Hints for families of GRBs improving the Hubble diagram
  As soon as their extragalactic origins were established, the hope to make
Gamma - Ray Bursts (GRBs) standardizeable candles to probe the very high - z
universe has opened the search for scaling relations between redshift
independent observable quantities and distance dependent ones. Although some
remarkable success has been achieved, the empirical correlations thus found are
still affected by a significant intrinsic scatter which downgrades the
precision in the inferred GRBs Hubble diagram. We investigate here whether this
scatter may come from fitting together objects belonging to intrinsically
different classes. To this end, we rely on a cladistics analysis to partition
GRBs in homogenous families according to their rest frame properties. Although
the poor statistics prevent us from drawing a definitive answer, we find that
both the intrinsic scatter and the coefficients of the $E_{peak}$\,-\,$E_{iso}$
and $E_{peak}$\,-\,$L$ correlations significantly change depending on which
subsample is fitted. It turns out that the fit to the full sample leads to a
scaling relation which approximately follows the diagonal of the region
delimited by the fits to each homogenous class. We therefore argue that a
preliminary identification of the class a GRB belongs to is necessary in order
to select the right scaling relation to be used in order to not bias the
distance determination and hence the Hubble diagram.
