scholarly journals High efficiency photospheric emission entailed by formation of a collimation shock in gamma-ray bursts

2019 ◽  
Vol 488 (1) ◽  
pp. 1416-1426 ◽  
Author(s):  
Ore Gottlieb ◽  
Amir Levinson ◽  
Ehud Nakar
Keyword(s):  
High Efficiency ◽  
Gamma Ray ◽  
Surrounding Medium ◽  
Emission Spectra ◽  
Lorentz Factor ◽  
Gamma Ray Bursts ◽  
Opening Angle ◽  
Radiative Efficiency ◽  
Relativistic Jet ◽  
Prompt Emission

ABSTRACT The primary dissipation mechanism in jets of gamma-ray bursts (GRBs), and the high efficiency of the prompt emission are long-standing issues. One possibility is strong collimation of a weakly magnetized relativistic jet by the surrounding medium, which can considerably enhance the efficiency of the photospheric emission. We derive a simple analytic criterion for the radiative efficiency of a collimated jet showing that it depends most strongly on the baryon loading. We confirm this analytic result by 3D numerical simulations, and further find that mixing of jet and cocoon material at the collimation throat leads to a substantial stratification of the outflow as well as sporadic loading, even if the injected jet is uniform and continuous. One consequence of this mixing is a strong angular dependence of the radiative efficiency. Another is large differences in the Lorentz factor of different fluid elements that lead to formation of internal shocks. Our analysis indicates that in both long and short GRBs a prominent photospheric component cannot be avoided when observed within an angle of a few degrees to the axis, unless the asymptotic Lorentz factor is limited by baryon loading at the jet base to Γ∞ < 100 (with a weak dependence on outflow power). Photon generation by newly created pairs behind the collimation shock regulates the observed temperature at $\sim 50~\theta _0^{-1}$ keV, where θ0 is the initial jet opening angle, in remarkable agreement with the observed peak energies of prompt emission spectra. Further consequences for the properties of the prompt emission are discussed at the end.

scholarly journals Interpreting the X-ray afterglows of gamma-ray bursts with radiative losses and millisecond magnetars

2020 ◽  
Vol 499 (4) ◽  
pp. 5986-5992
Author(s):  
Nikhil Sarin ◽  
Paul D Lasky ◽  
Gregory Ashton
Keyword(s):  
Gravitational Wave ◽  
Gamma Ray ◽  
Gamma Ray Bursts ◽  
Dipole Radiation ◽  
X Ray ◽  
Radiative Efficiency ◽  
Central Engine ◽  
Radiative Losses ◽  
Wave Emission ◽  
Prompt Emission

ABSTRACT The spin-down energy of millisecond magnetars has been invoked to explain X-ray afterglow observations of a significant fraction of short and long gamma-ray bursts. Here, we extend models previously introduced in the literature, incorporating radiative losses with the spin-down of a magnetar central engine through an arbitrary braking index. Combining this with a model for the tail of the prompt emission, we show that our model can better explain the data than millisecond-magnetar models without radiative losses or those that invoke spin-down solely through vacuum dipole radiation. We find that our model predicts a subset of X-ray flares seen in some gamma-ray bursts. We can further explain the diversity of X-ray plateaus by altering the radiative efficiency and measure the braking index of newly born millisecond magnetars. We measure the braking index of GRB061121 as $n=4.85^{+0.11}_{-0.15}$ suggesting the millisecond-magnetar born in this gamma-ray burst spins down predominantly through gravitational-wave emission.

scholarly journals Gamma-Ray Bursts Polarization

2016 ◽  
Vol 12 (S324) ◽  
pp. 54-61
Author(s):  
Diego Götz ◽  
Stefano Covino
Keyword(s):  
Black Hole ◽  
Kinetic Energy ◽  
Gamma Ray ◽  
Gamma Ray Bursts ◽  
Central Source ◽  
Poynting Flux ◽  
Relativistic Jet ◽  
Theoretical Status ◽  
Prompt Emission ◽  
Open Point

AbstractWe review the current observational and theoretical status of the polarization measurements of Gamma-ray Bursts at all wavelengths. Gamma-Ray Bursts are thought to be produced by an ultra-relativistic jet, possibly powered by a black hole. One of the most important open point is the composition of the jet: the energy may be carried out from the central source either as kinetic energy (of baryons and/or pairs), or in electromagnetic form (Poynting flux). The polarization properties are expected to help disentangling main energy carrier. The prompt emission and afterglow polarization are also a powerful diagnostic of the jet geometry.

scholarly journals Intermittent hydrodynamic jets in collapsars do not produce GRBs

2020 ◽  
Vol 495 (1) ◽  
pp. 570-577 ◽  
Author(s):  
Ore Gottlieb ◽  
Amir Levinson ◽  
Ehud Nakar
Keyword(s):  
High Efficiency ◽  
Gamma Ray ◽  
Heavy Baryon ◽  
Stellar Envelope ◽  
X Ray ◽  
Radiative Efficiency ◽  
Engine Model ◽  
Central Engine ◽  
Radio Transients ◽  
Prompt Emission

