Model Of GRBs Research Articles

Collimated Jet or Expanding Outflow: Possible Origins of Gamma‐Ray Bursts and X‐Ray Flashes

We investigate the dynamics of an outflow propagating in a progenitor in the context of the collapsar model for GRBs through two-dimensional axisymmetric relativistic hydrodynamic simulations. Initially, we locally inject an outflow near the center of a progenitor. We calculate 25 models, in total, by fixing its total input energy (1051 ergs s-1) and radius of the injected outflow (7 × 107 cm) while varying its bulk Lorentz factor, Γ0 = 1.05-5, and its specific internal energy, 0/c2 = 0.1-30 (with c being speed of light). We find a smooth but dramatic transition from a collimated jet to an expanding outflow among calculated models. The half opening angle of the outflow (θsim) is sensitive to Γ0; we find θsim < 2° for Γ0 ≳ 3. The maximum Lorentz factor is, on the other hand, sensitive to both Γ0 and 0: roughly Γmax ~ Γ0(1 + 0/c2). In particular, a very high Lorentz factor of Γmax ≳ 100 is achieved in one model. A variety of opening angles can arise by changing 0, even when the maximum Lorentz factor is fixed. The jet structure totally depends on Γ0. When Γ0 is high, high-pressure progenitor gas heated by the bow shock collimates the outflow to form a narrow, relativistic jet. When Γ0 is low, on the contrary, the outflow expands soon after the injection, since the bow shock is weak and thus the pressure of the progenitor gas is not high enough to confine the flow. Our finding will explain a smooth transition between the GRBs, X-ray-rich GRBs (XRRs), and X-ray flashes (XRFs) by the same model but with different 0 values.

A unified jet model of X-ray flashes and γ-ray bursts

HETE-2 has provided strong evidence that the properties of X-ray flashes (XRFs) and GRBs form a continuum, and therefore that these two types of bursts are the same phenomenon. We show that both the structured jet and the uniform jet models can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of both XRFs and GRBs in a unified picture, the uniform jet model works reasonably well while the structured jet model does not. The uniform jet model of XRFs and GRBs implies that most GRBs have very small jet opening angles (∼half a degree). This suggests that magnetic fields play a crucial role in GRB jets. The model also implies that the energy radiated in γ rays is ∼100 times smaller than has been thought. Most importantly, the model implies that there are ∼10 4–10 5 more bursts with very small jet opening angles for every such burst we see. Thus the rate of GRBs could be comparable to the rate of Type Ic core collapse supernovae. Determination of the spectral parameters and redshifts of many more XRFs will be required in order to confirm or rule out the uniform jet model and its implications. HETE-2 is ideally suited to do this (it has localized 16 XRFs in ∼2 years), whereas Swift is less so. The unique insights into the structure of GRBs jets, the rate of GRBs, and the nature of Type Ic supernovae that XRFs may provide therefore constitute a compelling scientific case for continuing HETE-2 during the Swift mission.

An Off-Axis Jet Model for GRB 980425 and Low Energy Gamma-Ray Bursts

Using a simple off-axis jet model of GRBs, we can reproduce the observed unusual properties of the prompt emission of GRB 980425, such as the extremely low isotropic γ-ray energy and the soft spectrum when the jet with the standard energy of ∼ 10 5 1 ergs and the opening half-angle of 10° < Δθ ≤ 30° is seen from the off-axis viewing angle θ v ∼ Δθ+10γ - 1 , where is a Lorentz factor of the jet. Our model may be able to explain the other unusual properties of this event. We also discuss the connection of GRB 980425 in our model with the X-ray flash, and the origin of a class of GRBs with small E γ .

