**4. Afterglow Emission from a Structured Jet**

As explained in Section 2.2, the expansion of the jet material in the "circum-burst medium" (the relic stellar wind from the progenitor massive star in long GRBs. In most long GRBs, the afterglow seems to be best modeled assuming a homogeneous interstellar medium (ISM), which poses a challenge to the massive star progenitor scenario for long GRBs [88], or a tenuous interstellar medium in short GRBs) leads to the formation of a shock, which is widely believed to be the main source of the broad-band GRB afterglow emission [89,226–228]. Hereon, we focus on the case of a homogeneous ISM composed of hydrogen, with number density *n*, and discuss the impact of jet structure on the expected afterglow emission.
