  We present numerical simulations of binary neutron star mergers, comparing
irrotational binaries to binaries of NSs rotating aligned to the orbital
angular momentum. For the first time, we study spinning BNSs employing nuclear
physics equations of state, namely the ones of Lattimer and Swesty as well as
Shen, Horowitz, and Teige. We study mainly equal mass systems leading to a
hypermassive neutron star (HMNS), and analyze in detail its structure and
dynamics. In order to exclude gauge artifacts, we introduce a novel coordinate
system used for post-processing. The results for our equal mass models show
that the strong radial oscillations of the HMNS modulate the instantaneous
frequency of the gravitational wave (GW) signal to an extend that leads to
separate peaks in the corresponding Fourier spectrum. In particular, the high
frequency peaks which are often attributed to combination frequencies can also
be caused by the modulation of the m=2 mode frequency in the merger phase. As a
consequence for GW data analysis, the offset of the high frequency peak does
not necessarily carry information about the radial oscillation frequency.
Further, the low frequency peak in our simulations is dominated by the
contribution of the plunge and the first 1-2 bounces. The amplitude of the
radial oscillations depends on the initial NS spin, which therefore has a
complicated influence on the spectrum. Another important result is that HMNSs
can consist of a slowly rotating core with an extended, massive envelope
rotating close to Keplerian velocity, contrary to the common notion that a
rapidly rotating core is necessary to prevent a prompt collapse. Finally, our
estimates on the amount of unbound matter show a dependency on the initial NS
spin, explained by the influence of the latter on the amplitude of radial
oscillations, which in turn cause shock waves.
