Gamma-Ray Bursts and Afterglows from Relativistic Fireballs

Open Access
Author:
Panaitescu, Alin-Daniel
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 25, 1999
Committee Members:
  • Steinn Sigurdsson, Committee Member
  • Pablo Laguna, Committee Member
  • Peter Istvan Meszaros, Committee Chair
  • George Pavlov, Committee Member
  • Samuel Finn, Committee Member
Keywords:
  • relativistic hydrodynamics
  • non-thermal emission
  • synchrotron
  • inverse Compton
Abstract:
The origin of Gamma-Ray Bursts, one of the long-standing mysteries of modern Astrophysics, has been recently established observationally by the discovery of the afterglows that follow them. The correlations between the temporal and spectral features manifested by these afterglows are in accord with the predictions of the relativistic fireball model, giving it thus a very strong support. Here we model the dynamics and radiation emission of fireballs interacting with an external medium and compare some of the results with the observations. We present a one-dimensional code to solve ultra-relativistic hydrodynamic problems, using the Glimm method (based on an exact Riemann solver) for an accurate treatment of shocks and contact discontinuities, and 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 strong shocks and Lorentz factors of up to ~2000. With the aid of the hydrodynamic code we model the interaction between an expanding fireball and a stationary external medium. We compute burst spectra and time structures arising from synchrotron radiation and inverse Compton scatterings by non-thermal electrons accelerated by the shocks which form during the fireball external medium interaction. 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. Multi-pulse structures are simulated using a variable magnetic field and anisotropic emission, and the most important spectral and temporal properties of the pulses are compared with observations. The fireball dynamics is further followed to study the spectral evolution of the remnant emission. We analyze the shape of the equal photon arrival time surfaces for different dynamic and radiative regimes and homogeneous or power-law external densities, and tabulate the most relevant parameters describing the source brightness distribution over these surfaces, which are useful for more accurate analytic estimates of the afterglow evolution. We also present an analytical approach to calculate the dynamics of the fireball–surrounding medium interaction. It is a 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, and is computationally much less expensive than hydrodynamic simulations. We investigate the effect of the relevant model parameters on the X-ray, optical and radio fluxes, and the effects of a refreshed shock energy input, anisotropy in the ejecta, and jet sideways expansion on the afterglow light-curves.