| 1. |
- Ryde, Felix, 1970-, et al.
(författare)
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Onset of Particle Acceleration during the Prompt Phase in Gamma-Ray Bursts as Revealed by Synchrotron Emission in GRB 160821A
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 932:2, s. L15-
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Tidskriftsartikel (refereegranskat)abstract
- The physical processes of gamma-ray emission and particle acceleration during the prompt phase in gamma-ray bursts (GRBs) are still unsettled. In order to perform unambiguous physical modeling of observations, a clear identification of the emission mechanism is needed. An instance of a clear identification is the synchrotron emission during the very strong flare in GRB 160821A, which occurred during the prompt phase at 135 s. Here we show that the distribution of the radiating electrons in this flare is initially very narrow but later develops a power-law tail of accelerated electrons. We thus identify for the first time the onset of particle acceleration in a GRB jet. The flare is consistent with a late energy release from the central engine causing an external shock as it encounters a preexisting ring nebula of a progenitor Wolf-Rayet star. Relativistic forward and reverse shocks develop, leading to two distinct emission zones with similar properties. The particle acceleration only occurs in the forward shock, moving into the dense nebula matter. Here, the magnetization also decreases below the critical value, which allows for Fermi acceleration to operate. Using this fact, we find a bulk Lorentz factor of 420 less than or similar to Gamma less than or similar to 770 and an emission radius of R similar to 10(18) cm, indicating a tenuous gas of the immediate circumburst surroundings. The observation of the onset of particle acceleration thus gives new and independent constraints on the properties of the flow as well as on theories of particle acceleration in collisionless astrophysical shocks.
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| 2. |
- Acuner, Zeynep, et al.
(författare)
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The Fraction of Gamma-Ray Bursts with an Observed Photospheric Emission Episode
- 2020
-
Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 893:2
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Tidskriftsartikel (refereegranskat)abstract
- There is no complete description of the emission physics during the prompt phase in gamma-ray bursts. Spectral analyses, however, indicate that many spectra are narrower than what is expected for nonthermal emission models. Here, we reanalyze the sample of 37 bursts in Yu et al. by fitting the narrowest time-resolved spectrum in each burst. We perform a model comparison between photospheric and synchrotron emission models based on Bayesian evidence. We compare the shapes of the narrowest expected spectra: emission from the photosphere in a non-dissipative flow and slow cooled synchrotron emission from a narrow electron distribution. We find that the photospheric spectral shape is preferred by 54% 8% of the spectra (20/37), while 38% 8% of the spectra (14/37) prefer the synchrotron spectral shape; three spectra are inconclusive. We hence conclude that GRB spectra are indeed very narrow and that more than half of the bursts have a photospheric emission episode. We also find that a third of all analyzed spectra, not only prefer, but are also compatible with a non-dissipative photosphere, confirming previous similar findings. Furthermore, we notice that the spectra that prefer the photospheric model all have low-energy power-law indices alpha greater than or similar to -0.5. This means that alpha is a good estimator for which model is preferred by the data. Finally, we argue that the spectra that statistically prefer the synchrotron model could equally as well be caused by subphotospheric dissipation. If that is the case, photospheric emission during the early, prompt phase would be even more dominant.
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| 3. |
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| 4. |
- Ahlgren, Björn, et al.
(författare)
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Investigating Subphotospheric Dissipation in Gamma-Ray Bursts Using Joint Fermi-Swift Observations
- 2019
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 880:2
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Tidskriftsartikel (refereegranskat)abstract
- The jet photosphere has been proposed as the origin for the gamma-ray burst (GRB) prompt emission. In many such models, characteristic features in the spectra appear below the energy range of the Fermi Gamma-ray Burst Monitor (GBM) detectors, so joint fits with X-ray data are important in order to assess the photospheric scenario. Here we consider a particular photospheric model which assumes localized subphotospheric dissipation by internal shocks in a non-magnetized outflow. We investigate it using Bayesian inference and a sample of eight GRBs with known redshifts which are observed simultaneously with Fermi GBM and the Swift X-ray Telescope (XRT). This provides us with an energy range of 0.3. keV-40. MeV and much tighter parameter constraints. We analyze 32 spectra and find that 16 are well described by the model. We also find that the estimates of the bulk Lorentz factor, Gamma, and the fireball luminosity, L-0,L-52, decrease while the fraction of dissipated energy, epsilon(d), increases in the joint fits compared to GBM-only fits. These changes are caused by a small excess of counts in the XRT data, relative to the model predictions from fits to GBM-only data. The fact that our limited implementation of the physical scenario yields 50% accepted spectra is promising, and we discuss possible model revisions in the light of the new data. Specifically, we argue that the inclusion of significant magnetization, as well as removing the assumption of internal shocks, will provide better fits at low energies.