GRBs-SN and SGR-X-Pulsar as Blazing Jets

Old and recent puzzles of GRBs and SGRs find a solution with a model based on the fast blazing of very collimated thin gamma Jets. Damped oscillating afterglows in GRB030329 find a natural explanation assuming a very thin Jet whose persistent activity and different angle of view maybe combined at once with the Supernovae power and the apparent huge GRBs output. The same thin beaming offer an understanding of the apparent SGR-Pulsar power connection. A thin collimated precessing Gamma Jet model for both GRBs and SGRs, at their different scaled luminosity (10^{44} - 10^{38}erg s^-1), explains the existence of few identical energy spectra and time evolution of these sources leading to a unified model. Their similarity with the huge precessing Jets in AGN, QSRs and Radio-Galaxies inspires this smaller scale SGR-GRB model. The spinning-precessing Jet explains the rare mysterious X-Ray precursors in GRBs and SGRs events. Any large Gamma Jet off-axis beaming to the observer might lead to the X-Flash events without any GRB signals, as the most recent XRF030723. Its possible re-brightening would confirm the evidence of the variable pointing of the jet in or off line towards the observer. Indeed a multi-precessing Jet at peak activity in all bands may explain the puzzling X or optical re-brightening bumps found in the GRB 021004, GRB030329 and the SGR 1900+ 14 on 27 August 1998 and once again on the 18 April 2001. Rarest micro-quasars neutron star in our galaxy as SS433, and Herbig Haro objects and Cir-X-1 describe these thin precessing Jet imprints in the spectacular shapes of their relic nebulae.

An Off-Axis Jet Model For GRB 980425 and Low-Energy Gamma-Ray Bursts

Using a simple off-axis jet model of GRBs, we can reproduce the observed unusual properties of the prompt emission of GRB980425, such as the extremely low isotropic equivalent gamma-ray energy, the low peak energy, the high fluence ratio, and the long spectral lag when the jet with the standard energy of ~10^{51}ergs and the opening half-angle of \Delta\theta=~10-30 degree is seen from the off-axis viewing angle ~\Delta\theta+10/\gamma, where \gamma is a Lorentz factor of the jet. For our adopted fiducial parameters, if the jet that caused GRB980425 is viewed from the on-axis direction, the intrinsic peak energy Ep(1+z) is ~2.0-4.0 MeV, which corresponds to those of GRB990123 and GRB021004. We also discuss the connection of GRB980425 in our model with the X-ray flash, and the origin of a class of GRBs with small E_gamma.

Polarization of Prompt Gamma‐Ray Burst Emission: Evidence for Electromagnetically Dominated Outflow

Observations by the RHESSI satellite of the large polarization of the prompt γ-ray emission from γ-ray burst (GRB) 021206 imply that the magnetic field coherence scale is larger than the size of the visible emitting region, ~R/Γ, where R is the radius of the flow and Γ is the associated Lorentz factor. Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs, in which large-scale, dynamically dominant, magnetic fields are present in the outflow from the very beginning, provide a natural explanation of this large reported linear polarization. We derive the Stokes parameters of the synchrotron emission of a relativistically moving plasma with a given magnetic field configuration and calculate the pulse-averaged polarization fraction of the emission from a relativistically expanding shell carrying a global toroidal magnetic field. For viewing angles larger than 1/Γ, the observed patch of the emitting shell has an almost homogeneous magnetic field, producing a large fractional polarization (56% for a power-law energy distribution of relativistic particles, dn/d -3). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in a uniform magnetic field because of relativistic kinematic effects.

Fireballs and cannonballs confront GRB 991208

The broad band afterglow of GRB 991208 provides a good case for a three-sided confrontation: the data, the generally-accepted fireball models of GRBs, and the cannonball (CB) model. Follow-up radio measurements for this GRB have been recently reported. These new data challenge the fireball models and disagree with their predictions, based on earlier broad-band observations, but they are in very good agreement with the predictions of the CB model, based on the same earlier data, and involving many fewer parameters.

Is the Afterglow of Gamma-Ray Burst GRB 021004 Unusual?

