Search

The oscillation modes of neutron star (NS) merger remnants, as encoded by the kHz postmerger gravitational wave (GW) signal, hold great potential for constraining the as-yet undetermined equation of state (EOS) of dense nuclear matter. Previous works have used numerical relativity simulations to derive quasi-universal relations for the key oscillation frequencies, but most of them omit the effects of a magnetic field. We conduct full general-relativistic magnetohydrodynamics simulations of NSNS mergers with two different masses and two different EOSs (SLy and ALF2) with three different initial magnetic field topologies (poloidal and toroidal only, confined to the interior, and "pulsar-like": dipolar poloidal extending from the interior to the exterior), with four different initial magnetic field strengths. We find that the magnetic braking and magnetic effective turbulent viscosity drives the merger remnants towards uniform rotation and increases their overall angular momentum loss. This causes the remnant to contract and the angular velocity of the quadrupole density oscillation to increase in such a way that it rotates faster than the fluid itself, in stark contrast with nonmagnetized simulations. As a result, the $f_2$ frequency of the dominant postmerger GW mode shifts upwards over time. The overall shift is up to ~ 200Hz for the strongest magnetic field we consider and ~ 50Hz for the median case and is therefore detectable in principle by future GW observatories, which should include the magnetic field in their analyses., Comment: 33 pages, 22 figures

Recent nonmagnetized studies of binary neutron star mergers have indicated the possibility of identifying equation of state features, such as a phase transition or a quark-hadron crossover, based on the frequency shift of the main peak in the postmerger gravitational wave spectrum. By performing a series of general relativistic, magnetohydrodynamic simulations we show that similar frequency shifts can be obtained due to the effect of the magnetic field. The existing degeneracy can either mask or nullify a shift due to a specific equation of state feature, and therefore the interpretation of observational data is more complicated than previously thought, requiring a more complete treatment that would necessarily include the neutron star's magnetic field., Comment: 6 pages, 4 figures

Lacking terrestrial experimental data, our best constraints on the behavior of matter at high densities up to and above nuclear density arise from observations of neutron stars. Current constraints include those based on measurements of stellar masses, radii, and tidal deformabilities. Here we explore how orbits of primordial black holes - should they exist - inside neutron stars could provide complementary constraints on the nuclear equation of state (EOS). Specifically, we consider a sample of candidate EOSs, construct neutron star models for these EOSs, and compute orbits of primordial black holes inside these stars. We discuss how the pericenter advance of eccentric orbits, i.e. orbital precession, results in beat phenomena in the emitted gravitational wave signal. Observing this beat frequency could constrain the nuclear EOS and break possible degeneracies arising from other constraints, as well as provide information about the host star., Comment: 12 pages, 9 figures; PRD in press

We present self-consistent numerical simulations in general relativity of putative primordial black holes inside neutron stars. Complementing a companion paper in which we assumed the black hole mass $m$ to be much smaller than the mass $M_*$ of the neutron star, thereby justifying a point-mass treatment, we here consider black holes with masses large enough so that their effect on the neutron star cannot be neglected. We develop and employ several new numerical techniques, including initial data describing boosted black holes in neutron-star spacetimes, a relativistic determination of the escape speed, and a gauge condition that keeps the black hole hole at a fixed coordinate location. We then perform numerical simulations that highlight different aspects of the capture of primordial black holes by neutron stars. In particular, we simulate the initial passage of the black hole through the star, demonstrating that the neutron star remains dynamically stable provided the black-hole mass is sufficiently small, $m \lesssim 0.05 M_*$. We also model the late evolution of a black hole oscillating about the center of an initially stable neutron star while accreting stellar mass and ultimately triggering gravitational collapse., Comment: 16 pages, 9 figures, animations at https://www.youtube.com/playlist?list=PLe7W1aFCp-Ocfs00Q3Z0fvkuULlznLi1u

