We study the secular evolution and gravitational wave signature of a newly-formed, rapidly rotating neutron star. The neutron star may arise from core collapse in a massive star or from the accretion-induced collapse of a white dwarf. After a brief dynamical phase, the nascent neutron star settles into an axisymmetric, secularly unstable equilibrium state. Gravitational radiation drives the star to a nonaxisymmetric, stationary equilibrium configuration via the bar-mode instability. The emitted quasi-periodic gravitational waves have a unique signature: the wave frequency sweeps downward from a few hundred Hertz to zero, while the wave amplitude increases from zero to a maximum and then decays back to zero. Such a wave signal could be detected by broad-band gravitational wave interferometers currently being constructed. We also characterize two other types of gravitational wave signals that could arise in principle from a rapidly rotating, secularly unstable neutron star: a high-frequency ($f\go 1000$ Hz) wave which increases the pattern-speed of the star, and a wave that actually increases the angular momentum of the star.Comment: 20 pages, plain TeX, CRSR-107

We perform general relativistic, magnetohydrodynamic simulations of merging binary neutron stars incorporating neutrino transport and magnetic fields. Our new radiative transport module for neutrinos adopts a general relativistic, truncated-moment formalism. The binaries consist of two identical, irrotational stars modeled by the Skyrme Lyon (SLy) nuclear equation of state. They are initially in quasicircular orbit and threaded with a poloidal magnetic field that extends from the stellar interior into the exterior, as in typical pulsars. We insert neutrino processes shortly after the merger and focus on the role of neutrinos in launching a jet following the collapse of the hypermassive neutron star (HMNS) remnant to a spinning black hole (BH). We treat two microphysical versions: one (a “warm-up”) evolving a single neutrino species and considering only charged-current processes and the other evolving three species $({\nu }_e,{\overline{\nu }}_e,{\nu }_{\mathrm{x}})$ and related processes. We trace the evolution until the system reaches a quasiequilibrium state after BH formation. We find that the $\mathrm{BH}+\text{disk}$ remnant eventually launches an incipient jet. The electromagnetic Poynting luminosity is $\sim {10}^{53}\mathrm{erg}{\mathrm{s}}^{-1}$, consistent with that of typical short gamma-ray bursts. The effect of neutrino cooling shortens the lifetime of the HMNS and lowers the amplitude of the major peak of the gravitational wave power spectrum somewhat. After BH formation, neutrinos help clear out the matter near the BH poles, resulting in lower baryon-loaded surrounding debris. The neutrino luminosity resides in the range $\sim {10}^{52-53}\mathrm{erg}{\mathrm{s}}^{-1}$ once quasiequilibrium is achieved. Comparing with the neutrino-free models, we observe that the inclusion of neutrinos yields similar ejecta masses and is inefficient in carrying off additional angular momentum.

We revisit the role that gravitational scattering off stars plays in establishing the steady-state distribution of collisionless dark matter (DM) around a massive black hole (BH). This is a physically interesting problem that has potentially observable signatures, such as $\gamma$ rays from DM annihilation in a density spike. The system serves as a laboratory for comparing two different dynamical approaches, both of which have been widely used: a Fokker-Planck treatment and a two-component conduction fluid treatment. In our Fokker-Planck analysis we extend a previous analytic model to account for a nonzero flux of DM particles into the BH, as well as a cutoff in the distribution function near the BH due to relativistic effects or, further out, possible DM annihilation. In our two-fluid analysis, following an approximate analytic treatment, we recast the equations as a “heated Bondi accretion” problem and solve the equations numerically without approximation. While both the Fokker-Planck and two-fluid methods yield basically the same DM density and velocity dispersion profiles away from the boundaries in the spike interior, there are other differences, especially the determination of the DM accretion rate. We discuss limitations of the two treatments, including the assumption of an isotropic velocity dispersion.

Adopting our new method for matching general relativistic, ideal magnetohydrodynamics to its force-free limit, we perform the first systematic simulations of force-free pulsar magnetospheres in general relativity. We endow the neutron star with a general relativistic dipole magnetic field, model the interior with ideal magnetohydrodynamics, and adopt force-free electrodynamics in the exterior. Comparing the spin-down luminosity to its corresponding Minkowski value, we find that general relativistic effects give rise to a modest enhancement: the maximum enhancement for $n=1$ polytropes is $$\sim${}23%$. Evolving a rapidly rotating $n=0.5$ polytrope we find an even greater enhancement of $$\sim${}35%$. Using our simulation data, we derive fitting formulas for the pulsar spin-down luminosity as a function of the neutron star compaction, angular speed, and dipole magnetic moment. We expect stiffer equations of state and more rapidly spinning neutron stars to lead to even larger enhancements in the spin-down luminosity.

