Rise and fall of the high-energy afterglow emission of GRB 180720B

Ronchi, M.; Fumagalli, Francesca; Ravasio, M. E.; Oganesyan, G.; Toffano, Mattia; Salafia, O. S.; Nava, Lara; Ascenzi, S.; Ghirlanda, G.; Ghisellini, G.

doi:10.1051/0004-6361/201936765

Cited by 25 publications

(26 citation statements)

References 73 publications

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“…A prototypical example, GRB 180720B at z = 0.65 (Fig. 6 top left panel) observed by Fermi/GBM, revealed [99] a steep electron energy distribution (inferred from the slope of the high energy power law spectrum) and a relatively small comoving frame magnetic field, which challenge the acceleration mechanisms, the common understanding of the jet composition, and the standard electron-synchrotron scenario [50].…”

Section: The Nature Of the Prompt Emissionmentioning

confidence: 99%

“…For instance, at z ∼2, corresponding to the peak of the THESEUS-detected population, the break energy will be constrained with an accuracy of 20% compared to >50% of Fermi (compare solid and dashed black lines Fig. 6 Simulated prompt emission spectrum of GRB 180720B [99] as observable by Fermi/GBM (top left panel) and by THESEUS/SXI+XGIS (bottom left panel). The assumed spectral model is a double break power-law [95] with parameters from [99].…”

Section: Accuracy Of Spectral Parametersmentioning

confidence: 99%

“…6 Simulated prompt emission spectrum of GRB 180720B [99] as observable by Fermi/GBM (top left panel) and by THESEUS/SXI+XGIS (bottom left panel). The assumed spectral model is a double break power-law [95] with parameters from [99]. The red line highlight the model in the instrumental energy range.…”

Section: Accuracy Of Spectral Parametersmentioning

confidence: 99%

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Gamma ray burst studies with THESEUS

et al. 2021

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Gamma-ray Bursts (GRBs) are the most powerful transients in the Universe, over–shining for a few seconds all other γ-ray sky sources. Their emission is produced within narrowly collimated relativistic jets launched after the core–collapse of massive stars or the merger of compact binaries. THESEUS will open a new window for the use of GRBs as cosmological tools by securing a statistically significant sample of high-z GRBs, as well as by providing a large number of GRBs at low–intermediate redshifts extending the current samples to low luminosities. The wide energy band and unprecedented sensitivity of the Soft X-ray Imager (SXI) and X-Gamma rays Imaging Spectrometer (XGIS) instruments provide us a new route to unveil the nature of the prompt emission. For the first time, a full characterisation of the prompt emission spectrum from 0.3 keV to 10 MeV with unprecedented large count statistics will be possible revealing the signatures of synchrotron emission. SXI spectra, extending down to 0.3 keV, will constrain the local metal absorption and, for the brightest events, the progenitors’ ejecta composition. Investigation of the nature of the internal energy dissipation mechanisms will be obtained through the systematic study with XGIS of the sub-second variability unexplored so far over such a wide energy range. THESEUS will follow the spectral evolution of the prompt emission down to the soft X–ray band during the early steep decay and through the plateau phase with the unique ability of extending above 10 keV the spectral study of these early afterglow emission phases.

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Section: The Nature Of the Prompt Emissionmentioning

confidence: 99%

Section: Accuracy Of Spectral Parametersmentioning

confidence: 99%

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Gamma ray burst studies with THESEUS

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“…The low-energy part of the observed spectrum, however, is harder than the predicted value, having α ∼−1. A major ease to this situation came with the discovery that many GRB spectra have a spectral break in the low-energy part [79,80]: in this case, the spectral regime α = −0.67 is visible inside the energy rage of the soft/hard X-ray instruments, and the spectral break (identified between 1-100 keV [79][80][81][82][83][84]) connects the segment α 1 = −0.67 with the segment α 2 = −1.5. In this configuration (sometimes named marginally fast cooling [85]), the break is identified with the cooling break frequency, and the radiation efficiency is still large and does not affect the estimates on the jet energetics.…”

Section: Prompt Emissionmentioning

confidence: 99%

Gamma-ray Bursts at the Highest Energies

Nava

2021

Universe

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Emission from Gamma-ray bursts is thought to be powered mainly by synchrotron radiation from energetic electrons. The same electrons might scatter these synchrotron seed photons to higher (>10 GeV) energies, building a distinct spectral component (synchrotron self-Compton, SSC). This process is expected to take place, but its relevance (e.g., the ratio between the SSC and synchrotron emitted power) is difficult to predict on the basis of current knowledge of physical conditions at GRB emission sites. Very high-energy radiation in GRBs can be produced also by other mechanisms, such as synchrotron itself (if PeV electrons are produced at the source), inverse Compton on external seed photons, and hadronic processes. Recently, after years of efforts, very high-energy radiation has been finally detected from at least four confirmed long GRBs by the Cherenkov telescopes H.E.S.S. and MAGIC. In all four cases, the emission has been recorded during the afterglow phase, well after the end of the prompt emission. In this work, I give an overview, accessible also to non-experts of the field, of the recent detections, theoretical implications, and future challenges, with a special focus on why very high-energy observations are relevant for our understanding of Gamma-ray bursts and which long-standing questions can be finally answered with the help of these observations.

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“…In addition, the authors did not attempt to fit this theoretical model to the data, which might introduce instrumental biases in the comparison with the Band fit results. 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: 99%

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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Rise and fall of the high-energy afterglow emission of GRB 180720B

Cited by 25 publications

References 73 publications

Gamma ray burst studies with THESEUS

Gamma ray burst studies with THESEUS

Gamma-ray Bursts at the Highest Energies

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

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