NS-NS merger simulations address a broad set of questions, which can be roughly summarized as follows (note the same questions apply to BH-NS mergers as well):
The influence of the gravitational formalism used in a numerical simulation on the answer one finds for
the questions above differs item by item. Determining the final fate of a merging system is highly dependent
on the gravitational formalism; NS-NS merger remnants only undergo collapse in quasi-relativistic and fully
GR schemes. Moreover, orbital dynamics at separations comparable to the ISCO and even somewhat larger
depend strongly on the gravitational scheme. In particular, mass loss rates into a disk are often suppressed
by orders of magnitude in GR calculations when compared to CF simulations, and even more so in
comparison to PN and Newtonian calculations. EM emission profiles from a disk are difficult to calculate
accurately without the use of full GR for this reason. On the other hand, while GR is required to
calculate the exact GW signal from a merger, even early Newtonian simulations predicted many
of the qualitative GW emission features correctly, and PN and CF schemes yielded results
with some degree of quantitative accuracy about the full wavetrain. B-fields have only begun
to be explored, but it already seems clear that they will affect the hydrodynamical evolution
primarily after the merger in cases where differential rotation in a HMNS or disk winds up
magnetic field strengths up to energy equipartition levels, vastly stronger than those found in
pre-merger NSs. For such configurations, non-relativistic calculations can often reproduce the basic
physical scenario but full GR is required to properly understand the underlying dynamics.
Finally, the production of r-process elements, which depends sensitively on the thermodynamic
evolution of the merger, seems to generally disfavor binary mergers as a significant source of the
observed stellar abundances since the temperature and thus the electron fraction of the fluid
remains too small [252], regardless of the nature of the gravitational treatment used in the
calculations. This picture may need to be revised if significant mass loss occurs from the hot accretion
disk that forms around the central post-merger object, possibly due to energy release from
the r-process itself, but numerical calculations do not currently predict sufficient mass loss to
match observations [297]. We will address each of these topics in greater detail in the sections
below.
Since the first NS-NS merger calculations, there have been two main directions for improvements: more
accurate relativistic gravitation, resulting in the current codes that operate using a self-consistent fully GR
approach, and the addition of microphysical effects, which now include treatments of magnetic fields and
neutrino/EM radiation. Noting that several of the following developments overlapped in time,
e.g., the first full GR simulations by Shibata and Uryū [287] are coincident with the first PN
SPH calculations, and predate the first CF SPH calculations, we consider in turn the original
Newtonian calculations, those performed using approximate relativistic schemes, the calculations
performed using full GR, and finally those that have included more advanced microphysical
treatments.
http://www.livingreviews.org/lrr-2012-8 |
Living Rev. Relativity 15, (2012), 8
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