The afterglow (AG) of gamma-ray burst GRB 021004 has been claimed to be unusual. In the cannonball (CB) model of GRBs, that is not the case. The very early AG's shape is, like for GRB 990123, a direct tracer of the expected circumburst density profile. The unprecedented precision of the data allows for the resolution of two CBs in the AG. These two CBs correspond to the two pulses in the GRB and to the two wide shoulders in the AG light curve. The smaller wiggles in the AG are, like for GRB 000301c and for GRB 970508, to be expected: they trace moderate deviations from a constant density interstellar medium. The observed evolution of the optical spectrum is that predicted in the CB model. The X-ray and radio emissions of GRB 021004 are also normal.

The pre-supernova evolution of rotating massive stars

Massive stars are born rotating rigidly with a significant fraction of critical rotation at the surface. Consequently, rotationally-induced circulation and instabilities lead to chemical mixing in regions that would otherwise be stable, as well as a redistribution of angular momentum. Differential rotation also winds up magnetic fields, causing instabilities that can power a dynamo and magnetic stresses that lead to additional angular momentum transport. We follow the evolution of typical massive stars, their structure and angular momentum distribution, from the zero-age main sequence until iron core collapse. Without the action of magnetic fields, the resulting angular momentum is sufficiently large to significantly affect the explosion mechanism and neutron star formation. Sub-millisecond pulsars result that could encounter the r-mode instability. In helium cores massive enough, at least at low metalicity, the angular momentum is also sufficiently great to form a centrifugally supported accretion disk around a central black hole, powering the engine of the ‘collapsar’ model for GRBs. Including current estimates of the effect of magnetic fields still allows the formation of rapidly rotating (~ 5-10 ms) pulsars, but might leave too little angular momentum for collapsars.

Temporal properties of gamma ray bursts as signatures of jets from the central engine

A comprehensive temporal analysis has been performed on the 319 brightest GRBs with T90 > 2s from the BATSE current catalog. The GRBs were denoised using wavelets and subjected to an automatic pulse selection algorithm as an objective way of identifying pulses and quantifying the eects of neighbouring pulses. The number of statistically signicant pulses selected from the sample was greater than 3000. The rise times, fall times, full-widths at half-maximum (FWHM), pulse amplitudes and pulse areas were measured and the frequency distributions are presented here. All are consistent with lognormal distributions provided the pulses are well separated. The distribution of time intervals between pulses is not random but compatible with a lognormal distribution when allowance was made for the 64 ms time resolution and a small excess (5%) of long duration intervals that is often referred to as a Pareto-L evy tail. The time intervals between pulses are most important because they may be an almost direct measure of the activity in the central engine. Lognormal distributions of time intervals also occur in pulsars and SGR sources and therefore provide indirect evidence that the time intervals between pulses in GRBs are also generated by rotation powered systems with super-strong magnetic elds. A range of correlations are presented on pulse and burst properties. The rise and fall times, FWHM and area of the pulses are highly correlated with each other. The pulse amplitudes are anticorrelated with the FWHM. The time intervals between pulses and pulse amplitudes of neighbouring pulses are correlated with each other. It was also found that the number of pulses, N, in GRBs is strongly correlated with the fluence and duration and that can explain the well known correlation between duration and fluence. The GRBs were sorted into three categories based on N i.e. 3 N 12, 13 N 24 and N 25. The properties of pulses before and after the strongest pulse were compared for three categories of bursts. No major dierences were found between the distributions of the pulse properties before and after the strongest pulse in the GRB. However there is a strong trend for pulses to have slower rise times and faster fall times in the rst half of the burst and this pattern is strongest for category 3 N 12. This analysis revealed that the GRBs with large numbers of pulses have narrower and faster pulses and also larger fluences, longer durations and higher hardness ratios than the GRBs with smaller numbers of pulses. These results may be explained by either homogeneous or inhomogeneous jet models of GRBs. The GRBs with larger number of pulses are closer to the axis if varies with the opening angle of the jet and the imprint of the jet is preserved in the pulse structure of the burst. The distribution of the number of pulses per GRB broadly reflects the beaming by the jet.

