A "BOOSTED FIREBALL" MODEL FOR STRUCTURED RELATIVISTIC JETS

We present amodel for relativistic jets which generates a particular angular distribution of Lorentz factor and energy per solid angle. We consider a fireball with specific internal energy E/M launched with bulk Lorentz factor.B. In its center-of-momentum frame the fireball expands isotropically, converting its internal energy into radially expanding flow with asymptotic Lorentz factor eta(0) similar to E/M. In the lab frame the flow is beamed, expanding with Lorentz factor =2.0.B in the direction of its initial bulk motion and with characteristic opening angle.eta(0) similar to /.B. The flow is jet-like with eta(0) similar to 2.0 such that jets with G > 1/.0 are naturally produced. The choice.eta(0) similar to.B similar to 10 yields a jet with eta(0) similar to 200 on-axis and angular structure characterized by opening angle.eta(0) similar to 0.1 of relevance for cosmological gamma-ray bursts (GRBs), while.eta(0) similar to 1 may be relevant for low-luminosity GRBs. The model produces a family of outflows, of relevance for different relativistic phenomena with structures completely determined by.0 and.B. We calculate the energy per unit solid angle for the model and use it to compute light curves for comparison with the widely used top-hat model. The jet break in the boosted fireball light curve is greatly subdued when compared to the top-hat model because the edge of the jet is smoother than for a top-hat. This may explain missing jet breaks in afterglow light curves.