arXiv subjects

A. Panaitescu

Publications and source records attributed to A. Panaitescu.

63 records · Page 4Linked to original sources

arXiv1999

Dynamical Evolution, Light-Curves and Spectra of Spherical and Collimated Gamma-Ray Burst Remnants

We present an analytical approach to the dynamical evolution of fireballs or axially symmetric jets expanding into an external medium, with application to Gamma-Ray Burst remnants. This method leads to numerical calculations of fireball dynamics that are computationally faster than hydrodynamic simulations. It is also a very flexible approach, that can be easily extended to include more complex situations, such as a continuous injection of energy at the reverse shock, and the sideways expansion in non-spherical ejecta. Some features of the numerical results for the remnant dynamical evolution are discussed and compared to the analytical results. We find that the ratio of the observer times when the jet half-angle reaches twice its initial value and when the light cone becomes wider than the jet is substantially smaller than predicted analytically. The effects arising from the remnant curvature and thickness further reduce this ratio, such that the afterglow light-curve break due to the remnant finite angular extent overlaps the weaker break due to the jet's sideways expansion. An analysis of the effects of the curvature and thickness of the remnant on the afterglow light-curves shows that these effects are important and should be taken into account for accurate calculations of the afterglow emission.

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arXiv1999

Power Density Spectra of Gamma-Ray Bursts in the Internal Shock Model

We simulate Gamma-Ray Bursts arising from internal shocks in relativistic winds, calculate their power density spectrum (PDS), and identify the factors to which the PDS is most sensitive: the wind ejection features, which determine the wind dynamics and its optical thickness, and the energy release parameters, which give the pulse 50-300 keV radiative efficiency. For certain combinations of ejection features and wind parameters the resulting PDS exhibits the features observed in real bursts. We found that the upper limit on the efficiency of conversion of wind kinetic energy into 50-300 keV photons is $\sim$ 1%. Winds with a modulated Lorentz factor distribution of the ejecta yield PDSs in accord with current observations and have efficiencies closer to $10^{-3}$, while winds with a random, uniform Lorentz factor ejection must be optically thick to the short duration pulses to produce correct PDSs, and have an overall efficiency around $10^{-4}$.

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arXiv1998

Hydrodynamical Simulations of Gamma Ray-Bursts from Internal Shocks in Relativistic Fireballs

We simulate the dynamics of the interaction between relativistically expanding spherically symmetric shells using a 1-dimensional hydrodynamic code and calculate spectra and light-curves arising from such collisions by integrating the synchrotron and inverse Compton emission of the shocked gas. The numerical results reflect the most important features observed in Gamma-Ray Bursts: the spectrum exhibits a progressive softening (its break energy decaying exponentially with the 50--300 keV photon fluence), and the pulses that form the burst appear narrower in higher energy bands. Analytical results for the most important physical parameters of the burst are obtained by solving the shock jump conditions for a pair of interacting shells, in the case when both the forward and reverse shocks are relativistic.

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arXiv1998

Multi-Wavelength Afterglows in Gamma-Ray Bursts: Refreshed Shock and Jet Effects

We present an analytical approach to calculate the hydrodynamics of the interaction between a relativistic ejecta and a surrounding medium, whose evolution serves as a model for Gamma-Ray Burst afterglows. We investigate the effect of the relevant model parameters on the X-ray, optical and radio fluxes, and the effect of a refreshed shock energy input and anisotropy in the ejecta on the shape of the light curves. We compare our numerical results to observed afterglows and give a quantitative description of the conditions (geometry and physical parameters) in the ejecta that are compatible with the light curves of the 970508 afterglow, for which a large number of accurate flux measurements are available.

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arXiv1998

Radiative Regimes in Gamma-Ray Bursts and Afterglows

We present numerical simulations of Gamma-Ray Bursts arising from external shocks in the impulsive and wind models, including a weak or a strong coupling between electrons and protons plus magnetic fields, and analyze the burst features in each scenario. The dynamics of the ejecta and external medium are followed into the late stages of deceleration, in order to study the hydrodynamics of the remnant and the temporal and spectral evolution of the afterglow. A brief comparison with the optical and radio afterglows of GRB 970228 and GRB 970508 is made.

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arXiv1997

Rings in Fireball Afterglows

We derive the equation for the surface of equal arrival times of radiation from a thin relativistic shell interacting with an external medium, representing the afterglow of a gamma-ray burst produced by a fireball. Due to the deceleration, these surfaces become distorted ellipsoids and, at sufficiently late times, most of the light (either bolometric or in a given band) comes from a ring-like region whose width depends only on age. We analyze the shape of these surfaces and the radiation received from different angles for different dynamic and radiative regimes and homogeneous or power-law external densities. We calculate angle-integrated bolometric and fixed frequency fluxes, and we tabulate the most relevant parameters that describe the equal arrival time surfaces and the source brightness distribution, quantities that are useful for more accurate analytic estimates of the afterglow evolution.

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arXiv1997

Simulations of Gamma-Ray Bursts from External Shocks: Time Variability and Spectral Correlations

We compute burst spectra and time structures arising from synchrotron and inverse Compton scattering by non-thermal electrons accelerated in shocks which form during the interaction between a thin ultra-relativistic fireball and a stationary external medium. We investigate the effect of varying the most important model parameters on the resulting burst spectra, and we present a set of correlations among the spectral and temporal features of the bursts. The spectral hardness, various spectral-temporal correlations and the spectral evolution of the simulated bursts are compared to those of observed bursts for a representative set of model parameters. Multi-pulse structures are simulated using a variable magnetic field and anisotropic emission, and the most important spectral and temporal properties of these pulses are compared with observations.

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arXiv1997

A Shock-Patching Code for Ultra-Relativistic Fluid Flows

We have developed a one-dimensional code to solve ultra-relativistic hydrodynamic problems, using the Glimm method for an accurate treatment of shocks and contact discontinuities. The implementation of the Glimm method is based on an exact Riemann solver and van der Corput sampling sequence. In order to improve computational efficiency, the Glimm method is replaced by a finite differencing scheme in those regions where the fluid flow is sufficiently smooth. The accuracy and convergence of this hybrid method is investigated in tests involving planar, cylindrically and spherically symmetric flows that exhibit strong shocks and Lorentz factors of up to ~2000. This hybrid code has proven to be successful in simulating the interaction between a thin, ultra-relativistic, spherical shell and a low density stationary medium, a situation likely to appear in Gamma-Ray Bursts, supernovae explosions, pulsar winds and AGNs.

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arXiv1997

Impact of Relativistic Fireballs on External Matter : Numerical Models of Cosmological Gamma-Ray Bursts

We numerically model the interaction between an expanding fireball and a stationary external medium whose density is either homogeneous or varies with distance as a power-law. The evolution is followed until most of the fireball kinetic energy is converted into internal energy. The density, pressure and flow Lorentz factor profiles are shown at different stages, including shock and rarefaction wave reflections, for a fireball of initial bulk Lorentz factor Gamma = 100, both in the adiabatic and non-adiabatic (radiative) regimes. For cooling times shorter than the dynamic time, bolometric light-curves are computed for values of Gamma = 50, 100 and 200. We compare the numerical light-curves with analytic results, and find that for a homogeneous external medium there is a simple scaling relationship among light-curves obtained for different parameters. The light-curves for power-law external densities are similar in shape to those in the homogeneous case. We discuss the implications of a comparison of the results with observed Gamma-Ray Burst time histories.

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