| 1. |
- Begue, Damien, et al.
(författare)
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Bethe-Heitler signature in proton synchrotron models for gamma-ray bursts
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We study the effect of Bethe-Heitler (BeHe) pair production on a proton synchrotron model for the prompt emission in gamma-ray bursts (GRBs). The possible parameter space of the model is constrained by consideration of the synchrotron radiation from the secondary BeHe pairs. We find two regimes of interest. 1) At high bulk Lorentz factor, large radius and low luminosity, proton synchrotron emission dominates and produces a spectrum in agreement with observations. For part of this parameter space, a subdominant (in the MeV band) power-law is created by the synchrotron emission of the BeHe pairs. This power-law extends up to few tens or hundreds of MeV. Such a signature is a natural expectation in a proton synchrotron model, and it is seen in some GRBs, including GRB 190114C recently observed by the MAGIC observatory. 2) At low bulk Lorentz factor, small radius and high luminosity, BeHe cooling dominates. The spectrum achieves the shape of a single power-law with spectral index α = −3/2 extending across the entire GBM/Swift energy window, incompatible with observations. Our theoretical results can be used to further constrain the spectral analysis of GRBs in the guise of proton synchrotron models.
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| 2. |
- Begué, Damien
(författare)
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Poynting flux dominated jets challenged by their photospheric emission
- 2015
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Ingår i: SECOND ICRANET CESAR LATTES MEETING. - : American Institute of Physics (AIP). - 9780735413405
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Konferensbidrag (refereegranskat)abstract
- One of the key open question for gamma-ray bursts (GRBs) jets, 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 far below the photosphere. Conservation of the total photon number above this radius, 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 greater than or similar to 8 MeV (observed energy) when decoupling the plasma at the photosphere. This result is weakly dependent on the free model parameters. 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. |
- Berthet, Gwenaël, et al.
(författare)
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Impact of a moderate volcanic eruption on chemistry in the lower stratosphere : balloon-borne observations and model calculations
- 2017
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 17:3, s. 2229-2253
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Tidskriftsartikel (refereegranskat)abstract
- The major volcanic eruption of Mount Pinatubo in 1991 has been shown to have significant effects on stratospheric chemistry and ozone depletion even at midlatitudes. Since then, only moderate but recurrent volcanic eruptions have modulated the stratospheric aerosol loading and are assumed to be one cause for the reported increase in the global aerosol content over the past 15 years. This particularly enhanced aerosol context raises questions about the effects on stratospheric chemistry which depend on the latitude, altitude and season of injection. In this study, we focus on the midlatitude Sarychev volcano eruption in June 2009, which injected 0.9 Tg of sulfur dioxide (about 20 times less than Pinatubo) into a lower stratosphere mainly governed by high-stratospheric temperatures. Together with in situ measurements of aerosol amounts, we analyse high-resolution in situ and/or remote-sensing observations of NO2, HNO3 and BrO from balloon-borne infrared and UV-visible spectrometers launched in Sweden in August-September 2009. It is shown that differences between observations and three-dimensional (3-D) chemistry-transport model (CTM) outputs are not due to transport calculation issues but rather reflect the chemical impact of the volcanic plume below 19 km altitude. Good measurement-model agreement is obtained when the CTM is driven by volcanic aerosol loadings derived from in situ or space-borne data. As a result of enhanced N2O5 hydrolysis in the Sarychev volcanic aerosol conditions, the model calculates reductions of similar to 45% and increases of similar to 11% in NO2 and HNO3 amounts respectively over the August-September 2009 period. The decrease in NOx abundances is limited due to the expected saturation effect for high aerosol loadings. The links between the various chemical catalytic cycles involving chlorine, bromine, nitrogen and HOx compounds in the lower stratosphere are discussed. The increased BrO amounts (similar to 22 %) compare rather well with the balloon-borne observations when volcanic aerosol levels are accounted for in the CTM and appear to be mainly controlled by the coupling with nitrogen chemistry rather than by enhanced BrONO2 hydrolysis. We show that the chlorine partitioning is significantly controlled by enhanced BrONO2 hydrolysis. However, simulated effects of the Sarychev eruption on chlorine activation are very limited in the high-temperature conditions in the stratosphere in the period considered, inhibiting the effect of ClONO2 hydrolysis. As a consequence, the simulated chemical ozone loss due to the Sarychev aerosols is low with a reduction of -22 ppbv (-1.5 %) of the ozone budget around 16 km. This is at least 10 times lower than the maximum ozone depletion from chemical processes (up to -20 %) reported in the Northern Hemisphere lower stratosphere over the first year following the Pinatubo eruption. This study suggests that moderate volcanic eruptions have limited chemical effects when occurring at midlatitudes (restricted residence times) and outside winter periods (high-temperature conditions). However, it would be of interest to investigate longer-lasting tropical volcanic plumes or sulfur injections in the wintertime low-temperature conditions.
