Evidence of two spectral breaks in the prompt emission of gamma-ray bursts

Ravasio, M. E.; Ghirlanda, G.; Nava, Lara; Ghisellini, G.

doi:10.1051/0004-6361/201834987

Cited by 77 publications

(95 citation statements)

References 41 publications

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“…The fact that the majority of the spectra of bright long GRBs can be fitted with the above mentioned three-power law model (Oganesyan et al 2017;Ravasio et al 2018Ravasio et al , 2019 indeed suggests that the synchrotron process is the radiative mechanism originating the prompt emission. This implies that the emitting particles do not cool completely (Daigne, Bosnjak & Dubus 2011), but "remain" at the energy γ cool for a timescale comparable to the typical time bin of the time resolved spectral analysis (∼ 1 s).…”

Section: Introductionmentioning

confidence: 95%

Proton–synchrotron as the radiation mechanism of the prompt emission of gamma-ray bursts?

Ghisellini

Ghirlanda

Oganesyan

et al. 2020

A&A

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We discuss the new surprising observational results that indicate quite convincingly that the prompt emission of Gamma-Ray Bursts (GRBs) is due to synchrotron radiation produced by a particle distribution that has a low energy cut-off. The evidence of this is provided by the low energy part of the spectrum of the prompt emission, that shows the characteristic F ν ∝ ν 1/3 shape followed by F ν ∝ ν −1/2 up to the peak frequency. This implies that although the emitting particles are in fast cooling, they do not cool completely. This poses a severe challenge to the basic ideas about how and where the emission is produced, because the incomplete cooling requires a small value of the magnetic field, to limit synchrotron cooling, and a large emitting region, to limit the self-Compton cooling, even considering Klein-Nishina scattering effects. Some new and fundamental ingredient is required for understanding the GRBs prompt emission. We propose proton-synchrotron as a promising mechanism to solve the incomplete cooling puzzle.

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Section: Introductionmentioning

confidence: 95%

Proton–synchrotron as the radiation mechanism of the prompt emission of gamma-ray bursts?

Ghisellini

Ghirlanda

Oganesyan

et al. 2020

A&A

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“…So, after a decade from 2009, the main component that shapes the GRB prompt emission spectra is identified 23,25,27,28,30 . With detailed observations and data analyses of the broad-band prompt emission data (γ-rays, X-rays and optical) of many GRBs using the current and future GRB observatories, it would be possible to quantify the distribution of the GRB jet composition in the future.…”

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confidence: 99%

Synchrotron radiation in γ-ray bursts prompt emission

Zhang

2020

Nat Astron

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Growing evidence suggests that synchrotron radiation plays a significant role in shaping the spectra of most γ-ray bursts. The relativistic jets producing them likely carry a significant fraction of energy in the form of a Poynting flux.The electromagnetic spectra of γ-ray bursts (GRBs) are usually characterized by a mathematical function invoking an exponentially-connected two-power-law function first proposed in a paper by the Burst And Transient Source Experiment (BATSE) team led by David Band (1957-2009) 1 . In the special session in memory of Band at the 2009 Fermi Symposium, Josh Grindlay, his PhD advisor from Harvard University, challenged GRB theorists to "find the physical meaning" of the "Band function" in 10 years.In the GRB field there was never a lack of models to interpret data. The challenge is rather to "identify" than to "find" the physical meaning of the Band function. Indeed, two leading models proposed long ago could both roughly account for the the general shape of the spectra but both models have one critical flaw regarding the low-energy photon index of the Band function, denoted as α. The measured values of α peak around ∼ −1 with a broad distribution 2, 3 . The first model invokes synchrotron radiation 4, 5 of the electrons accelerated in the energy dissipation regions (internal shocks or magnetic reconnection sites) to account for the observed γ-rays. However, for a typical GRB environment the electrons are in the so-called "fast-cooling" regime that demands 6, 7 α = −1.5. This is too "soft" to account for the data. The second model invokes quasi-thermal emission from a relativistic fireball 8, 9 .The model typically predicts 10, 11 α ∼ +1.5. This becomes too "hard".

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“…Direct fits of the synchrotron emission model to GRB prompt spectra have been performed by Zhang et al (2016) and Burgess (2019), who showed that the line-of-death and spectralsharpness issues are likely artefacts due to the use of the Band function (see also Ronchi et al 2020). Fitting the spectra with simpler versions of the synchrotron emission model or with empirical functions featuring a low-energy spectral break, especially on a broad energy range extending down to the X-ray and/or optical domain, appears able to reconcile the observations with the synchrotron theory as well, showing the expected transition from fast to slow cooling (Oganesyan et al 2017(Oganesyan et al , 2018(Oganesyan et al , 2019Ravasio et al 2018Ravasio et al , 2019.…”

Section: Introductionmentioning

confidence: 91%

A new fitting function for GRB MeV spectra based on the internal shock synchrotron model

Yassine

Piron

Daigne

et al. 2020

A&A

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Aims. The physical origin of the gamma-ray burst (GRB) prompt emission is still a subject of debate. Internal shock models have been widely explored, owing to their ability to explain most of the high-energy properties of this emission phase. While the Band function or other phenomenological functions are commonly used to fit GRB prompt emission spectra, we propose a new parametric function that is inspired by an internal shock physical model. We use this function as a proxy of the model to compare it easily to GRB observations. Methods. We built a parametric function that represents the spectral form of the synthetic bursts provided by our internal shock synchrotron model (ISSM). We simulated the response of the Fermi instruments to the synthetic bursts and fit the obtained count spectra to validate the ISSM function. Then, we applied this function to a sample of 74 bright GRBs detected by the Fermi GBM, and we computed the width of their spectral energy distributions around their peak energy. For comparison, we also fit the phenomenological functions that are commonly used in the literature. Finally, we performed a time-resolved analysis of the broadband spectrum of GRB 090926A, which was jointly detected by the Fermi GBM and LAT. This spectrum has a complex shape and exhibits a power-law component with an exponential cutoff at high energy, which is compatible with inverse Compton emission attenuated by gamma-ray internal absorption. Results. This work proposes a new parametric function for spectral fitting that is based on a physical model. The ISSM function reproduces 81% of the spectra in the GBM bright GRB sample, versus 59% for the Band function, for the same number of parameters. It gives also relatively good fits to the GRB 090926A spectra. The width of the MeV spectral component that is obtained from the fits of the ISSM function is slightly larger than the width from the Band fits, but it is smaller when observed over a wider energy range. Moreover, all of the 74 analyzed spectra are found to be significantly wider than the synthetic synchrotron spectra. We discuss possible solutions to reconcile the observations with the internal shock synchrotron model, such as an improved modeling of the shock microphysics or more accurate spectral measurements at MeV energies.

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Evidence of two spectral breaks in the prompt emission of gamma-ray bursts

Cited by 77 publications

References 41 publications

Proton–synchrotron as the radiation mechanism of the prompt emission of gamma-ray bursts?

Proton–synchrotron as the radiation mechanism of the prompt emission of gamma-ray bursts?

Synchrotron radiation in γ-ray bursts prompt emission

A new fitting function for GRB MeV spectra based on the internal shock synchrotron model

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