  Aims. 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 in two classes of short- and long-duration GRBs correspond also to
the existence of two distinct spatial populations. Methods. The angular
distribution is analyzed by using multifractal analysis and characterized by a
multifractal spectrum of dimensions. Different spectra of dimensions indicate
different angular distributions. Results. The spectra of dimensions of short
and long bursts indicate that the two populations have two different angular
distributions. Both Swift and BATSE long bursts appear to be homo- geneously
distributed in the sky with a monofractal distribution. Short GRBs follow
instead a multifractal distribution for both the two samples. Even if BATSE
data may not give a secure in- terpretation of their angular distribution
because of the instrumental selection effects that mainly favor the detection
of near GRBs, the results from Swift short GRBs confirm this behavior, also
when are included 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 what
expected.
