A "Boosted Fireball" Model for Structured Relativistic Jets
  We present a model 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
\gamma_B. This "boosted fireball" model is motivated by the phenomenology of
collapsar jets, but is applicable to a wide variety of relativistic flows. In
its center-of-momentum frame the fireball expands isotropically, converting its
internal energy into radially expanding flow with asymptotic Lorentz factor
\eta_0 ~ E/M. In the lab frame the flow is beamed, expanding with Lorentz
factor \Gamma = 2 \eta_0 \gamma_B in the direction of its initial bulk motion
and with characteristic opening angle \theta_0 ~ 1/\gamma_B. The flow is
jet-like with \Gamma \theta_0 ~ 2 \eta_0 such that jets with \Gamma >
1/\theta_0 are naturally produced. The choice \eta_0 ~ \gamma_B ~ 10 yields a
jet with \Gamma ~ 200 on-axis and angular structure characterized by opening
angle \theta_0 ~ 0.1 of relevance for cosmological GRBs, while \gamma_B >~ 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 \eta_0 and \gamma_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.
