External Shock Scenario Research Articles

GRB 200829A: External-shock Origin of the Very Early Prompt Emission?

Long-duration GRB 200829A was detected by Fermi-GBM and Swift-BAT/XRT, and then rapidly observed by other ground-based telescopes. It has a weak γ-ray emission in the very early phase and is followed by a bright spiky γ-ray emission pulse. The radiation spectrum of the very early emission is best fitted by a power-law function with index ∼−1.7. However, the bright spiky γ-ray pulse, especially the time around the peak, exhibits a distinct two-component radiation spectrum, i.e., Band function combined with a blackbody radiation spectrum. We infer the photospheric properties and reveal a medium magnetization at a photospheric position by adopting the initial size of the outflow as r 0 = 109 cm. It implies that the Band component in this pulse may be formed during the dissipation of the magnetic field. The power-law radiation spectra found in the very early prompt emission may imply the external-shock origination of this phase. Then, we perform the Markov Chain Monte Carlo method fitting on the light curves of this burst, where the jet corresponding to the γ-ray pulse at around 20 s is used to refresh the external shock. It is shown that the light curves of the very early phase and X-ray afterglow after 40 s, involving the X-ray bump at around 100 s, can be well modeled in the external-shock scenario. For the obtained initial outflow, we estimate the minimum magnetization factor of the jet based on the fact that the photospheric emission of this jet is missed in the very early phase.

Exchange-Rate Policy in a Dollarized Economy: Implications for Growth and Employment in Bolivia

We analyzed the impact of currency devaluation on the Bolivian economy, employing a dynamic and extended version of the PEP 1-1 standard model to simulate effects impact on both the main macroeconomic aggregates and the financial stocks and flows of economic agents. We built a new Financial Social Accounting Matrix for the year 2014 and calibrated the model to it. Besides simulating a devaluation of the nominal exchange rate, we also analyzed a policy-response scenario, an external-shock scenario, and a gradual-devaluation scenario. In the policy-response scenario, devaluation was accompanied by a reduction in government expenses (fiscal adjustment); in the external-shock scenario, devaluation came with an increase in the export price of gas (main export commodity); and, in the gradual-devaluation scenario, the exchange-rate policy relaxed gradually. The external-shock scenario dominated the other scenarios in terms of higher average growth and less average unemployment. The fiscal-adjustment scenario, however, dominated in terms of inflation, though it implied an inflationary shock in 2020.

Ultrahigh-energy cosmic-ray nuclei and neutrinos from engine-driven supernovae

Transrelativistic supernovae (SNe), which are likely driven by central engines via jets or winds, have been among candidate sources of ultrahigh-energy cosmic rays (UHECRs). We investigate acceleration and survival of UHECR nuclei in the external reverse shock scenario. With composition models used in Zhang et al. (2018), we calculate spectra of escaping cosmic rays and secondary neutrinos. If their local rate is $\sim1$% of the core-collapse supernova rate, the observed UHECR spectrum and composition can be explained with the total cosmic-ray energy ${\mathcal E}_{\rm cr}\sim10^{51}$ erg. The maximum energy of UHECR nuclei can reach $\sim 10^{20}-{10}^{21}\rm~eV$. The diffuse flux of source neutrinos is predicted to be $\sim 10^{-11} - 10^{-10}~{\rm GeV}~{\rm cm}^{-2}~{\rm s}^{-1}~{\rm sr}^{-1}$ in the 0.1-1 EeV range, satisfying nucleus-survival bounds. The associated cosmogenic neutrino flux is calculated, and shown to be comparable or even higher than the source neutrino flux. These ultrahigh-energy neutrinos can be detected by ultimate detectors such as the Giant Radio Askaryan Neutrino Detector and Probe Of Extreme Multi-Messenger Astrophysics.

From the earliest pulses to the latest flares in long gamma-ray bursts

The prompt emission of gamma-ray bursts extends from the early pulses observed in γ-rays (>15 keV) to very late flares of X-ray photons (0.3–10 keV). The duration of prompt γ-ray pulses is rather constant, while the width of X-ray flares correlates with their peak time, suggesting a possibly different origin. However, pulses and flares have similar spectral properties. Considering internal and external shock scenarios, we derive how the energy and duration of pulses scale with their time of occurrence, and we compare this with observations. The absence of an observed correlation between the prompt emission pulse duration and its time of occurrence favours an “internal” origin and confirms earlier results. We show that the energetic and temporal properties of X-ray flares are also consistent with being produced by internal shocks between slow fireballs with a small contrast between their bulk Lorentz factors. These results relax the requirement of a long-lasting central engine to explain the latest X-ray flares.

