UNC PHYSICS AND ASTRONOMY MASTER’S DEFENSE
Nathan Frank, UNC-Chapel Hill
“A Toy Model for Jet Structure in the Afterglow of Gamma-Ray Burst 080710”
Gamma-ray Bursts (GRBs) are the most energetic explosions in the universe since the Big Bang and are associated with the deaths of massive stars. Material not immediately consumed by the resulting black hole is ejected at relativistic speeds and releases a fleeting, collimated pulse of gamma rays. The prompt burst of emission is followed by a longer duration afterglow radiating from X-ray to radio frequencies; the result of shocks formed by rapid deceleration of the ultra-relativistic jet upon interaction with the circumburst medium. In the years since the first afterglows were observed in the late 90’s, a theoretical synchrotron emission model has been developed which well approximates the spectral flux by power-law components in frequency and time with indices that vary smoothly across various break frequencies. We present results from modeling the afterglow GRB 080710, a burst with a particularly rich, high S/N data set including X-ray, optical, and near-IR observations whose lightcurve is rising for the first ~ 30 minutes, a feature not well explained by standard GRB afterglow models. This so-called “slow rise” behavior distinguishes this burst from most others which are, in general, only observed to fade with time. We have fit flux values obtained from the observations of this afterglow to a simple model which allows for temporal variations in the equivalent isotropic energy (bulk kinetic energy) as well as the fractions thereof in shocked electrons and in the magnetic field. If the emission cone of a burst is directed away from our line of sight (“off-axis”) then we will see the fringes of the emission beam at early times. As the jet slows and the Lorentz factor drops the emission beam will widen and our line of site will begin to encompass regions closer to the center of the jet, possibly with greater energy. We find that an off-axis jet scenario is consistent with the early rising component seen in this GRB afterglow, warranting a more detailed modeling of the event. In future work, we seek to develop a model for energy variations across the surface of the jet and will integrate this model over larger and larger regions around a parameterized viewing angle as the jet slows, allowing for the creation of model lightcurves for any viewing angle.