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 } .
In its center-of-momentum frame the fireball expands isotropically , converting its internal energy into radially expanding flow with asymptotic Lorentz factor \eta _ { 0 } \sim 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 } \sim 1 / \gamma _ { B } .
The flow is jet-like with \Gamma \theta _ { 0 } \sim 2 \eta _ { 0 } such that jets with \Gamma > 1 / \theta _ { 0 } are naturally produced .
The choice \eta _ { 0 } \sim \gamma _ { B } \sim 10 yields a jet with \Gamma \sim 200 on-axis and angular structure characterized by opening angle \theta _ { 0 } \sim 0.1 of relevance for cosmological GRBs , while \gamma _ { B } \gtrsim 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 .