Broadband modeling of gamma-ray burst afterglows(*)

According to the fireball model gamma-ray burst (GRB) afterglows are the result of a shock pushed into the surrounding medium by an extremely relativistic outflow from the GRB. By modeling the broadband spectrum, ranging from X-ray to radio wavelengths, on time scales of hours to years after the GRB, we can determine several burst parameters, e.g. the burst energy, the circumburst medium density (and its gradient) and the energy contained in electrons and the magnetic field. We have developed a code to do broadband modeling and determine the model parameters. We also performed Monte Carlo simulations with synthetic data sets in order to derive an estimate of the accuracy of the best fit parameters. RB970508 was taken as a case study and we present our fitting results. We also present our WSRT radio observations of GRB030329 at centimeter wavelengths and the results from modeling this afterglow. The low-frequency WSRT radio observations provide a unique possibility to study the late stages of the fireball evolution and examine the jet structure. PACS 98.70. Rz - gamma-ray sources; gamma-ray bursts. PACS 95.30. Gv - Radiation mechanisms; polarization. PACS 95.85. Bh - Radio, microwave (> 1 mm). PACS 01.30. Cc - Conference proceedings.