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| 5. |
- Ahlgren, Björn, et al.
(författare)
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Testing a model for subphotospheric dissipation in GRBs: fits to Fermi data constrain the dissipation scenario
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 485, s. 474-497
-
Tidskriftsartikel (refereegranskat)abstract
- It has been suggested that the prompt emission in gamma-ray bursts (GRBs) could be described by radiation from the photosphere in a hot fireball. Such models must be tested by directly fitting them to data. In this work we use data from the Fermi Gamma-ray Space Telescope and consider a specific photospheric model, in which the kinetic energy of a low-magnetization outflow is dissipated locally by internal shocks below the photosphere. We construct a table model with a physically motivated parameter space and fit it to time-resolved spectra of the 36 brightest Fermi GRBs with a known redshift. We find that about two-thirds of the examined spectra cannot be described by the model, as it typically underpredicts the observed flux. However, since the sample is strongly biased towards bright GRBs, we argue that this fraction will be significantly lowered when considering the full population. From the successful fits we find that the model can reproduce the full range of spectral slopes present in the sample. For these cases we also find that the dissipation consistently occurs at a radius of ∼1012 cm and that only a few per cent efficiency is required. Furthermore, we find a positive correlation between the fireball luminosity and the Lorentz factor. Such a correlation has been previously reported by independent methods. We conclude that if GRB spectra are due to photospheric emission, the dissipation cannot only be the specific scenario we consider here.
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| 6. |
- 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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| 7. |
- Bret, Antoine, et al.
(författare)
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Departure from MHD prescriptions in shock formation over a guiding magnetic field
- 2017
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Ingår i: Laser and particle beams (Print). - : Cambridge University Press. - 0263-0346 .- 1469-803X. ; 35, s. 513-519
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Tidskriftsartikel (refereegranskat)abstract
- In plasmas where the mean-free-path is much larger than the size of the system, shock waves can arise with a front much shorter than the mean-free path. These so-called "collisionless shocks" are mediated y collective plasma interactions. Studies conducted so far on these shocks found that although binary collisions are absent, the distribution functions are thermalized downstream by scattering on the fields, so that magnetohydrodynamic prescriptions may apply. Here we show a clear departure from this pattern in the case of Weibel shocks forming over a flow-aligned magnetic field. A micro-physical analysis of the particle motion in the Weibel filaments shows how they become unable to trap the flow in the presence of too strong a field, inhibiting the mechanism of shock formation. Particle-in-cell simulations confirm these results.
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| 8. |
- 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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| 9. |
- Burgess, J. Michael, et al.
(författare)
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AN OBSERVED CORRELATION BETWEEN THERMAL AND NON-THERMAL EMISSION IN GAMMA-RAY BURSTS
- 2014
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 784:2
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Tidskriftsartikel (refereegranskat)abstract
- Recent observations by the Fermi Gamma-ray Space Telescope have confirmed the existence of thermal and non-thermal components in the prompt photon spectra of some gamma-ray bursts (GRBs). Through an analysis of six bright Fermi GRBs, we have discovered a correlation between the observed photospheric and non-thermal gamma-ray emission components of several GRBs using a physical model that has previously been shown to be a good fit to the Fermi data. From the spectral parameters of these fits we find that the characteristic energies, E-p and kT, of these two components are correlated via the relation E-p proportional to T-alpha which varies from GRB to GRB. We present an interpretation in which the value of the index alpha indicates whether the jet is dominated by kinetic or magnetic energy. To date, this jet composition parameter has been assumed in the modeling of GRB outflows rather than derived from the data.
