5 Clumpiness of the GRB ambient medium

The above discussion shows that the emission lines detected in several GRB afterglows can have a thermal origin only if the GRB surroundings are extremely clumpy, with density and geometric contrasts of order of a hundred thousand or more. We now discuss the reliability of these conditions in different GRB progenitor scenario.

In the hypernova scenario, the GRB sets on simultaneously to a supernova explosion, and the ambient medium is the result of the interaction of the pre-SN star with its surroundings. In particular, the nearby ambient medium will be dominated by the late stages of the mass ejection history of the star. These stages are known to be unsteady and clumps or shell-like structures can be envisaged. If the mass ejection is caused by radiative effects, however, extreme structure cannot be produced, since the stellar luminosity varies on time scales comparable to the Kelvin time-scale, which is of the order of hundreds of years, yielding a thick shell. In the wind environment of SN1998bw, for example, Li & Chevalier (1999) found that inhomogeneities up to a factor of a few were present. A more appealing scenario, in this perspective, is the supranova model by Vietri & Stella (1998), where the GRB is supposed to explode several weeks to years after a supernova. The supernova explosion, having a much smaller time scale, can generate a more extreme geometry. The clumpiness of SN ejecta has been investigated by numerical simulations. It is found that Rayleigh-Taylor instabilities can produce high density clumps with angular scales $\ga$$1^\circ$ (Böttcher et al. 2002). Such structures have $\eta_R\equiv{a}/R\sim10^{-2}$ where a is the clump radius, much larger than the value required to produce sizable line emission from collisional excitation. It is however possible, in the supranova scenario, that the remnant is illuminated by a super-Eddington relativistic wind (Vietri & Stella 1998; Konigl & Granot 2002) in the time span between the SN and GRB explosions. The interaction of this wind with the ejected SN shell may increase the inhomogeneities originally present in the shell (Guetta & Granot 2003; Lazzati & Rees, in preparation).