| 1. |
- Begue, Dereli, et al.
(författare)
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A Study of GRBs with Low-luminosity Afterglows
- 2017
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 850:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a sample composed of the 41 faintest X-ray afterglows of the population of long gamma-ray bursts (lGRBs) with known redshift. We study their intrinsic properties (spectral index, decay index, distance, luminosity, isotropic radiated energy, and peak energy) and their luminosity distribution functions to assess whether they belong to the same population as the brighter afterglow events. We find that these events belong to a population of nearby ones, different from the general population of lGRBs. In addition, these events are faint during their prompt phase, and include the few possible outliers of the Amati relation.
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| 2. |
- Begue, Dereli, et al.
(författare)
-
Poynting-flux-dominated jets challenged by their photospheric emission
- 2015
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 802:2
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Tidskriftsartikel (refereegranskat)abstract
- One of the key open question in the study of jets, in general, and jets in gamma-ray bursts (GRBs), in particular, is the magnetization of the outflow. Here we consider the photospheric emission of Poynting-flux-dominated outflows, when the dynamics is mediated by magnetic reconnection. We show that thermal three-particle processes, responsible for the thermalization of the plasma, become inefficient at a radius of rsup ∼ 109.5 cm, far below the photosphere, at ∼1011.5 cm. Conservation of the total photon number above rsup combined with Compton scattering below the photosphere enforces kinetic equilibrium between electrons and photons. This, in turn, leads to an increase in the observed photon temperature, which reaches ≳8 MeV (observed energy) when decoupling the plasma at the photosphere. This result is weakly dependent on the free model parameters. We show that in this case, the expected thermal luminosity is a few percent of the total luminosity, and could therefore be detected. The predicted peak energy is more than an order of magnitude higher than the observed peak energy of most GRBs, which puts strong constraints on the magnetization of these outflows.
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| 3. |
- Begue, Dereli, et al.
(författare)
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Radiative striped wind model for gamma-ray bursts
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 467:3, s. 2594-2611
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we revisit the striped wind model in which the wind is accelerated by magnetic reconnection. In our treatment, radiation is included as an independent component, and two scenarios are considered. In the first one, radiation cannot stream efficiently through the reconnection layer, while the second scenario assumes that radiation is homogeneous in the striped wind. We show how these two assumptions affect the dynamics. In particular, we find that the asymptotic radial evolution of the Lorentz factor is not strongly modified whether radiation can stream through the reconnection layer or not. On the other hand, we show that the width, density and temperature of the reconnection layer are strongly dependent on these assumptions. We then apply the model to the gamma-ray burst context and find that photons cannot diffuse efficiently through the reconnection layer below radius r(D)(Delta) similar to 10(10.5) cm, which is about an order of magnitude below the photospheric radius. Above r(D)(Delta) , the dynamics asymptotes to the solution of the scenario in which radiation can stream through the reconnection layer. As a result, the density of the current sheet increases sharply, providing efficient photon production by the Bremsstrahlung process that could have profound influence on the emerging spectrum. This effect might provide a solution to the soft photon problem in gamma-ray bursts.
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| 4. |
- Bégué, Hüsne Dereli, et al.
(författare)
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The anatomy of a long gamma-ray burst : a simple classification scheme for the emission mechanism(s)
- 2016
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Ingår i: Astrophysical Journal. - : The American Astronomical Society. - 0004-637X .- 1538-4357. ; 820:1
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Tidskriftsartikel (refereegranskat)abstract
- Ultra-relativistic motion and efficient conversion of kinetic energy to radiation are required by gamma-ray burst (GRB) observations, yet they are difficult to simultaneously achieve. Three leading mechanisms have been proposed to explain the observed emission emanating from GRB outflows: radiation from either relativistic internal or external shocks, or thermal emission from a photosphere. Previous works were dedicated to independently treating these three mechanisms and arguing for a sole, unique origin of the prompt emission of GRBs. In contrast, herein, we first explain why all three models are valid mechanisms and that a contribution from each of them is expected in the prompt phase. Additionally, we show that a single parameter, the dimensionless entropy of the GRB outflow, determines which mechanism contributes the most to the emission. More specifically, internal shocks dominate for low values of the dimensionless entropy, external shocks for intermediate values, and finally, photospheric emission for large values. We present a unified framework for the emission mechanisms of GRBs with easily testable predictions for each process.
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| 5. |
- Dereli-Begue, Husne, et al.