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| 10. |
- 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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| 11. |
- 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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| 12. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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Change of a Weibel-type to an Alfvénic shock in pair plasma by upstream waves
- 2020
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 27:6
-
Tidskriftsartikel (refereegranskat)abstract
- We examine with particle-in-cell simulations how a parallel shock in pair plasma reacts to upstream waves, which are driven by escaping downstream particles. Initially, the shock is sustained in the two-dimensional simulation by a magnetic filamentation (beam-Weibel) instability. Escaping particles drive an electrostatic beam instability upstream. Modifications of the upstream plasma by these waves hardly affect the shock. In time, a decreasing density and an increasing temperature of the escaping particles quench the beam instability. A larger thermal energy along than perpendicular to the magnetic field destabilizes the pair-Alfvén mode. In the rest frame of the upstream plasma, the group velocity of the growing pair-Alfvén waves is below that of the shock and the latter catches up with the waves. Accumulating pair-Alfvén waves gradually change the shock in the two-dimensional simulation from a Weibel-type shock into an Alfvénic shock with a Mach number that is about 6 for our initial conditions.
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| 13. |
- Gowri, A., et al.
(författare)
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Exploring multi-pulse GRB prompt emission via novel pulse shape model
- 2024
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Ingår i: 8th High Energy Phenomena in Relativistic Outflows, HEPRO 2023. - : Sissa Medialab Srl.
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Konferensbidrag (refereegranskat)abstract
- The light curves of the prompt phase of gamma-ray bursts (GRBs) exhibit erratic and diverse behaviour, often with multiple pulses. The temporal shape of individual pulses is often modelled as ‘fast rise exponential decay’ (FRED). Here, we propose a novel fitting function to measure pulse asymmetry. We perform a time-resolved spectrum analysis on a sample of 75 pulses from twenty-seven GRBs that the Fermi Gamma-ray Burst Monitor has identified. When multi-pulse bursts are taken into account, a distinct behaviour becomes evident: the first pulses have the most symmetric-like lightcurve, while subsequent pulses show an increase in the asymmetry parameter, leading to a more FRED-like form. Furthermore, we correlate pulse temporal and spectral shapes after fitting the spectra with the classical “Band" function. A moderate positive Spearman correlation between pulse asymmetry and the low-energy spectral index αmax (where the maximum is taken over all time bins that cover the pulse shape) is identified. An overlapping emission mechanism is indicated by the fact that ∼ 64% of the GRB pulses fall within the limits of the slow-cooling synchrotron and non-dissipative photospheric emission models. Thus, our findings offer a compelling hint towards understanding the origin of GRB pulses.
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| 14. |
- Li, Liang, et al.
(författare)
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A Cosmological Fireball with 16% Gamma-Ray Radiative Efficiency
- 2023
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 944:2
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. How efficiently the jet converts its energy to radiation is a long-standing problem, which is poorly constrained. The standard model invokes a relativistic fireball with a bright photosphere emission component. A definitive diagnosis of GRB radiation components and the measurement of GRB radiative efficiency require prompt emission and afterglow data, with high resolution and wide band coverage in time and energy. Here, we present a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its fluence is one of the highest for all the GRBs that have been detected so far, which allows us to perform a high-resolution study of the prompt emission spectral properties and their temporal evolutions, down to a timescale of about 0.1 s. We observe that each of the initial pulses has a thermal component contributing ∼20% of the total energy and that the corresponding temperature and inferred Lorentz factor of the photosphere evolve following broken power-law shapes. From the observation of the nonthermal spectra and the light curve, the onset of the afterglow corresponding to the deceleration of the fireball is considered to start at ∼6 s. By incorporating the thermal and nonthermal observations, as well as the photosphere and synchrotron radiative mechanisms, we can directly derive the fireball energy budget with little dependence on hypothetical parameters, measuring a ∼16% radiative efficiency for this GRB. With the fireball energy budget derived, the afterglow microphysics parameters can also be constrained directly from the data.
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| 15. |
- Li, Liang, et al.
