeb (1999), Lloyd & Petrosian (2000), Ghisellini et al. (2000),Panaitescu & Mészáros (2000), Medvedev (2000), Mészáros & Rees (2000), Spruit et al. (2001), Drenkhahn & Spruit (2002), Mészáros et al. (2002), Zhang & Mészáros (2002c),…”
• Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation? (2010, background): “…the standard synchrotron radiation, related to the timescale of the acceleration process (Stern & Poutanen 2004; Asano & Terasawa 2009), the pitch-angle distributionof electrons (Lloyd-Ronning & Petrosian 2002) or the small scale structure of the magnetic field (Medvedev 2000; Pe’er & Zhang 2006).”
• Synchrotron Radiation as the Source of Gamma-Ray Burst Spectra (2000, background): “Emission due to these fluctuations is very similiar to the small pitch angle case and therefore also produces a spectrum Fν ∝ ν at low energies (Medvedev, 2000).”
• Particle Acceleration in Relativistic Jets Due to Weibel Instability (2003, background): “The generation of magnetic fields both with and without an initial magnetic field suggests that synchrotron emission (or jitter radiation) is relevant in GRB afterglows and Crab-like pulsar winds (Medvedev 2000).”
• SIMULATIONS OF PARTICLE ACCELERATION BEYOND THE CLASSICAL SYNCHROTRON BURNOFF LIMIT IN MAGNETIC RECONNECTION: AN EXPLANATION OF THE CRAB FLARES (2013, background): “If the magnetic turbulence in the nebula occurs on a length scale shorter than the synchrotron photon formation length mc2/eB, then the particles could emit jitter radiation (Medvedev 2000), with typical energy greater than the classical synchrotron limit (Teraki & Takahara 2013).”
• THE INTERNAL-COLLISION-INDUCED MAGNETIC RECONNECTION AND TURBULENCE (ICMART) MODEL OF GAMMA-RAY BURSTS (2010, methodology): “…and Loeb 1999; Nishikawa et al. 2005, 2009; Spitkovsky 2008) in relativistic shocks, in which Fermi-accelerated electrons cool via synchrotron/jitter (Mészáros et al. 1994; Tavani 1996; Medvedev 2000) or synchrotron self-Compton (SSC) (Mészáros et al. 1994; Kumar and McMahon 2008) radiation.”
• A COMPREHENSIVE ANALYSIS OF FERMI GAMMA-RAY BURST DATA. I. SPECTRAL COMPONENTS AND THE POSSIBLE PHYSICAL ORIGINS OF LAT/GBM GRBs (2010, background): “(3) Is the radiation mechanism synchrotron/jitter radiation (Mészáros et al. 1994; Medvedev 2000), synchrotron selfCompton (Panaitescu & Mészáros 2000; Kumar & McMahon 2008), or Comptonization of thermal photons (e.g. Thompson 1994; Pe’er et al. 2005, 2006; Beloborodov 2010; Lazzati & Begelman…”
• Theories of Gamma-Ray Bursts (2001, methodology): “…shock properties, specifically time-dependent effects would be expected to affect the electron energy distribution and photon spectral slopes, leading to time-integrated observed spectra which could differ from those in the simple time-averaged picture (Medvedev 2000,Lloyd & Petrosian 2001a).”
• Short-hard gamma-ray bursts (2007, methodology): “Some of these models are: random magnetic fields on very short length-scales, leading to jitter radiation instead of synchrotron (Medvedev, 2000), effects of synchrotron self absorption or anisotropic electron pitching angles (Lloyd & Petrosian, 2000; Granot, Piran & Sari , 2000), Upscatter of…”
• Interpenetrating Plasma Shells: Near-Equipartition Magnetic Field Generation and Nonthermal Particle Acceleration (2003, background): “Such a behavior may explain the observed evolution of the soft spectral indexa in time-resolved GRB spectra (Ghirlanda, Celotti, & Ghisellini 2002; M. V. Medvedev 2003, in preparation) as a jitter-to-synchrotron spectral transition (Medvedev 2000).”
• Relativistic Shocks: Particle Acceleration and Magnetization (2015, background): “The possibility that the small scale nature of the turbulence gives rise to diffusive (or jitter) synchrotron radiation rather than conventional synchrotron radiation has also attracted attention (e.g. Medvedev 2000, 2006; Fleishman 2006; Mao and Wang 2011; Medvedev et al. 2011; Kelner et al. 2013).”