Iron Kα Emission from X-Ray Reflection: Predictions for Gamma-Ray Burst Models

Recent observations of several γ-ray burst (GRB) afterglows have shown evidence for a large amount of X-ray line-emitting material, possibly arising from ionized iron. A significant detection of an X-ray spectral feature, such as that found in the Chandra observation of GRB 991216, may provide important constraints on the immediate environment of the burst and hence on progenitor models. The large Fe Kα equivalent widths inferred from the X-ray observations favor models in which the line is produced when the primary X-ray emission from the source strikes Thomson-thick material and Compton scatters into our line of sight. We present such reflection spectra here, computed in a fully self-consistent manner, and discuss the range of ionization parameters that may be relevant to different models of GRBs. We argue that the presence of a strong hydrogen-like Kα line is unlikely, because Fe XXVI photons would be trapped resonantly and removed from the line core by Compton scattering. In contrast, a strong narrow emission line from He-like Fe XXV is prominent in the model spectra. We briefly discuss how these constraints may affect the line energy determination in GRB 991216.

Efficiency and Spectrum of Internal Gamma‐Ray Burst Shocks

We present an analysis of the Internal Shock Model of GRBs, where gamma-rays are produced by internal shocks within a relativistic wind. We show that observed GRB characteristics impose stringent constraints on wind and source parameters. We find that a significant fraction, of order 20 %, of the wind kinetic energy can be converted to radiation, provided the distribution of Lorentz factors within the wind has a large variance and provided the minimum Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time R/c < 1 ms, suggesting a possible explanation for the observed clustering of spectral break energies in this range. The lower limit to wind Lorenz factor and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies are set by Thomson optical depth due to electron positron pairs produced by synchrotron photons. Natural consequences of the model are absence of bursts with peak emission energy significantly exceeding 1 MeV, and existence of low luminosity bursts with low, 1 keV to 10 keV, break energies.

On the Neutrino Flux from Gamma‐Ray Bursts

Observations imply that γ-ray bursts (GRBs) are produced by the dissipation of the kinetic energy of a highly relativistic fireball. Photomeson interactions of protons with γ-rays within the fireball dissipation region are expected to convert a significant fraction of fireball energy to greater than 1014 eV neutrinos. We present an analysis of the internal shock model of GRBs, where production of synchrotron photons and photomeson neutrinos are self-consistently calculated, and show that the fraction of fireball energy converted to high-energy neutrinos is not sensitive to uncertainties in fireball model parameters, such as the expansion Lorentz factor and characteristic variability time. This is due, in part, to the constraints imposed on fireball parameters by observed GRB characteristics and, in part, to the fact that, for parameter values for which the photomeson optical depth is high (implying high proton energy loss to pion production), neutrino production is suppressed by pion and muon synchrotron losses. The neutrino flux is, therefore, expected to be correlated mainly with the observed γ-ray flux. The time-averaged neutrino intensity predicted by the model, ~10-8.5 GeV cm-2 s-1 sr-1, is consistent with the flux predicted by the assumption that GRBs are the sources of greater than 1019 eV cosmic rays.

An Interpretation of the Evidence for T[CLC]e[/CLC]V Emission from GRB 970417[CLC]a[/CLC

The Milagrito collaboration recently reported evidence for emission of very high energy gamma rays in the TeV range from one of the BATSE gamma-ray bursts, GRB 970417a. Here I discuss possible interpretations of this result. Taking into account the intergalactic absorption of TeV gamma rays by the cosmic infrared background, I found that the detection rate (one per 54 gamma-ray bursts [GRBs] observed by the Milagrito) and energy fluence can be consistently explained with the redshift of this GRB at z approximately 0.7 and the isotropic total energy in the TeV range, ETeV,iso greater, similar1054 ergs. This energy scale is not unreasonably large, but interestingly similar to the maximum total GRB energy in the sub-MeV range observed to date for GRB 990123. On the other hand, the energy emitted in the ordinary sub-MeV range becomes EMeV,iso approximately 1051 ergs for GRB 970417a, which is much smaller than the total energy in the TeV range by a factor of about 10(3). I show that the proton-synchrotron model of GRBs provides a possible explanation for these observational results. I also discuss some observational signatures expected in future experiments from this model.

