Analysis of Fermi gamma-ray burst duration distribution
  Two classes of GRBs, short and long, have been determined without any doubts,
and are usually prescribed to different physical scenarios. A third class,
intermediate in $T_{90}$ durations, has been reported to be present in the
datasets of BATSE, Swift, RHESSI and possibly BeppoSAX. The latest release of
$>1500$ GRBs observed by Fermi gives an opportunity to further investigate the
duration distribution. The aim of this paper is to investigate whether a third
class is present in the $\log T_{90}$ distribution, or is it described by a
bimodal distribution. A standard $\chi^2$ fitting of a mixture of Gaussians is
applied to 25 histograms with different binnings. Different binnings give
various values of the fitting parameters, as well as the shape of the fitted
curve. Among five statistically significant fits none is trimodal. Locations of
the Gaussian components are in agreement with previous works. However, a
trimodal distribution, understood in the sense of having three separated peaks,
is not found for any binning. It is concluded that the duration distribution in
Fermi data is well described by a mixture of three log-normal distributions,
but it is intrinsically bimodal, hence no third class is present in the
$T_{90}$ data of Fermi. It is suggested that the log-normal fit may not be an
adequate model.
