  The detection of GW170817/AT2017gfo inaugurated an era of multimessenger
astrophysics, in which gravitational wave and multiwavelength photon
observations complement one another to provide unique insight on astrophysical
systems. A broad theoretical consensus exists in which the photon phenomenology
of neutron star mergers largely rests upon the evolution of the small amount of
matter left on bound orbits around the black hole or massive neutron star
remaining after the merger. Because this accretion disk is far from inflow
equilibrium, its subsequent evolution depends very strongly on its initial
state, yet very little is known about how this state is determined. Using both
snapshot and tracer particle data from a numerical relativity/MHD simulation of
an equal-mass neutron star merger that collapses to a black hole, we show how
gravitational forces arising in a non-axisymmetric, dynamical spacetime
supplement hydrodynamical effects in shaping the initial structure of the bound
debris disk. The work done by hydrodynamical forces is ${\sim}10$ times greater
than that due to time-dependent gravity. Although gravitational torques prior
to remnant relaxation are an order of magnitude larger than hydrodynamical
torques, their intrinsic sign symmetry leads to strong cancellation; as a
result, hydrodynamical and gravitational torques have comparable effect. We
also show that the debris disk's initial specific angular momentum distribution
is sharply peaked at roughly the specific angular momentum of the merged
neutron star's outer layers, a few $r_g c$, and identify the regulating
mechanism.