Observational status of Gamma-Ray bursts with BeppoSAX

An overview of the results on Gamma-Ray Bursts obtained with the X-ray Astronomy satellite BeppoSAX is given. Eighteen events were detected and localized with the wide field instruments, with sixteen of them that were also promptly followed-up with the BeppoSAX Narrow Field Instruments (NFIs). X-ray afterglow emission was discovered in twelve cases. Most of the GRBs with X-ray afterglow also show optical emission, while radio emission has been observed in about half of these cases. There is a GRB (981226), from which radio but not optical afterglow emission has been discovered. I will discuss the properties of these events in the light of the fireball models of GRBs.

Cosmological versus Intrinsic: The Correlation between Intensity and the Peak of the νFνSpectrum of Gamma‐Ray Bursts

We present results of correlation studies, examining the association between the peak of the nu F_nu spectrum of gamma ray bursts, E_p, with the burst's energy fluence and photon peak flux. We discuss methods to account for data truncation in E_p and fluence or flux when performing the correlation analyses. However, because bursts near the detector threshold are not usually able to provide reliable spectral parameters, we focus on results for the brightest bursts in which we can better understand the selection effects relevant to E_p and burst strength. We find that there is a strong correlation between total fluence and E_p. We discuss these results in terms of both cosmological and intrinsic effects. In particular, we show that for realistic distributions of the burst parameters, cosmological expansion alone cannot account for the correlation between E_p and total fluence; the observed correlation is likely a result of an intrinsic relation between the burst rest-frame peak energy and the total radiated energy. We investigate this latter scenario in the context of synchrotron radiation from external and internal shock models of GRBs. We find that the internal shock model is consistent with our interpretation of the correlation, while the external shock model cannot easily explain this intrinsic relation between peak energy and burst radiated energy.

Beamed Model of GRBs and Central Engine

Beamed Model of GRBs and Central Engine

On the nature of GRB-SGRs blazing jets

The GRB980425-SN1998bw association put in severe strain and contradiction the simplest "candle" fireball model for GRBs. We probed that statistically the association is reliable, the energy luminosity and probability puzzles between cosmic and near by GRBs find a solution within a precessing gamma jet model either for GRBs and SGRs. The expected repetitivity for GRB 980425 has been already, probably, observed on GRB980712.

Temporal evolution of the pulse width in GRBs

Many cosmological models of GRBs envision the energy source to be a cataclysmic stellar event leading to a relativistically expanding fireball. Particles are thought to be accelerated at shocks and produce nonthermal radiation. The highly variable temporal structure observed in most GRBs has significantly constrained models. By using different methods of statistical analysis in the time domain we show that the width of the pulses in GRBs time histories remain remarkably constant throughout the classic GRB phase. Peaks at the end of a burst have the same average duration to within a few percent as the peaks at the start of the burst. For emission sites that lie on a relativistically expanding shell, peaks should grow in duration because of deceleration. We find no deceleration over at least 2/3 of the burst duration. For emission sites that occupy a spread of angles on a shell, the curvature should cause the later peaks to grow in duration. Since we see no such growth, we can limit the total angular size of the shell to be substantially smaller than \Gamma^{-1} where \Gamma is the bulk Lorentz factor. This lack of temporal evolution of the pulse width should be explained by any fireball shock scenario.

Gamma-Ray Burst Spectral Features: Interpretation as X-Ray Emission from a Photoionized Plasma

Numerous reports have been made of features, either in emission or absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally interpreted in the context of Galactic neutron star models as cyclotron line emission and $e^+ - e^-$ annihilation features, the recent demonstration that the majority of GRBs lie at cosmological distances make these explanations unlikely. In this letter, we adopt a relativistic fireball model for cosmological GRBs in which dense, metal rich blobs or filaments of plasma are entrained in the relativistic outflow. In the context of this model, we investigate the conditions under which broadband features, similar to those detected, can be observed. We find a limited region of parameter space capable of reproducing the observed GRB spectra. Finally, we discuss possible constraints further high-energy spectral observations could place on fireball model parameters.