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| 4. |
- Burgess, J. Michael, et al.
(författare)
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AN EXTERNAL SHOCK ORIGIN OF GRB 141028A
- 2016
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 822:2
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Tidskriftsartikel (refereegranskat)abstract
- The prompt emission of the long, smooth, and single-pulsed gamma-ray burst, GRB. 141028A, is analyzed under the guise of an external shock model. First, we fit the gamma-ray spectrum with a two-component photon model, namely, synchrotron+blackbody, and then fit the recovered evolution of the synchrotron nu F-nu peak to an analytic model derived considering the emission of a relativistic blast. wave expanding into an external medium. The prediction of the model for the nu F-nu peak evolution matches well with the observations. We observe the blast. wave transitioning into the deceleration phase. Furthermore, we assume the expansion of the blast. wave to be nearly adiabatic, motivated by the low magnetic field deduced from the observations. This allows us to recover within an order of magnitude the flux density at the nu F-nu peak, which is remarkable considering the simplicity of the analytic model. Under this scenario we argue that the distinction between prompt and afterglow emission is superfluous as both early-time emission and late-time emission emanate from the same source. While the external shock model is clearly not a universal solution, this analysis opens the possibility that at least some fraction of GRBs can be explained with an external shock origin of their prompt phase.
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| 5. |
- Samuelsson, Filip, et al.
(författare)
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Constraining Low-luminosity Gamma-Ray Bursts as Ultra-high-energy Cosmic Ray Sources Using GRB 060218 as a Proxy
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We study the connection between low-luminosity gamma-ray bursts (llGRBs) and ultra-high-energy cosmic rays using the canonical low-luminosity GRB 060218 as a proxy. We focus on the consequen- tial synchrotron emission from electrons that are co-accelerated in the UHECR acceleration region, comparing this emission to observations. Both the prompt and afterglow phases are considered. For the prompt phase, we find that bright optical-UV emission is inevitable if the co-accelerated electrons are instantaneously injected into a power-law distribution. To enable acceleration of UHECRs while accommodating the optical-UV emission, it is necessary to keep the electrons from fast cooling (e.g., via reheating). Yet, the energetics of such models are independently constrained from our analysis of the afterglow. For the afterglow phase, we consider mildly relativistic outflows with bulk Lorentz factor $\Gamma \gtrsim 2$. Using thermal synchrotron radiation, we show that the initial kinetic energy of the afterglow blast wave of GRB 060218 was 10 times lower than the minimum energy required to satisfy the observed flux of UHECRs. Indeed, a blast wave with sufficient energy and where electrons carry 10–20% of the energy as suggested by particle-in-cell simulations, would typically overshoot the available radio data at ∼ 3 days by more than an order of magnitude. If GRB 060218 is representative of the llGRB population as a whole, then our results show that their relativistic afterglows are unlikely to be the dominant sources of UHECRs. It also implies that for the prompt phase to be the main origin of UHECRs, a majority of the energy would need to escape as cosmic rays, neutrinos, or radiation before the onset of the afterglow, independent of the prompt emission mechanism. More generally, our study demonstrates that synchrotron emission from thermal electrons is a powerful diagnostic of the physics of mildly relativistic shocks.
