2010A&A...517A..84T

We analyze and characterize the angular distribution of selected samples of gamma ray bursts (GRBs) from BATSE and Swift data to confirm that the division into two classes of short- and long-duration GRBs also correspond to two distinct spatial populations. The angular distribution is analyzed using multifractal analysis and characterized by a multifractal spectrum of dimensions. Different spectra of dimensions relate to different angular distributions. The spectra of dimensions of short and long bursts indicate that the two populations indeed have two different angular distributions. Both Swift and BATSE long bursts appear to be homogeneously distributed across the sky with a monofractal distribution. In contrast, short GRBs follow a multifractal distribution for both the two samples. Even if BATSE data do not enable a unique interpretation of their angular distribution to be made because of the instrumental selection effects that mainly favor the detection of nearby GRBs, the results from Swift short GRBs confirm this behavior, also when including GRBs corrected by the redshift factor. The distributions traced by short GRBs, up to z=1, depict a universe with a structure similar to that of a disordered porous material with uniformly distributed heterogeneous irregular structures, appearing more clustered than expected.