Klein–nishina Effects on Optically Thin Synchrotron and Synchrotron Self-Compton Spectrum

Nakar, Ehud; Ando, Shinʼichiro; Sari, Re’em

doi:10.1088/0004-637x/703/1/675

Cited by 197 publications

(270 citation statements)

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“…hardness-intensity and hardness-fluence correlations, evolution of the pulse width with energy channel, time lags (Daigne & Mochkovitch 1998Ramirez-Ruiz & Fenimore 2000;Bošnjak 2010, in preparation) In this paper, we investigate a solution to steepen the low-energy slope of the synchrotron component and possibly reconcile the synchrotron process with observed GRB prompt spectra. This solution is related to the steepening of the low-energy synchrotron slope by moderately efficient inverse Compton scatterings in Klein-Nishina regime, as suggested by Derishev et al (2001); Bošnjak et al (2009) ;Nakar et al (2009); see also Rees (1967) where this is mentionned in the general context of SSC radiation. It is an alternative to the SSC scenario (see e.g.…”

Section: Introductionmentioning

confidence: 84%

“…Interestingly, these scatterings affect the spectral shape of the synchrotron component, due to a better efficiency for low frequency photons. This results in a steepening of the lowenergy photon index α, with α → −1 (Derishev et al 2001;Bošnjak et al 2009;Nakar et al 2009). For a large range of parameters regarding the dynamics of internal shocks (variability timescale, bulk Lorentz factor, amplitude of fluctuations of the Lorentz factor, kinetic energy flux), we produce synthetic pulses with peak energies of a few hundred keV in the observer frame, and steep slopes in the range −3/2 ≤ α ≤ −1, at the peak of the lightcurve.…”

Section: Discussionmentioning

confidence: 99%

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Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Daigne

Bošnjak

Dubus

2011

A&A

200

253

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Context. Gamma-ray burst emission in the prompt phase is often interpreted as synchrotron radiation from high-energy electrons accelerated in internal shocks. Fast synchrotron cooling of a power-law distribution of electrons leads to the prediction that the slope below the spectral peak has a photon index α = −3/2 (N(E) ∝ E α ). However, this differs significantly from the observed median value α ≈ −1. This discrepancy has been used to argue against this scenario. Aims. We quantify the influence of inverse Compton (IC) and adiabatic cooling on the low energy slope to understand whether these processes can reconcile the observed slopes with a synchrotron origin. Methods. We use a time-dependent code developed to calculate the GRB prompt emission within the internal shock model. The code follows both the shock dynamics and electron energy losses and can be used to generate lightcurves and spectra. We investigate the dependence of the low-energy slope on the parameters of the model and identify parameter regions that lead to values α > −3/2. Results. Values of α between −3/2 and −1 are reached when electrons suffer IC losses in the Klein-Nishina regime. This does not necessarily imply a strong IC component in the Fermi/LAT range (GeV) because scatterings are only moderately efficient. Steep slopes require that a large fraction (10−30%) of the dissipated energy is given to a small fraction ( < ∼ 1%) of the electrons and that the magnetic field energy density fraction remains low ( < ∼ 0.1%). Values of α up to −2/3 can be obtained with relatively high radiative efficiencies (>50%) when adiabatic cooling is comparable with radiative cooling (marginally fast cooling). This requires collisions at small radii and/or with low magnetic fields. Conclusions. Amending the standard fast cooling scenario to account for IC cooling naturally leads to values α up to −1. Marginally fast cooling may also account for values of α up to −2/3, although the conditions required are more difficult to reach. About 20% of GRBs show spectra with slopes α > −2/3. Other effects, not investigated here, such as a thermal component in the electron distribution or pair production by high-energy gamma-ray photons may further affect α. Still, the majority of observed GRB prompt spectra can be reconciled with a synchrotron origin, constraining the microphysics of mildly relativistic internal shocks.

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Section: Introductionmentioning

confidence: 84%

Section: Discussionmentioning

confidence: 99%

Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Daigne

Bošnjak

Dubus

2011

A&A

200

253

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“…, and γ max ∼ 10 8 / √ B, while the synchrotron characteristic frequencies ν c , ν m , ν max can be calculated by ν i = γ 2 i DeB/[2πm e c(1 + z)] (where i = c, m, max), where Y (γ * ) is the Compton factor for electrons with the Lorentz factor γ * , defined as the ratio of the self-inverse Compton (SSC) to synchrotron emission (Nakar et al 2009). …”

Section: Radiationmentioning

confidence: 99%

“…To confirm whether SSC make a significant contribution to the GeV afterglow or Klein-Nishina effect changes the synchrotron and SSC spectra, we need to discuss the value of Y (γ c ) in the slow cooling regime or Y (γ m ) in the fast cooling regime (Nakar et al 2009), therefore Y (γ GeV ) can be estimated (Wang et al 2010). If no solution is found in the range of (10 53 ergs E K,iso 10 56 ergs and 10 −2…”

Section: Constraints On Parametersmentioning

confidence: 99%

Multiband fitting to three long GRBs with Fermi/LAT data: structured ejecta sweeping up a density-jump medium

Feng¹,

Dai²

2011

Res. Astron. Astrophys.

