  Recently, GRB 170817A was confirmed to be associated with GW 170817, which
was produced by a neutron star - neutron star merger. It indicates that at
least some short gamma-ray bursts come from binary neutron star mergers.
Theoretically, it is widely accepted that short gamma-ray bursts can be
produced by two distinctly different mechanisms, binary neutron star mergers
and neutron star - black hole mergers. These two kinds of bursts should be
different observationally due to their different trigger mechanisms. Motivated
by this idea, we collect a universal data set constituted of 51 short gamma-ray
bursts observed by $Swift$/BAT, among which 14 events have extended emission
component. We study the observational features of these 51 events
statistically. It is found that our samples are consisted of two distinct
groups. They clearly show a bimodal distribution when their peak photon fluxes
at 15-150 keV band are plotted against the corresponding fluences. Most
interestingly, all the 14 short bursts with extended emission lie in a
particular region. When the fluences are plotted against the burst durations,
short bursts with extended emission again tend to concentrate in the long
duration segment. These features strongly indicate that short gamma-ray bursts
really may come from two distinct types of progenitors. We argue that those
short gamma-ray bursts with extended emission come from the coalescence of
neutron stars, while the short gamma-ray bursts without extended emission come
from neutron star - black hole mergers.
