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General Relativity and Quantum Cosmology, gr-qc, Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
We calculate quasi-equilibrium sequences of equal-mass, irrotational binary
neutron stars (BNSs) in a scalar-tensor (ST) theory of gravity that admits
dynamical scalarization. We model neutron stars with realistic equations of
state (notably through piecewise polytropic equations of state). Using these
quasi-equilibrium sequences we compute the binary's scalar charge and binding
energy versus orbital angular frequency. We find that the absolute value of the
binding energy is smaller than in general relativity (GR), differing at most by
~14% at high frequencies for the cases considered. We use the newly computed
binding energy and the balance equation to estimate the number of
gravitational-wave (GW) cycles during the adiabatic, quasi-circular inspiral
stage up to the end of the sequence, which is the last stable orbit or the mass
shedding point, depending on which comes first. We find that, depending on the
ST parameters, the number of GW cycles can be substantially smaller than in GR.
In particular, we obtain that when dynamical scalarization sets in around a GW
frequency of ~130 Hz, the sole inclusion of the ST binding energy causes a
reduction of GW cycles from ~120 Hz up to the end of the sequence (~1200 Hz) of
~11% with respect to the GR case. We estimate that when the ST energy flux is
also included the reduction in GW cycles becomes of ~24%. Quite interestingly,
dynamical scalarization can produce a difference in the number of GW cycles
with respect to the GR point-particle case that is much larger than the effect
due to tidal interactions, which is on the order of only a few GW cycles. These
results further clarify and confirm recent studies that have evolved BNSs
either in full numerical relativity or in post-Newtonian theory, and point out
the importance of developing accurate ST-theory waveforms for systems composed
of strongly self-gravitating objects, such as BNSs.