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We present broadband (radio, optical, X-ray and GeV) fits to the afterglow light curves and spectra of three long-duration gamma-ray bursts (GRBs 080916C, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on the Fermi satellite. Using the observed broadband data, we study the origin of the high energy emission, and suggest that the early-time GeV emission and the late-time radio, optical, and X-ray afterglows can be understood as being due to synchrotron emission from an external forward shock caused by structured ejecta propagating in a wind bubble jumping to a homogeneous density medium. If the ceasing time for majority of the energy injection is assumed to be close to the deceleration time of the forward shock, the structured ejecta with continuous energy injection to the forward shock can well explain the early rising feature of the GeV mission from these burst, and the density-jump medium can account for some certain plateaus or flares in the late afterglows. From our fits, we find that, on one hand, the external shock origin of the GeV photons will make the optical depth have not significant contribution to the early LAT rising part, which will loosen strong constraint of lower limits of Lorentz factor. On the other hand, these Fermi-LAT events preferentially occur in a low-density circumburst environment, in which case the Klein-Nishina cutoff will significantly suppress the Self-Synchrotron Compton (SSC) radiation. Such an environment might result from superbubbles or low-metallicity progenitor stars (which have a low mass-loss rate at late times of stellar evolution) of type Ib/c supernovae.

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“…Among the first class of models, we recall scenarios invoking Comptonization and/or thermal components Blinnikov et al 1999;Ghisellini & Celotti 1999;Lazzati et al 2000;Mészáros & Rees 2000;Stern & Poutanen 2004;Rees & Mészáros 2005;Ryde & Pe'er 2009;Guiriec et al 2011Guiriec et al , 2015aGuiriec et al ,b, 2016aGhirlanda et al 2013;Burgess et al 2014). For the second class of models (studies that consider synchrotron radiation) effects producing a hardening of the low-energy spectral index have been invoked, such as Klein-Nishina effects, marginally fast cooling regime, and anisotropic pitch angle distributions (Lloyd & Petrosian 2000;Derishev 2001Derishev , 2007Bosnjak et al 2009;Nakar et al 2009;Daigne et al 2011;Uhm & Zhang 2014). In spite of all theoretical efforts, there is still no consensus on the origin of the prompt emission.…”

Section: Introductionmentioning

confidence: 99%

Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

Oganesyan

Nava

Ghirlanda

et al. 2017

ApJ

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The radiative process responsible for gamma-Ray Burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the νF ν peak energy should display a power-law behavior with slope α 2 = −3/2, which breaks to a higher value α 1 = −2/3 (i.e. to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to ∼ 10−20 keV) are consistent with one single power-law behavior below the peak, with typical slope α = −1, higher than (and then inconsistent with) the expected value α 2 = −3/2. To better characterize the spectral shape at low energy, we analyzed 14 GRBs for which the Swift X-ray Telescope started observations during the prompt. When available, Fermi-GBM observations have been included in the analysis. For 67% of the spectra, models that usually give a satisfactory description of the prompt (e.g., the Band model) fail in reproducing the 0.5 − 1000 keV spectra: low-energy data outline the presence of a spectral break around a few keV. We then introduce an empirical fitting function that includes a low-energy power law α 1 , a break energy E break , a second power law α 2 , and a peak energy E peak . We find α 1 = −0.66 (σ = 0.35), log(E break /keV) = 0.63 (σ = 0.20), α 2 = −1.46 (σ = 0.31), and log(E peak /keV) = 2.1 (σ = 0.56). The values α 1 and α 2 are very close to expectations from synchrotron radiation. In this context, E break corresponds to the cooling break frequency. The relatively small ratio E peak /E break ∼ 30 suggests a regime of moderately fast cooling, which might solve the long-lasting problem of the apparent inconsistency between measured and predicted low-energy spectral index.

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Klein–nishina Effects on Optically Thin Synchrotron and Synchrotron Self-Compton Spectrum

Cited by 197 publications

References 30 publications

Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation?

Multiband fitting to three long GRBs with Fermi/LAT data: structured ejecta sweeping up a density-jump medium

Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

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