(författare)
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Bayesian Time-resolved Spectroscopy of Multipulse GRBs : Variations of Emission Properties among Pulses
- 2021
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Ingår i: Astrophysical Journal Supplement Series. - : IOP PUBLISHING LTD. - 0067-0049 .- 1538-4365. ; 254:2
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral evolution. In particular, the emission properties vary from pulse to pulse in multipulse bursts. Here we present a time-resolved Bayesian spectral analysis of a compilation of GRB pulses observed by the Fermi/Gamma-ray Burst Monitor. The pulses are selected to have at least four time bins with a high statistical significance, which ensures that the spectral fits are well determined and spectral correlations can be established. The sample consists of 39 bursts, 117 pulses, and 1228 spectra. We confirm the general trend that pulses become softer over time, with mainly the low-energy power-law index alpha becoming smaller. A few exceptions to this trend exist, with the hardest pulse occurring at late times. The first pulse in a burst is clearly different from the later pulses; three-fourths of them violate the synchrotron line of death, while around half of them significantly prefer photospheric emission. These fractions decrease for subsequent pulses. We also find that in two-thirds of the pulses, the spectral parameters (alpha and peak energy) track the light-curve variations. This is a larger fraction compared to what is found in previous samples. In conclusion, emission compatible with the GRB photosphere is typically found close to the trigger time, while the chance of detecting synchrotron emission is greatest at late times. This allows for the coexistence of emission mechanisms at late times.
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| 16. |
- Lundman, Christoffer, et al.
(författare)
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A theory of photospheric emission from collimated outflows
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle dependent velocity profiles, Gamma = Gamma(r, theta, phi), where Gamma is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Gamma \approx Gamma_0/[(theta/theta_j)^p + 1], were theta_j is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, theta_v. We show that for narrow jets (theta_j Gamma_0 \lesssim few), the obtained low energy photon index is alpha \approx -1 (dN/dE \propto E^alpha), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at theta_v \approx theta_j. This result is surprisingly similar to the average low energy photon index seen in gamma-ray bursts. For wide jets (theta_j Gamma_0 \gtrsim few) observed at theta_v \ll theta_j, a multicolor blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.
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| 17. |
- Martin, Francis, et al.
(författare)
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The genome of Laccaria bicolor provides insights into mycorrhizal symbiosis
- 2008
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 452:7183, s. 7-88
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Tidskriftsartikel (refereegranskat)abstract
- Mycorrhizal symbioses -- the union of roots and soil fungi -- are universal in terrestrial ecosystems and may have been fundamental to land colonization by plants1,2. Boreal, temperate, and montane forests all depend upon ectomycorrhizae1. Identification of the primary factors that regulate symbiotic development and metabolic activity will therefore open the door to understanding the role of 2 ectomycorrhizae in plant development and physiology, allowing the full ecological significance of this symbiosis to be explored. Here, we report the genome sequence of the ectomycorrhizal basidiomycete Laccaria bicolor (Fig. 1) and highlight gene sets involved in rhizosphere colonization and symbiosis. This 65-million-base genome assembly contains ~ 20,000 predicted protein-encoding genes and a very large number of transposons and repeated sequences. We detected unexpected genomic features most notably a battery of effector-type small secreted proteins (SSP) with unknown function, several of which are only expressed in symbiotic tissues. The most highly expressed SSP accumulates in the proliferating hyphae colonizing the host root. The ectomycorrhizae-specific proteins likely play a decisive role in the establishment of the symbiosis. The unexpected observation that the genome of L. bicolor lacks carbohydrate-active enzymes involved in degradation of plant cell walls, but maintains the ability to degrade non-plant cell walls, reveals the dual saprotrophic and biotrophic lifestyle of the mycorrhizal fungus which enables it to grow within both soil and living plant roots. The predicted gene inventory of the L. bicolor genome, therefore, points to previously unknown mechanisms of symbiosis operating in biotrophic mycorrhizal fungi. The availability of this genome provides an unparalleled opportunity to develop a deeper understanding of the processes by which symbionts interact with plants within their ecosystem in order to perform vital functions in the carbon and nitrogen cycles that are fundamental to sustainable plant productivity.
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| 18. |
- Pe'er, Asaf, et al.
(författare)
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A new method of determining the initial size and Lorentz factor of gamma-ray burst fireballs using a thermal emission component
- 2007
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 664:1, s. L1-L4
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, increasing evidence has emerged for a thermal component in the gamma- and X- ray spectrum of the prompt emission phase in gamma- ray bursts. The temperature and flux of the thermal component show a characteristic break in the temporal behavior after a few seconds. We show here that measurements of the temperature and flux of the thermal component at early times ( before the break) allow the determination of the values of two of the least restricted fireball model parameters: the size at the base of the flow and the outflow bulk Lorentz factor. Relying on the thermal emission component only, this measurement is insensitive to the inherent uncertainties of previous estimates of the bulk motion Lorentz factor. We give specific examples of the use of this method: for GRB 970828 at redshift z = 0.9578, we show that the physical size at the base of the flow is r(0) = (2.9 +/- 1.8) x 10(8) Y-0(-3/2) cm and the Lorentz factor of the flow is Gamma = (305 +/- 28) Y-0(1/4), and for GRB 990510 at z= 1.619, r(0) = (1.7 +/- 1.7) x 10(8)Y(0)(-3/2) cm and Gamma = (384 +/- 71)y(0)(1/4), where Y= 1Y(0) is the between the total fireball energy and the energy emitted in g- rays.