ABSTRACT Strong variability is a common characteristic of the prompt emission of gamma-ray bursts (GRB). This observed variability is widely attributed to an intermittency of the central engine, through formation of strong internal shocks in the GRB-emitting jet expelled by the engine. In this paper, we study numerically the propagation of hydrodynamic jets, injected periodically by a variable engine, through the envelope of a collapsed star. By post-processing the output of 3D numerical simulations, we compute the net radiative efficiency of the outflow. We find that all intermittent jets are subject to heavy baryon contamination that inhibits the emission at and above the photosphere well below detection limits. This is in contrast to continuous jets that, as shown recently, produce a highly variable gamma-ray photospheric emission with high efficiency, owing to the interaction of the jet with the stellar envelope. Our results challenge the variable engine model for hydrodynamic jets, and may impose constraints on the duty cycle of GRB engines. If such systems exist in nature, they are not expected to produce bright gamma-ray emission, but should appear as X-ray, optical, and radio transients that resemble a delayed GRB afterglow signal.

scholarly journals HIGH ENERGETIC GAMMA RAY BURSTS AND THEIR SPECTRAL PROPERTIES WITHIN THE FIRESHELL MODEL

2012 ◽  
Vol 12 ◽  
pp. 385-389
Author(s):  
B. PATRICELLI ◽  
M.G. BERNARDINI ◽  
C.L. BIANCO ◽  
L. CAITO ◽  
G. DE BARROS ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Observational Data ◽  
Spectral Properties ◽  
Gamma Ray ◽  
Emission Spectra ◽  
Gamma Ray Bursts ◽  
Low Energy ◽  
Isotropic Energy ◽  
Thermal Spectrum ◽  
Prompt Emission

The analysis of various Gamma Ray Bursts (GRBs) characterized by an isotropic energy Eiso ≲ 1053 ergs within the fireshell model has shown how that the observed N(E) spectrum of their prompt emission can be reproduced in a satisfactory way by assuming a thermal spectrum in the comoving frame of the fireshell. Nevertheless, from the study of higher energetic bursts (Eiso ≳ 1054 ergs ) such as, for example, GRB 080319B, some discrepancies between the numerical simulations and the observational data have been observed. We investigate a different spectrum of photons in the comoving frame of the fireshell in order to better reproduce the spectral properties of GRB prompt emission within the fireshell model. We introduce a phenomenologically modified comoving thermal spectrum: a spectrum characterized by a different asymptotic low energy slope with respect to the thermal one. We test this spectrum by comparing the numerical simulations with the observed prompt emission spectra of various GRBs; we present, as an exaple, the case of GRB 080319B.

scholarly journals Linking extended and plateau emissions of short gamma-ray bursts

2020 ◽  
Vol 493 (1) ◽  
pp. 783-791 ◽  
Author(s):  
Tatsuya Matsumoto ◽  
Shigeo S Kimura ◽  
Kohta Murase ◽  
Peter Mészáros
Keyword(s):  
Gamma Ray ◽  
Lorentz Factor ◽  
Gamma Ray Bursts ◽  
Radiation Efficiency ◽  
High Radiation ◽  
Average Value ◽  
Forward Shock ◽  
Compactness Arguments ◽  
Prompt Emission ◽  
Extended Emission

ABSTRACT Some short gamma-ray bursts (SGRBs) show a longer lasting emission phase, called extended emission (EE) lasting ${\sim}10^{2\!-\!3}\, \rm s$, as well as a plateau emission (PE) lasting ${\sim}10^{4\!-\!5}\, \rm s$. Although a long-lasting activity of the central engines is a promising explanation for powering both emissions, their physical origin and their emission mechanisms are still uncertain. In this work, we study the properties of the EEs and their connection with the PEs. First, we constrain the minimal Lorentz factor Γ of the outflows powering EEs, using compactness arguments and find that the outflows should be relativistic, Γ ≳ 10. We propose a consistent scenario for the PEs, where the outflow eventually catches up with the jet responsible for the prompt emission, injecting energy into the forward shock formed by the prior jet, which naturally results in a PE. We also derive the radiation efficiency of EEs and the Lorentz factor of the outflow within our scenario for 10 well-observed SGRBs accompanied by both EE and PE. The efficiency has an average value of ${\sim}3\, {{\ \rm per\ cent}}$ but shows a broad distribution ranging from ∼0.01 to ${\sim}100{{\ \rm per\ cent}}$. The Lorentz factor is ∼20–30, consistent with the compactness arguments. These results suggest that EEs are produced by a slower outflow via more inefficient emission than the faster outflow that causes the prompt emission with a high radiation efficiency.

scholarly journals A Cosmological Fireball with Thirty-Percent Gamma-Ray Radiative Efficiency

