  We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective
Bayesian Block procedure to extract temporal descriptors of the bursts' initial
pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without
extended emission (EE) components, respectively. IPCs of non-EE bursts are
dominated by single pulse structures, while EE bursts tend to have two or more
pulse structures. The medians of characteristic timescales - durations, pulse
structure widths, and peak intervals - for EE bursts are factors of ~ 2-3
longer than for non-EE bursts. A trend previously reported by Hakkila and
colleagues unifying long and short bursts - the anti-correlation of pulse
intensity and width - continues in the two short burst groups, with non-EE
bursts extending to more intense, narrower pulses. In addition we find that
preceding and succeeding pulse intensities are anti-correlated with pulse
interval. We also examine the short burst X-ray afterglows as observed by the
Swift/XRT. The median flux of the initial XRT detections for EE bursts (~ 6 x
10^-10 erg cm^-2 s^-1) is ~> 20 x brighter than for non-EE bursts, and the
median X-ray afterglow duration for EE bursts (~ 60,000 s) is ~ 30 x longer
than for non-EE bursts.
  The tendency for EE bursts toward longer prompt-emission timescales and
higher initial X-ray afterglow fluxes implies larger energy injections powering
the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest
that a significant fraction explode into more dense environments than non-EE
bursts, or that the sometimes-dominant EE component efficiently powers the
afterglow. Combined, these results favor different progenitors for EE and
non-EE short bursts.

High Energy Astrophysical Phenomena