We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of binary neutron stars in quasi-circular orbit that merge and undergo delayed or prompt collapse to a black hole (BH). The stars are irrotational and modeled using an SLy or an H4 nuclear equation of state. To assess the impact of the initial magnetic field configuration on jet launching, we endow the stars with a purely poloidal magnetic field that is initially unimportant dynamically and is either confined to the stellar interior or extends from the interior into the exterior as in typical pulsars. Consistent with our previous results, we find that only the $\mathrm{BH}+\mathrm{disk}$ remnants originating from binaries that form hypermassive neutron stars (HMNSs) and undergo delayed collapse can drive magnetically powered jets. We find that the closer the total mass of the binary is to the threshold value for prompt collapse, the shorter is the time delay between the gravitational wave peak amplitude and jet launching. This time delay also strongly depends on the initial magnetic field configuration. We also find that seed magnetic fields confined to the stellar interior can launch a jet over $\sim 25\mathrm{ms}$ later than those with pulsarlike magnetic fields. The lifetime of the jet [$\mathrm{\Delta }t\lesssim 150\mathrm{ms}$] and its outgoing Poynting luminosity [$L_{\mathrm{EM}}\sim {10}^{52\pm 1}\mathrm{erg}/\mathrm{s}$] are consistent with typical short gamma-ray burst central engine lifetimes, as well as with the Blandford–Znajek mechanism for launching jets and their associated Poynting luminosities. Our numerical results also suggest that the dynamical ejection of matter can be enhanced by the magnetic field. Therefore, GRMHD studies are required to fully understand kilonova signals from GW170818-like events.

Black hole-neutron star (BHNS) mergers are thought to be sources of gravitational waves (GWs) with coincident electromagnetic (EM) counterparts. To further probe whether these systems are viable progenitors of short gamma-ray bursts (SGRBs) and kilonovas, and how one may use (the lack of) EM counterparts associated with LIGO/Virgo candidate BHNS GW events to sharpen parameter estimation, we study the impact of neutron star spin in BHNS mergers. Using dynamical spacetime magnetohydrodynamic simulations of BHNSs initially on a quasicircular orbit, we survey configurations that differ in the BH spin ($a_{\mathrm{BH}}/M_{\mathrm{BH}}=0$ and 0.75), the NS spin ($a_{\mathrm{NS}}/M_{\mathrm{NS}}=-0.17$, 0, 0.23, and 0.33), and the binary mass ratio ($q\equiv M_{\mathrm{BH}}:M_{\mathrm{NS}}=3:1$ and $5:1$). The general trend we find is that increasing the NS prograde spin increases both the rest mass of the accretion disk onto the remnant black hole, and the rest mass of dynamically ejected matter. By a time $\mathrm{\Delta }t\sim 3500–5500M\sim 88–138(M_{\mathrm{NS}}/1.4M_{\odot })\mathrm{ms}$ after the peak gravitational-wave amplitude, a magnetically driven jet is launched only for $q=3:1$ regardless of the initial NS spin. The lifetime of the jets [$\mathrm{\Delta }t\sim 0.5–0.8(M_{\mathrm{NS}}/1.4M_{\odot })\mathrm{s}$] and their outgoing Poynting luminosity [$L_{\text{Poyn}}\sim {10}^{51.5\pm 0.5}\mathrm{erg}/\mathrm{s}$] are consistent with typical SGRBs’ luminosities and expectations from the Blandford-Znajek mechanism. By the time we terminate our simulations, we do not observe either an outflow or a large-scale magnetic-field collimation for the other systems we consider. The mass range of dynamically ejected matter is ${10}^{-4.5}–{10}^{-2}(M_{\mathrm{NS}}/1.4M_{\odot })M_{\odot }$, which can power kilonovas with peak bolometric luminosities $L_{\text{knova}}\sim {10}^{40}–{10}^{41.4}\mathrm{erg}/\mathrm{s}$ with rise times $\lesssim 6.5\mathrm{h}$ and potentially detectable by the LSST.