2021 ◽  
Author(s):  
Liang Li ◽  
Yu Wang ◽  
Felix Ryde ◽  
Asaf Pe'er ◽  
Bing Zhang ◽  
...  
Keyword(s):  
High Efficiency ◽  
Gamma Ray ◽  
Thermal Component ◽  
Independent Manner ◽  
Radiative Efficiency ◽  
Poynting Flux ◽  
Emission Component ◽  
The Past ◽  
The Universe ◽  
Prompt Emission

Abstract Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. The composition of the jets is, however, subject to debate\cite{Peer2015,Zhang2018}. Whereas the traditional model invokes a relativistic matter-dominated fireball with a bright photosphere emission component\cite{Meszaros2000}, the lack of the detection of such a component in some GRBs\cite{Abdo2009} has led to the conclusion that GRB jets may be Poynting-flux-dominated\cite{Zhang2009}. Furthermore, how efficiently the jet converts its energy to radiation is poorly constrained. A definitive diagnosis of the GRB jet composition and measurement of GRB radiative efficiency requires high-quality prompt emission and afterglow data, which has not been possible with the sparse observations in the past. Here we report a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its fluence is one of the highest of all GRBs detected so far, which allows us to perform a high-significance study on the prompt emission spectral properties and their variations down to a very short timescale of about 0.1 s. We identify a clear thermal component during the first two prompt emission episodes, which is fully consistent with the prediction of the fireball photosphere model. The third episode of the prompt emission is consistent with synchrotron radiation from the deceleration of the fireball. This allows us to directly dissect the fireball energy budget in a parameter-independent manner\cite{Zhang2021} and robustly measure a nearly $30\%$ radiative efficiency for this GRB. The afterglow microphysics parameters can be also well constrained from the data. GRB 190114C, therefore, exhibits the evolution of a textbook-version relativistic fireball, suggesting that fireballs can indeed power at least some GRBs with high efficiency.

scholarly journals Testing a model for subphotospheric dissipation in GRBs: fits to Fermi data constrain the dissipation scenario

2019 ◽  
Vol 485 (1) ◽  
pp. 474-497 ◽  
Author(s):  
Björn Ahlgren ◽  
Josefin Larsson ◽  
Erik Ahlberg ◽  
Christoffer Lundman ◽  
Felix Ryde ◽  
...  
Keyword(s):  
Gamma Ray ◽  
Full Range ◽  
Lorentz Factor ◽  
Gamma Ray Bursts ◽  
Time Resolved ◽  
Cent Efficiency ◽  
Table Model ◽  
Photospheric Model ◽  
Prompt Emission ◽  
Specific Scenario

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.

scholarly journals Bulk Lorentz factors of gamma-ray bursts

2018 ◽  
Vol 609 ◽  
pp. A112 ◽  
Author(s):  
G. Ghirlanda ◽  
F. Nappo ◽  
G. Ghisellini ◽  
A. Melandri ◽  
G. Marcarini ◽  
...  
Keyword(s):  
Density Profile ◽  
Rest Frame ◽  
Gamma Ray ◽  
Lorentz Factor ◽  
Gamma Ray Bursts ◽  
Opening Angle ◽  
Isotropic Energy ◽  
Upper Limits ◽  
Peak Energy ◽  
Short Event

Knowledge of the bulk Lorentz factor Γ0 of gamma-ray bursts (GRBs) allows us to compute their comoving frame properties shedding light on their physics. Upon collisions with the circumburst matter, the fireball of a GRB starts to decelerate, producing a peak or a break (depending on the circumburst density profile) in the light curve of the afterglow. Considering all bursts with known redshift and with an early coverage of their emission, we find 67 GRBs (including one short event) with a peak in their optical or GeV light curves at a time tp. For another 106 GRBs we set an upper limit tpUL. The measure of tp provides the bulk Lorentz factor Γ0 of the fireball before deceleration. We show that tp is due to the dynamics of the fireball deceleration and not to the passage of a characteristic frequency of the synchrotron spectrum across the optical band. Considering the tp of 66 long GRBs and the 85 most constraining upper limits, we estimate Γ0 or a lower limit Γ0LL. Using censored data analysis methods, we reconstruct the most likely distribution of tp. All tp are larger than the time Tp,γ when the prompt γ-ray emission peaks, and are much larger than the time Tph when the fireball becomes transparent, that is, tp>Tp,γ>Tph. The reconstructed distribution of Γ0 has median value ~300 (150) for a uniform (wind) circumburst density profile. In the comoving frame, long GRBs have typical isotropic energy, luminosity, and peak energy ⟨ Eiso ⟩ = 3(8) × 1050 erg, ⟨ Liso ⟩ = 3(15) × 1047 erg s-1, and ⟨ Epeak ⟩ = 1(2) keV in the homogeneous (wind) case. We confirm that the significant correlations between Γ0 and the rest frame isotropic energy (Eiso), luminosity (Liso), and peak energy (Ep) are not due to selection effects. When combined, they lead to the observed Ep−Eiso and Ep−Liso correlations. Finally, assuming a typical opening angle of 5 degrees, we derive the distribution of the jet baryon loading which is centered around a few 10-6M⊙.

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