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| 19. |
- Pe'er, Asaf, et al.
(författare)
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A THEORY OF MULTICOLOR BLACKBODY EMISSION FROM RELATIVISTICALLY EXPANDING PLASMAS
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 732:1, s. 49-
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Tidskriftsartikel (refereegranskat)abstract
- We consider the emission of photons from the inner parts of a relativistically expanding plasma outflow, characterized by a constant Lorentz factor, Gamma. Photons that are injected in regions of high optical depth are advected with the flow until they escape at the photosphere. Due to multiple scattering below the photosphere, the locally emerging comoving photon distribution is thermal. However, as an observer simultaneously sees photons emitted from different angles, hence with different Doppler boosting, the observed spectrum is a multicolor blackbody. We calculate here the properties of the observed spectrum at different observed times. Due to the strong dependence of the photospheric radius on the angle to the line of sight, for parameters characterizing gamma-ray bursts (GRBs) thermal photons are seen up to tens of seconds following the termination of the inner engine. At late times, following the inner engine termination, both the number flux and energy flux of the thermal spectrum decay as F proportional to t(-2). At these times, the multicolor blackbody emission results in a power law at low energies (below the thermal peak), with power-law index F-nu proportional to nu(0). We discuss the implications and limitations of this result in the study of GRBs.
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| 20. |
- Pe'er, Asaf, et al.
(författare)
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HYDRODYNAMIC PROPERTIES of GAMMA-RAY BURST OUTFLOWS DEDUCED from the THERMAL COMPONENT
- 2015
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 813:2
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Tidskriftsartikel (refereegranskat)abstract
- We study the properties of a significant thermal emission component that was identified in 47 gamma-ray bursts observed by different instruments. Within the framework of the "fireball" model, we deduce the values of the Lorentz factor Gamma, and the acceleration radius, r(0), for these bursts. We find that all the values of Gamma in our sample are in the range 10(2) <= Gamma <= 10(3), with = 370. We find a very weak dependence of Gamma on the acceleration radius r0, Gamma proportional to r(0)(alpha) with alpha - -0.10 +/- 0.09 at the sigma - 2.1 confidence level. The values of r(0) span a wide range, 10(6.5) <= r(0) <= 10(9.5) cm, with a mean value of < r(0)> similar to 10(8) cm. This is higher than the gravitational radius of a 10 M-circle dot black hole by a factor of approximate to 30. We argue that this result provides indirect evidence for jet propagation inside a massive star, and suggests the existence of recollimation shocks that take place close to this radius.
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| 21. |
- Pe'er, Asaf, et al.
(författare)
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Photospheric emission in gamma-ray bursts
- 2017
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Ingår i: International Journal of Modern Physics D. - : WORLD SCIENTIFIC PUBL CO PTE LTD. - 0218-2718. ; 26:10
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Forskningsöversikt (refereegranskat)abstract
- A major breakthrough in our understanding of gamma-ray bursts (GRB) prompt emission physics occurred in the last few years, with the realization that a thermal component accompanies the over-all nonthermal prompt spectra. This thermal part is important by itself, as it provides direct probe of the physics in the innermost outflow regions. It further has an indirect importance, as a source of seed photons for inverse-Compton scattering, thereby it contributes to the nonthermal part as well. In this short review, we highlight some key recent developments. Observationally, although so far it was clearly identified only in a minority of bursts, there is indirect evidence that a thermal component exists in a very large fraction of GRBs, possibly close to 100%. Theoretically, the existence of a thermal component has a large number of implications as a probe of underlying GRB physics. Some surprising implications include its use as a probe of the jet dynamics, geometry and magnetization.
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| 22. |
- Ryde, Felix, et al.
