SIMBAD references

In a cosmological context dust has been always poorly understood. This is true also for the statistics of Gamma-Ray Bursts (GRBs). We therefore started a program to understand the role of dust both in GRBs and as function of z. This paper presents a composite model that considers a rather generic distribution of dust in a spiral galaxy and considers the effect of changing some of the parameters characterizing the dust grains, size in particular. We first simulated 500 GRBs distributed as the host galaxy mass distribution, using as a model the Milky Way. If we consider dust with the same properties as those we observe in the Milky Way, we find that due to absorption we miss ∼10% of the afterglows assuming we observe the event within 100hr/s. In our second set of simulations we placed GRBs randomly inside giant molecular clouds, considering different kinds of dust inside and outside the host cloud and the effect of dust sublimation caused by the GRB inside the clouds. In this case absorption is mainly due to the host cloud and the physical properties of dust play a strong role. Computations from this model agree with the hypothesis of host galaxies with an extinction curve similar to that of the Small Magellanic Cloud, but the host cloud could be characterized also by dust with larger grains. Unfortunately, the present statistics lack significance, being based on incompatible observations, at different times from the burst and with different limiting magnitudes. To confirm our findings we need a set of homogeneous infrared observations. The use of forthcoming dedicated infrared telescopes, like REM, will provide a wealth of new afterglow observations.