  Neutron star mergers where both stars have negligible spins are commonly
considered as the most likely, "standard" case. But based on observed systems,
we estimate that actually a non-negligible fraction of all double neutron star
mergers ($\sim$ 5 %) may contain one millisecond component. We use the
Lagrangian Numerical Relativity code SPHINCS_BSSN to simulate mergers where one
star has no spin and the other has a dimensionless spin parameter of
$\chi=0.5$. These mergers exhibit several distinct signatures compared to
irrotational cases. Morphologically, they are similar to unequal mass mergers
and they form in particular only one, very pronounced spiral arm. Compared to
the non-spinning cases, they dynamically eject an order of magnitude more mass
of unshocked material at the original low electron fraction of the neutron
stars and therefore produce particularly bright, red kilonovae and brighter
kilonova afterglows months after the merger. We also find that the spinning
cases have significantly more fallback accretion, with implications for
late-time X-ray flares and the duration of the associated gamma-ray burst.
Overall, the spinning case collisions are substantially less violent and they
emit smaller amounts of shock-generated semi-relativistic material and
therefore produce less pronounced blue/UV kilonova precursor signals. Their
post-merger gravitational wave signal is weaker and, during the simulated time,
substantially smaller amounts of energy and angular momentum are emitted.
Therefore the central remnant contains a larger angular momentum reservoir and
could remain an "active engine" for a longer time.

High Energy Astrophysical Phenomena