(författare)
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Observational evidence of dissipative photospheres in gamma-ray bursts
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 415:4, s. 3693-3705
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Tidskriftsartikel (refereegranskat)abstract
- The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B. The main gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later, the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening mainly depends on the strength and location of the dissipation, the magnetic field strength and the relation between the energy densities of thermal photons and electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease in the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also explain the correlation observed between the peak energy of the spectrum and low-energy power-law slope, a, a correlation commonly observed in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role in creating the diverse spectral shapes and spectral evolutions that are observed.
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| 23. |
- 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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| 24. |
- Ryde, Felix, et al.
(författare)
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QUASI-BLACKBODY COMPONENT AND RADIATIVE EFFICIENCY OF THE PROMPT EMISSION OF GAMMA-RAY BURSTS
- 2009
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 702:2, s. 1211-1229
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Tidskriftsartikel (refereegranskat)abstract
- We perform time-resolved spectroscopy on the prompt emission in gamma-ray bursts (GRBs) and identify a thermal, photospheric component peaking at a temperature of a few hundreds keV. This peak does not necessarily coincide with the broad-band (keV-GeV) power peak. We show that this thermal component exhibits a characteristic temporal behavior. We study a sample of 56 long bursts, all strong enough to allow time-resolved spectroscopy. We analyze the evolution of both the temperature and flux of the thermal component in 49 individual time-resolved pulses, for which the temporal coverage is sufficient, and find that the temperature is nearly constant during the first few seconds, after which it decays as a power law with a sample-averaged index of -0.68. The thermal flux first rises with an averaged power-law index of 0.63 after which it decays with an averaged index of -2. The break times are the same to within errors. We find that the ratio of the observed to the emergent thermal flux typically exhibits a monotoneous power-law increase during the entire pulse as well as during complex bursts. Thermal photons carry a significant fraction (similar to 30% to more than 50%) of the prompt emission energy (in the observed 25-1900 keV energy band), thereby significantly contributing to the high radiative efficiency. Finally, we show here that the thermal emission can be used to study the properties of the photosphere, hence the physical parameters of the GRB fireball.
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| 25. |
- Ryde, Felix, et al.
(författare)
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Quasi-blackbody component in the prompt emission of gamma-ray bursts
- 2009
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Ingår i: AIP Conference Proceedings. - : AIP. - 0094-243X .- 1551-7616. ; 1133, s. 312-316
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Tidskriftsartikel (refereegranskat)abstract
- The characteristic sub-MeV peak in the energy spectrum of the prompt GRB emission can be attributed to blackbody emission of the photosphere of the outflow, having a temperature of approximately 10(9) K. An additional non-thermal spectral component is then attributed to additional dissipation of the kinetic energy in the outflow. Here we study this interpretation and analyze instantantaneous spectra in the 20- 2000 keV range and show that such a two-component model can be fit to most spectra well. Interestingly, the thermal component exhibits a recurring behaviour over emission pulse structures. Both the temperature and the energy flux vary as broken power-laws. During the pre-break phase the temperature is approximately constant while the energy flux rises. Furthermore, the ratio of the observed thermal flux to the emergent flux increases as a power law over the whole pulse. It is argued that these observations hold the key to our understanding of the prompt emission and of the properties of the site from which it emanates.
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| 26. |
- 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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| 27. |
- 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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| 28. |
- 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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| 29. |
- 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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| 30. |
- Sharma, Vidushi, et al.
(författare)
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Time-varying Polarized Gamma-Rays from GRB 160821A : Evidence for Ordered Magnetic Fields
- 2019
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Ingår i: Astrophysical Journal Letters. - : IOP PUBLISHING LTD. - 2041-8205 .- 2041-8213. ; 882:1
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Tidskriftsartikel (refereegranskat)abstract
- GRB 160821A is the third most energetic gamma-ray burst observed by the Fermi gamma-ray space telescope. Based on the observations made by the Cadmium Zinc Telluride Imager on board AstroSat, here we report the most conclusive evidence to date of (i) high linear polarization (66(27)(+26)%; 5.3 sigma detection), and (ii) variation of polarization angle with time, occurring twice during the rise and decay phase of the burst at 3.5 sigma and 3.1 sigma detections, respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines that are highly ordered on angular scales of 1/Gamma, where Gamma is the Lorentz factor of the outflow.
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