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| 6. |
- Samuelsson, Filip, et al.
(författare)
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Constraining Low-luminosity Gamma-Ray Bursts as Ultra-high-energy Cosmic Ray Sources Using GRB 060218 as a Proxy
- 2020
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 902:2
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Tidskriftsartikel (refereegranskat)abstract
- We study the connection between low-luminosity gamma-ray bursts (llGRBs) and ultra-high-energy cosmic rays (UHECRs) using the canonical low-luminosity GRB 060218 as a proxy. We focus on the consequential synchrotron emission from electrons that are coaccelerated in the UHECR acceleration region, comparing this emission to observations. Both the prompt and afterglow phases are considered. For the prompt phase, we assume the coaccelerated electrons are injected with a power-law distribution instantaneously (without additional heating or reacceleration), which results in bright optical-UV emission in tension with observations. For the afterglow phase, we constrain the total kinetic energy of the blast wave by comparing electron thermal synchrotron radiation to available radio data at similar to 3 days. Considering mildly relativistic outflows with bulk Lorentz factor Gamma greater than or similar to 2 (slower transrelativistic outflows are not treated), we find that the limited available energy does not allow for GRB 060218-like afterglows to be the main origin of UHECRs. This analysis independently constrains the prompt phase as a major UHECR source as well, given that the prompt energy budget is comparable to that of the afterglow kinetic energy. More generally, our study demonstrates that synchrotron emission from thermal electrons is a powerful diagnostic of the physics of mildly relativistic shocks.
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| 7. |
- Samuelsson, Filip, et al.
(författare)
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The Limited Contribution of Low- and High-luminosity Gamma-Ray Bursts to Ultra-high-energy Cosmic Rays
- 2019
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 876:2
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Tidskriftsartikel (refereegranskat)abstract
- The acceleration site for ultra-high-energy cosmic rays (UHECRs) is still an open question despite extended research. In this paper, we reconsider the prompt phase of gamma-ray bursts (GRBs) as a possible candidate for this acceleration and constrain the maximum proton energy in optically thin synchrotron and photospheric models, using properties of the prompt photon spectra. We find that neither of the models favors acceleration of protons to 10(20) eV in high-luminosity bursts. We repeat the calculations for low-luminosity GRBs (llGRBs) considering both protons and completely stripped iron and find that the highest obtainable energies are < 10(19) eV and < 10(20) eV for protons and iron respectively, regardless of the model. We conclude therefore that for our fiducial parameters, GRBs, including low-luminosity bursts, contribute little to nothing to the UHECRs observed. We further constrain the conditions necessary for an association between UHECRs and llGRBs and find that iron can be accelerated to 1020 eV in photospheric models, given very efficient acceleration and/or a small fractional energy given to a small fraction of accelerated electrons. This will necessarily result in high prompt optical fluxes, and the detection of such a signal could therefore be an indication of successful UHECR acceleration at the source.
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| 8. |
- Samuelsson, Filip, et al.
(författare)
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The problematic connection between low-luminosity gamma-ray bursts and ultra-high-energy cosmic rays
- 2022
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Konferensbidrag (refereegranskat)abstract
- The origin of ultra-high-energy cosmic rays (UHECRs) remains debated. The prompt and afterglow phases of low-luminosity gamma-ray bursts (LLGRBs) are seen as promising candidates for this acceleration. Here, we investigate this connection by looking at the unavoidable emission from the electrons that are co-accelerated together with UHECRs. Specifically, we use the data from the archetypical low-luminosity GRB 060218. We find that if acceleration of UHECRs occurred during the prompt phase, the emission from the electrons would be orders of magnitude brighter than the observations in the optical band. For the afterglow phase, we limit the total available kinetic energy by comparing the emission from the thermal electrons to the radio data at three days. We find that the total energy in the afterglow is not sufficient to supply the UHECR flux observed at Earth. These results challenge the mildly relativistic outflows of LLGRBs as the main sources of UHECRs.
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