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Re'em Sari

Publications and source records attributed to Re'em Sari.

153 records · Page 9Linked to original sources
arXiv1997

The Observed Size and Shape of GRB Afterglow

The detection of delayed emission in X-ray, optical and radio wave length, ``afterglow'', following a gamma-ray burst can be described by the emission of a relativistic shell decelerating upon collision with the ISM. We show that the observed radiation surface have well defined bright edges. We derive an explicit expression for the size as a function of time, and obtain the surface brightness distribution. This might be directly observed if the burst occurs at small redshift so that its radio signal can be resolved. The size and shape are relevant for detailed analysis of scintillation or microlensing. We show that the effective Lorentz factor depends on the observed frequency and it is higher for frequencies above the synchrotron typical frequency (optical and X-ray) than for low frequencies (radio). Consequently transition to non relativistic evolution, will be observed first in low frequencies and only a factor of ~2 later in the high frequencies.

astro-ph↗
arXiv1997

Hydrodynamics of GRB After-Glow

The detection of delayed emission at X-ray optical and radio wavelengths, ``after-glow'', suggests that the relativistic shell which emitted the initial GRB due to internal shocks decelerates on encountering an external medium, giving rise to the after-glow. At a very early stage (few seconds), the observed bolometric luminosity increases as t^2. On longer time scales (more than about 10s), the luminosity drops as t^{-1}. If the main burst is long enough, an intermediate stage of constant luminosity will form. In this case, the after-glow overlaps the main burst, otherwise there is a time separation between the two. On the long time scale, the flow decelerate in a self similar way, reaching non relativistic velocities after about 30days. Due to the deceleration and the accumulation of ISM material, the relation between the observed time, the shock radius, and its Lorentz factor, is given by t=R/16γ^2c which is a factor of 8 different from the usual expression. The majority of particles are those of the original ejecta (and not the ISM) up to about 900s. These particles reach sub-relativistic velocities on a time scale of about 2hours, well before the flow becomes sub-relativistic. Therefore the ejecta particles are probably unimportant for most of the after-glow radiation. We show that even though only a small fraction of the energy is given to the electrons, most of the energy can be radiated over time. If this fraction is greater than 10% radiation losses will significantly influence the hydrodynamical evolution at early times (less than 1day).

astro-ph↗
arXiv1997

Can internal shocks produce the variability in GRBs?

We discuss the possibility that gamma-ray bursts result from internal shocks in an ultra-relativistic matter. Using a simple model we calculate the temporal structure and we estimate the efficiency of this process. In this model the ultra-relativistic matter flow is represented by a succession of shells with random values of the Lorentz factor. We calculate the shocks that take place between those shells and we estimate the resulting emission. Internal shocks can produce the highly variable temporal structure observed in most of the bursts provided that the source emitting the relativistic flow is highly variable. The observed peaks are in almost one to one correlation to the activity of the emitting source. A large fraction of the kinetic energy is converted to radiation. The most efficient case is when an inner engine produces shells with comparable energy but very different Lorentz factors. It also gives the most preferable temporal structure.

astro-ph↗
arXiv1997

Implication of Temporal Structure in GRB

The highly variable temporal structure observed in most GRBs provides us with unexpected clues. We show that variable GRBs cannot be produced by external shocks models and consequently cannot be produced by an ``explosive'' inner engine. The observed temporal structure must reflect the activity of the inner engine that must be producing unsteady and irregular ``wind'' which is converted to radiation via internal shocks.

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arXiv1997

Variability in GRBs - A Clue

We show that external shocks cannot produce a variable GRB, unless they are produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only a small fraction of the shell emits the radiation and the process is very inefficient. Internal shocks can produce the observed complex temporal structure provided that the source itself is variable. In this case, the observed temporal structure reflects the activity of the ``inner engine'' that drives the bursts. This sets direct constraints on it.

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arXiv1996

Cosmological GRBs: Internal vs. External Shocks

Internal (IS) or external (ES) shocks in relativistic expanding shells are currently the best known mechanism for producing GRBs. We calculate the hydrodynamic conditions and the cooling processes in internal shocks, that occur when one layer overtakes another. We compare these conditions with the conditions in external shocks that occur when the shell encounters the ISM. We find that the complicated temporal structure observed in GRBs could be easily explained in IS scenario. Unlike ES, the observed temporal structure simply reflects the temporal behavior of the internal engine that produces the shell. We also find that opacity and in particular the opacity for pair production poses strong constraints on the parameter space and consequently bursts with very narrow peaks are expected to be optically thick and would not contain GeV photons. Finally we find that as in ES synchrotron is the most likely radiation process.

astro-ph↗
arXiv1996

Cooling Time Scales and Temporal Structure of Gamma-Ray Bursts

A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultra-relativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is radiated via synchrotron emission and Comptonization. We estimate the synchrotron cooling time scale of the shocked regions for electrons of various Lorentz factors, focusing in particular on those electrons whose radiation falls within the energy detection range of the BATSE detectors. We find that in order to produce the rapid variability observed in most bursts the magnetic energy density in the shocked material must be greater than about 1\% of the thermal energy density. Additionally, the electrons must be nearly in equipartition with the protons, otherwise the radiative efficiency is low. Inverse Compton scattering can increase the cooling rate of the relevant electrons but the Comptonized emission itself is never within the BATSE range. These arguments allow us to pinpoint the conditions within the radiating regions in GRBs and to determine the important radiation processes. In addition, the model predicts that the duty cycle of intensity variations in GRB light curves should be nearly independent of burst duration, and should scale inversely as the square root of the observed photon energy. Both correlations are in agreement with observations. The model also provides a plausible explanation for the bimodal distribution of burst durations. There is no explanation, however, for the characteristic break energy in GRB spectra.

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arXiv1995

Hydrodynamic Time Scales and Temporal Structure in GRBs

We calculate the hydrodynamic time scales for a spherical ultra-relativistic shell that is decelerated by the ISM and discuss the possible relations between these time scales and the observed temporal structure in $γ$-ray bursts. We suggest that the bursts' duration is related to the deceleration time, the variability is related to the ISM inhomogeneities and precursors are related to internal shocks within the shell. Good agreement can be achieved for these quantities with reasonable, not fined tuned, astrophysical parameters. The difference between Newtonian and relativistic reverse shocks may lead to the observed bimodal distribution of bursts' durations.

astro-ph↗
arXiv1995

Hydrodynamic Time Scales and Temporal Structure of GRBs

We calculate the hydrodynamic time scales for a spherical ultra-relativistic shell that is decelerated by the ISM and discuss the possible relations between these time scales and the observed temporal structure in $γ$-ray bursts. We suggest that the bursts' duration is related to the deceleration time, the variability is related to the ISM inhomogeneities and precursors are related to internal shocks within the shell. Good agreement can be achieved for these quantities with reasonable, not fined tuned, astrophysical parameters. The difference between Newtonian and relativistic reverse shocks may lead to the observed bimodal distribution of bursts' durations.

astro-ph↗