Physical insight into the Combo-relation

The Combo-relation is a tight relation involving the prompt emission spectral peak energy Ep,i and the X-ray afterglow plateau luminosity L0 and its rest-frame duration τ, and the late power-law decay index α. To explain its features, in this work we investigate the external shock scenario, i.e. the synchrotron emission from the electrons accelerated in a relativistic shock propagating through a cold external medium with density n and interacting with the magnetic field B associated with the turbulent plasma. Three assumptions were introduced: (1) the homologous shock expansion; (2) an electron power-law distribution over a large range of frequencies; and (3) a power-law dependence on the shock radius of B and n. First, we show how the values of Ep,i, L0 and τ depend on combinations of the initial values of the magnetic field B0, shock radius R0 and medium density n0, as well as the shock Lorentz factor Γ0 and the electron distribution. Then, we prove that the Combo-relation results from the relationship between Ep,i, L0 and τ and the corresponding comoving quantities that scale with Γ0. Finally, through Monte Carlo simulations, we deduce the typical model parameters, for example, |$B_0=4.6^{+253.9}_{-4.5}$| G, |$R_0=7.0^{+34.4}_{-5.8}\times 10^{13}$| cm and |$n_0=10.00^{+7694.62}_{-9.99}$| cm−3, all in agreement with the results obtained in the literature. We conclude that, within the external shock scenario and its flaws, the Combo-relation represents a new tool for investigating the physics of the prompt and afterglow emissions and for inferring the properties of the gamma-ray-burst external medium.

Dynamical macroprudential stress testing using network theory

The increasing frequency and scope of nancial crises have made global nancial stability one of the major concerns of economic policy and decision makers. This has led to the understanding that nancial and banking supervision has to be thought of as a systemic task, focusing on the interdependent relations among the institutions. Using network theory, we develop a dynamic model that uses a bipartite network of banks and their assets to analyze the system’s sensitivity to external shocks in individual asset classes and to evaluate the presence of features underlying the system that could lead to contagion. As a case study, we apply the model to stress test the Venezuelan banking system from 1998 to 2013. The introduced model was able to capture monthly changes in the structure of the system and the sensitivity of bank portfolios to dierent external shock scenarios and to identify systemic vulnerabilities and their time evolution. The model provides new tools for policy makers and supervision agencies to use for macroprudential dynamical stress testing.

Afterglow rebrightenings as a signature of a long-lasting central engine activity?

In the past few years the number of well-sampled optical to NIR light curves of long Gamma-Ray Bursts (GRBs) has greatly increased particularly due to simultaneous multi-band imagers such as GROND. Combining these densely sampled ground-based data sets with the Swift UVOT and XRT space observations unveils a much more complex afterglow evolution than what was predicted by the most commonly invoked theoretical models. GRB 100814A represents a remarkable example of these interesting well-sampled events, showing a prominent late-time rebrightening in the optical to NIR bands and a complex spectral evolution. This represents a unique laboratory to test the different afterglow emission models. Here we study the nature of the complex afterglow emission of GRB 100814A in the framework of different theoretical models. Moreover, we compare the late-time chromatic rebrightening with those observed in other well-sampled long GRBs. We analysed the optical and NIR observations obtained with the seven-channel Gamma-Ray burst Optical and Near-infrared Detector at the 2.2 m MPG/ESO telescope together with the X-ray and UV data detected by the instruments onboard the Swift observatory. The broad-band afterglow evolution, achieved by constructing multi-instrument light curves and spectral energy distributions, will be discussed in the framework of different theoretical models. We find that the standard models that describe the broad-band afterglow emission within the external shock scenario fail to describe the complex evolution of GRB 100814A, and therefore more complex scenarios must be invoked. [abridged]

Dynamical Macro-Prudential Stress Testing Using Network Theory

The increasing frequency and scope of financial crises have made global financial stability one of the major concerns of economic policy and decision makers. This has led to the understanding that financial and banking supervision has to be thought of as a systemic task, focusing on the interdependent relations among the institutions. Using network theory, we develop a dynamic model that uses a bipartite network of banks and their assets to analyze the system’s sensitivity to external shocks in individual asset classes and to evaluate the presence of features underlying the system that could lead to contagion. As a case study, we apply the model to stress test the Venezuelan banking system from 1998 to 2013. The introduced model was able to capture monthly changes in the structure of the system and the sensitivity of bank portfolios to different external shock scenarios and to identify systemic vulnerabilities and their time evolution. The model provides new tools for policy makers and supervision agencies to use for macroprudential dynamical stress testing.

