Black Hole and Neutron Star Mergers in Galactic Nuclei. (arXiv:1811.10627v1 [astro-ph.GA])

Black Hole and Neutron Star Mergers in Galactic Nuclei. (arXiv:1811.10627v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fragione_G/0/1/0/all/0/1">Giacomo Fragione</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Grishin_E/0/1/0/all/0/1">Evgeni Grishin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leigh_N/0/1/0/all/0/1">Nathan W. C. Leigh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Perets_H/0/1/0/all/0/1">Hagai. B. Perets</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Perna_R/0/1/0/all/0/1">Rosalba Perna</a>

Nuclear star clusters surrounding supermassive black holes (SMBHs) in
galactic nuclei contain large numbers of stars, black holes (BHs) and neutron
stars (NSs), a fraction of which are likely to form binaries. These binaries
were suggested to form a triple system with the SMBH, which acts as a perturber
and may enhance BH and NS mergers via the Lidov-Kozai mechanism. We follow-up
previous studies, but for the first time perform an extensive statistical study
of BH-BH, NS-NS and BH-NS binary mergers by means of direct high-precision
regularized $N$-body simulations, including Post-Newtonian (PN) terms up to
order PN2.5. We consider different SMBH masses, slopes for the BH mass
function, binary semi-major axis and eccentricity distributions, and different
spatial distributions for the binaries. We find that the merger rates are a
decreasing function of the SMBH mass and are in the ranges $sim 0.17$-$0.52
mathrm{Gpc}^{-3} mathrm{yr}^{-1}$, $sim 0.06$-$0.10 mathrm{Gpc}^{-3}
mathrm{yr}^{-1}$ and $sim 0.04$-$0.16 mathrm{Gpc}^{-3} mathrm{yr}^{-1}$
for BH-BH, BH-NS and NS-NS binaries, respectively. These rates are typically
smaller than previous studies, where the supply rates of BHs and NSs were
probably overestimated. Most of the mergers enter the LIGO band with very high
eccentricities, approaching almost unity. We also compare our results to the
secular approximation, and show that $N$-body simulations generally predict a
larger number of mergers. Finally, these events can also be observable via
their electromagnetic counterparts, thus making these compact object mergers
especially valuable for cosmological and astrophysical purposes.

Nuclear star clusters surrounding supermassive black holes (SMBHs) in
galactic nuclei contain large numbers of stars, black holes (BHs) and neutron
stars (NSs), a fraction of which are likely to form binaries. These binaries
were suggested to form a triple system with the SMBH, which acts as a perturber
and may enhance BH and NS mergers via the Lidov-Kozai mechanism. We follow-up
previous studies, but for the first time perform an extensive statistical study
of BH-BH, NS-NS and BH-NS binary mergers by means of direct high-precision
regularized $N$-body simulations, including Post-Newtonian (PN) terms up to
order PN2.5. We consider different SMBH masses, slopes for the BH mass
function, binary semi-major axis and eccentricity distributions, and different
spatial distributions for the binaries. We find that the merger rates are a
decreasing function of the SMBH mass and are in the ranges $sim 0.17$-$0.52
mathrm{Gpc}^{-3} mathrm{yr}^{-1}$, $sim 0.06$-$0.10 mathrm{Gpc}^{-3}
mathrm{yr}^{-1}$ and $sim 0.04$-$0.16 mathrm{Gpc}^{-3} mathrm{yr}^{-1}$
for BH-BH, BH-NS and NS-NS binaries, respectively. These rates are typically
smaller than previous studies, where the supply rates of BHs and NSs were
probably overestimated. Most of the mergers enter the LIGO band with very high
eccentricities, approaching almost unity. We also compare our results to the
secular approximation, and show that $N$-body simulations generally predict a
larger number of mergers. Finally, these events can also be observable via
their electromagnetic counterparts, thus making these compact object mergers
especially valuable for cosmological and astrophysical purposes.

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