(författare)
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A wind environment and Lorentz factors of tens explain gamma-ray bursts X-ray plateau
- 2022
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- The origin of the plateau observed in the early X-ray light curves of gamma ray bursts (GRBs) is debated. Here, the authors show that the observed plateau can be explained within the classical GRB model by considering expanding shell with initial Lorentz factor of a few tens. Gamma-ray bursts (GRBs) are known to have the most relativistic jets, with initial Lorentz factors in the order of a few hundreds. Many GRBs display an early X-ray light-curve plateau, which was not theoretically expected and therefore puzzled the community for many years. Here, we show that this observed signal is naturally obtained within the classical GRB fireball model, provided that the initial Lorentz factor is rather a few tens, and the expansion occurs into a medium-low density wind. The range of Lorentz factors in GRB jets is thus much wider than previously thought and bridges an observational gap between mildly relativistic jets inferred in active galactic nuclei, to highly relativistic jets deduced in few extreme GRBs. Furthermore, long GRB progenitors are either not Wolf-Rayet stars, or the wind properties during the final stellar evolution phase are different than at earlier times. Our model has predictions that can be tested to verify or reject it in the future, such as lack of GeV emission, lack of strong thermal component and long (few seconds) variability during the prompt phase characterizing plateau bursts.
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| 6. |
- Dereli-Begue, Huesne, et al.
(författare)
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Classification of Photospheric Emission in Short GRBs
- 2020
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 897:2
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Tidskriftsartikel (refereegranskat)abstract
- In order to better understand the physical origin of short-duration gamma-ray bursts (GRBs), we perform a time-resolved spectral analysis on a sample of 70 pulses in 68 short GRBs with burst durations T-90 less than or similar to 2 s detected by the Fermi/Gamma-ray Burst Monitor. We apply a Bayesian analysis to all spectra that have statistical significance S >= 15 within each pulse and apply a cutoff power-law model. We then select in each pulse the time bin that has the maximum value of the low-energy spectral index for further analysis. Under the assumption that the main emission mechanism is the same throughout each pulse, this analysis is indicative of pulse emission. We find that about 1/3 of the short GRBs are consistent with a pure, nondissipative photospheric model, at least around the peak of the pulse. This fraction is larger than the corresponding fraction (1/4) obtained for long GRBs. For these bursts, we find (i) a bimodal distribution in the values of the Lorentz factors and the hardness ratios and (ii) an anticorrelation between T-90 and the peak energy, E-pk: T-90 proportional to E-pk(-0.50 +/- 0.19). This correlation disappears when we consider the entire sample. Our results thus imply that the short GRB population may in fact be composed of two separate populations: one that is a continuation of the long GRB population to shorter durations, and another that is distinctly separate with different physical properties. Furthermore, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron).
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| 7. |
- Ryde, Felix, 1970-, et al.
(författare)
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On the alpha-intensity correlation in gamma-ray bursts : subphotospheric heating with varying entropy
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 484:2, s. 1912-1925
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Tidskriftsartikel (refereegranskat)abstract
- The emission mechanism during the prompt phase in gamma-ray bursts (GRBs) can be investigated through correlations between spectral properties. Here, we revisit the correlation relating the instantaneous flux, F, and the photon index below the spectral break, alpha, in individual emission pulses, by studying the 38 most prominent pulses in the Fermi/Gamma-ray Burst Monitor GRB catalogue. First, we search for signatures of the bias in the determination of alpha due to the limited spectral coverage (window effect) expected in the synchrotron case. The absence of such a characteristic signature argues against the simplest synchrotron models. We instead find that the observed correlation between F and alpha can, in general, be described by the relation F(t) proportional to e(k alpha(t)), for which the median k = 3. We suggest that this correlation is a manifestation of subphotospheric heating in a flow with a varying entropy. Around the peak of the light curve, a large entropy causes the photosphere to approach the saturation radius, leading to an intense emission with a narrow spectrum. As the entropy decreases the photosphere secedes from the saturation radius, and weaker emission with a broader spectrum is expected. This simple scenario naturally leads to a correlated variation of the intensity and spectral shape, covering the observed range.
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| 8. |
- Yu, Hoi-Fung, et al.
(författare)
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Bayesian Time-resolved Spectroscopy of GRB Pulses
- 2019
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 886:1
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Tidskriftsartikel (refereegranskat)abstract
- We performed time-resolved spectroscopy on a sample of 38 single pulses from 37 gamma-ray bursts detected by the Fermi/Gamma-ray Burst Monitor during the first 9 yr of its mission. For the first time a fully Bayesian approach is applied. A total of 577 spectra are obtained and their properties studied using two empirical photon models, namely the cutoff power law (CPL) and Band model. We present the obtained parameter distributions, spectral evolution properties, and parameter relations. We also provide the result files containing this information for usage in further studies. It is found that the CPL model is the preferred model, based on the deviance information criterion and the fact that it consistently provides constrained posterior density maps. In contrast to previous works, the high-energy power-law index of the Band model, ?, has in general a lower value for the single pulses in this work. In particular, we investigate the individual spectrum in each pulse, that has the largest value of the low-energy spectral indexes, ?. For these 38 spectra, we find that 60% of the ? values are larger than ?2/3, and thus incompatible with synchrotron emission. Finally, we find that the parameter relations show a variety of behaviors. Most noteworthy is the fact that the relation between ? and the energy flux is similar for most of the pulses, independent of any evolution of the other parameters.
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