DETECTION OF HIGH-ENERGY GAMMA-RAY EMISSION DURING THE X-RAY FLARING ACTIVITY IN GRB 100728A

We present the simultaneous Swift and Fermi observations of the bright GRB100728A and its afterglow. The early X-ray emission is dominated by a vigorous flaring activity continuing until 1 ks after the burst. In the same time interval high energy emission is significantly detected by the Fermi/LAT. Marginal evidence of GeV emission is observed up to later times. We discuss the broadband properties of this burst within both the internal and external shock scenarios, with a particular emphasis on the relation between X-ray flares, the GeV emission and a continued long-duration central engine activity as their power source.

Where are the prompt emitting regions of long GRBs?

We have used a simple method to estimate the prompt emission radii of 27 Swift and 37 pre-Swift long gamma-ray bursts with known redshift and jet break time. The prompt γ-rays are found to emit mainly from a beamed jet with dynamic open angle narrower than its geometric open angle, suggesting an obviously dynamic evolution from prompt to follow-up phases for the jetted outflow. Under condition of the external shock scenario, we confirm that long gamma-ray bursts are produced at a larger radius of ∼1016 cm, which puts a strict constraint on the currently theoretical models.

GeV emission from short gamma-ray bursts: the case of GRB 081024B

We investigate whether the high energy tail detected by the Fermi/LAT for the short GRB 081024B can be caused by synchrotron and self-Compton emission in the context of either the internal or external shock models. For the internal shock scenario, we explore the possibility of generating the high energy photons directly by means of the synchrotron process, or inverse Compton emission in which target photons are synchrotron photons produced in internal shocks taking place in a lately emitted shell (delayed internal shocks). In the external shock scenario, we test whether the high energy tail can be an extension of the afterglow synchrotron emission, or alternatively the inverse Compton component associated with the afterglow synchrotron photons. For the internal shock scenario, we conclude that only an inverse Compton component from delayed internal shocks can explain the high energy tail that extends to the GeV range. In the external shock scenario, we show that the high energy tail may be interpreted as synchrotron afterglow emission, if the slow cooling phase starts as early as a few seconds after the trigger. On the other hand, the observed high energy tail is consistent with an inverse Compton component of the afterglow in the fast cooling regime.

GRB 070311: A COMMON ORIGIN FOR THE PROMPT AND AFTERGLOW EMISSION

GRB 070311 was a long burst that triggered INTEGRAL. We present prompt γ-ray, early NIR/optical, late optical and X-ray data on this burst and its afterglow. Interestingly, the H-band light curve acquired with REM exhibits two pulses at 80 and 140 s after the peak of the γ-ray burst, with possible evidence for a contemporaneous faint γ-ray tail. The late optical and X-ray afterglow underwent a rebrightening between 3 × 104 and 2 × 105 s after the burst with energy comparable with that of the prompt emission extrapolated in the X-ray band. After fitting the early γ-ray and optical light curves, we modelled the time profile of the late rebrightening as the time-rescaled version of the prompt γ-ray pulse over an underlying power law. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario.

Prospects for detection of very high-energy emission from GRB in the context of the external shock model

The detection of the 100 GeV-TeV emission by a gamma-ray burst (GRB) will provide an unprecedented opportunity to study the nature of the central engine and the interaction between the relativistic flow and the environment of the burst's progenitor. In this paper we show that there are exciting prospects of detecting from the burst by MAGIC high-energy (HE) emission during the early X-ray flaring activity and, later, during the normal afterglow phase. We also identify the best observational strategy, trigger conditions and time period of observation. We determine the expected HE emission from the flaring and afterglow phases of GRBs in the context of the external shock scenario and compare them with the MAGIC threshold. We find that an X-ray flare with the average properties of the class can be detected in the 100 GeV range by MAGIC, provided that z a few cm^-3. In this case, the MAGIC observation shall extend to about 10-20 ks. We provide recipes for tailoring this prediction to the observational properties of each burst,in particular the fluence in the prompt emission and the redshift, thus allowing an almost real time decision procedure to decide whether to continue the follow-up observation of a burst at late times.

GRB 070311: a direct link between the prompt emission and the afterglow

Context. The prompt emission of gamma-ray bursts is mostly thought to be produced in internal shocks of relativistic shells emitted by the progenitor at different times, whereas the late multi-band afterglow is interpreted as the synchrotron emission of electrons swept up by the fireball expanding through the surrounding interstellar medium. The short timescale variability observed in flares superimposed on the X-ray/optical afterglow of several bursts, recently made possible by Swift, has been interpreted as evidence for prolonged activity of the inner engine through internal shocks. Yet, it is not clear whether this applies to all the observed bursts and, in particular, whether the bursts exhibiting single γ-ray pulses with no short timescale variability at late times could also be entirely interpreted as external shocks. Aims. We present prompt γ-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt γ-ray pulse as well as for the early and late multiband afterglow of GRB 070311. Methods. We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays. Results. The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the γ-ray burst and possibly accompanied by a faint γ-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3 × 10 4 and 2 × 10 5 s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt γ-ray pulse and an underlying power law. Conclusions. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock.

Implications of Variability Patterns Observed in TeV Blazars on the Structure of the Inner Jet

The recent long look X-ray observations of TeV blazars have revealed many important new features concerning their time variability. In this paper, we suggest a physical interpretation for those features based on the framework of the internal and external shock scenarios. We present a simplified model applicable to TeV blazars, and investigate through simulations how each of the model parameters would affect to the observed light curve or spectrum. In particular, we show that the internal shock scenario naturally leads to all the observed variability properties including the structure function, but for it to be applicable, the fractional fluctuation of the initial bulk Lorentz factors must be small, with sigma_gamma / gamma_average < 0.01. This implies very low dynamical efficiency of the internal shock scenario. We also suggest that several observational quantities -- such as the characteristic time scale, the relative amplitude of flares as compared to the steady (``offset'') component, and the slope of the structure function -- can be used to probe the inner jet. The results are applied to the TeV blazar Mrk421, and this, within the context of the model, leads to the determination of several physical parameters: the ejection of a shell with average thickness of ~1E13 cm occurs on average every 10 minutes, and the shells collide ~1E17 cm away from the central source.

Beaming, Baryon Loading, and the Synchrotron Self‐Compton Component in Gamma‐Ray Bursts

We present detailed calculations of nonthermal synchrotron and synchrotron self-Compton (SSC) spectra radiated by blast waves that are energized by interactions with a uniform surrounding medium. Radio, optical, X-ray and gamma-ray light curves and spectral indices are calculated for a standard parameter set that yields hard GRB spectra during the prompt emission phase. Because no lateral spreading of the blast-wave is assumed, the calculated temporal breaks represent the sharpest breaks possible from collimated outflows in a uniform surrounding medium. Absence of SSC hardenings in observed GRB X-ray afterglows indicates magnetic field generation toward equipartition as the blast wave evolves. EGRET detections of 100 MeV-GeV photons observed promptly and 90 minutes after GRB 940217 are attributed to nonthermal synchrotron radiation and SSC emission from a decelerating blast wave, respectively. The SSC process will produce prompt TeV emission that could be observed from GRBs with redshifts $z \lesssim 0.1$, provided $\gamma$-$\gamma$ opacity in the source is small. Measurements of the time dependence of the 100 MeV-GeV spectral indices with the planned {\it GLAST} mission will chart the evolution of the SSC component and test the external shock scenario. Transient optical and X-ray emissions from misaligned GRBs are generally much weaker than on-axis emissions produced by dirty and clean fireballs that would themselves not trigger a GRB detector; thus detection of long wavelength transients not associated with GRBs will not unambiguously demonstrate GRB beaming.

Gamma‐Ray Bursts Have Millisecond Variability

We have performed searches for isolated flares and for steady flickering in the initial ~1 s of gamma-ray burst light curves on the microsecond to millisecond timescales. Two bursts among our sample of 20 revealed four isolated flares with timescales from 256 to 2048 μs. A wavelet analysis for our sample showed low-level flickering for all bursts on timescales from 256 μs to 33 ms, with the majority of bursts containing rise times faster than 4 ms and 30% having rise times faster than 1 ms. These results show that millisecond variability is common in classical bursts and not some exceptional activity by a possibly separate class of bursts. These fast rise times can be used to place the following severe limits on burst models. (1) The characteristic thickness of the energy generation region must be less than 1200 km along the line of sight. (2) The angular size of the gamma-ray emission region as subtended from the central source must be less than 42''. (3) The expanding ejecta must have a range of Lorentz factors along a radius line with a dispersion of less than roughly 2%. (4) Within the external shock scenario, the characteristic dimension of the impacted cloud must be smaller than 16 AU on average. (5) Within the collimated jet scenario, the collimation angle must be smaller than 42''.

A SAM-based General Equilibrium Model of the Pakistan Economy 1983-84

Interest in Social Accounting Matrices as extensions of input-output tables has grown tremendously in the last ten years both for the purpose of obtaining a better descriptive analysis of the many intersect oral interdependencies in an economy and of household income distribution , and as data framework for macroeconomic modelling. The objective of this paper is to make use of a social accounting matrix for Pakistan on one hand and a general equilibrium model framework on the other hand to gauge the quantitative impact on the country as a whole of alternative policy options and external shock scenarios. The policy options and external shocks simulated here and the economic indicators used for this purpose are shown in Table I.