GW170817 and GRB 170817A provided direct evidence that binary neutron star (NSNS) mergers can produce short gamma-ray bursts (sGRBs). However, questions remain about the nature of the central engine. Depending on the mass, the remnant from a NSNS merger may promptly collapse to a black hole (BH), form a hypermassive neutron star (HMNS) which undergoes a delayed collapse to a BH, a supramassive neutron star (SMNS) with a much longer lifetime, or an indefinitely stable NS. There is strong evidence that a BH with an accretion disk can launch a sGRB-compatible jet via the Blandford-Znajek mechanism, but whether a supramassive star can do the same is less clear. We have performed general relativistic magnetohydrodynamics simulations of the merger of both irrotational and spinning, equal-mass NSNSs constructed from a piecewise polytropic representation of the SLy equation of state, with a range of gravitational masses that yield remnants with mass above and below the supramassive limit. Each NS is endowed with a dipolar magnetic field extending from the interior into the exterior, as in a radio pulsar. We examine cases with different initial binary masses, including a case which produces a HMNS which collapses to a BH, and lower mass binaries that produce SMNS remnants. We find similar jetlike structures for both the SMNS and HMNS remnants that meet our basic critera for an incipient jet. The outflow for the HMNS case is consistent with a Blandford-Znajek (BZ) jet. There is sufficient evidence that such BZ-powered outflows can break out and produce ulrarelativistic jets so that we can describe the HMNS system as a sGRB progenitor. However, the incipient jets from the SMNS remnants have much more baryon pollution and we see indications of inefficient acceleration and mixing with the surrounding debris. Therefore, we cannot conclude that SMNS outflows are the progenitors of sGRBs., Comment: 25 pages, 15 figures

Primordial black holes (PBHs), if they exist, may collide with and be captured by neutron stars. We adopt a relativistic point-mass approximation to study this capture, the subsequent confinement of the PBH of mass $m$ inside the neutron star of mass $M_* \gg m$, and the PBH's growth by accretion of stellar material. Building on earlier treatments we systematically study the capture, confinement, and accretion process, characterize the emitted quasiperiodic continuous gravitational-wave signal, track the evolution of the PBH's orbital parameters, and compare the effects of different choices for the prescription of the dissipative forces. Our point-mass treatment here is applicable in the limit of small PBH masses, for which its effects on the neutron star can be ignored., Comment: 23 pages, 13 figures; matches version published in PRD

Long-period radio transients have unusual properties that challenge their interpretation as pulsars or magnetars. We examine whether they might instead be powered by primordial black holes (PBHs) making repeated passages through a host star, thereby providing a signature of elusive dark-matter candidates. We demonstrate that constraints derived from the transients' period and period derivative alone already rule out this scenario for most potential host stars. While white dwarfs may satisfy these constraints, they are unlikely to capture PBHs in the required mass range., Comment: 5 pages, 1 figure, PRD in press

Primordial black holes (PBHs), if captured by neutron stars (NSs), would emit a characteristic gravitational wave (GW) signal as they orbit inside the host star. We identify a specific and qualitatively new feature of these signals, namely quasi-periodic beats caused by the precession of noncircular PBH orbits. We demonstrate numerically and analytically that the beat frequency depends rather sensitively on the NS structure, so that hypothetical future observations with next-generation GW detectors would provide valuable constraints on the nuclear equation of state., Comment: 13 pages (including 6 pages of supplemental material), 1 figure

LISA, the Laser Interferometer Space Antenna, will usher in a new era in gravitational-wave astronomy. As the first anticipated space-based gravitational-wave detector, it will expand our view to the millihertz gravitational-wave sky, where a spectacular variety of interesting new sources abound: from millions of ultra-compact binaries in our Galaxy, to mergers of massive black holes at cosmological distances; from the beginnings of inspirals that will venture into the ground-based detectors' view to the death spiral of compact objects into massive black holes, and many sources in between. Central to realising LISA's discovery potential are waveform models, the theoretical and phenomenological predictions of the pattern of gravitational waves that these sources emit. This white paper is presented on behalf of the Waveform Working Group for the LISA Consortium. It provides a review of the current state of waveform models for LISA sources, and describes the significant challenges that must yet be overcome., Comment: 239 pages, 11 figures, white paper from the LISA Consortium Waveform Working Group, invited for submission to Living Reviews in Relativity, updated with comments from community

We consider the steady-state density and velocity dispersion profiles of collisionless matter around a Schwarzschild black hole (BH) and its associated rate of accretion onto the BH. We treat matter, which could be stars or dark matter particles, whose orbits are {\it unbound} to the BH, but still governed by its gravitational field. We consider two opposite spatial geometries for the matter distributions: an infinite, 3D cluster and a 2D razor-thin disk, both with zero net angular momentum. We demonstrate that the results depend critically on the adopted geometry, even in the absence of angular momentum. We adopt a simple monoenergetic, isotropic, phase-space distribution function for the matter as a convenient example to illustrate this dependence. The effect of breaking strict isotropy by incorporating an unreplenished loss cone due to BH capture of low-angular momentum matter is also considered. Calculations are all analytic and performed in full general relativity, though key results are also evaluated in the Newtonian limit. We consider one application to show that the rate of BH accretion from an ambient cluster can be significantly less than that from a thin disk to which it may collapse, although both rates are considerably smaller than Bondi accretion for a (collisional) fluid with a similar asymptotic particle density and velocity dispersion (i.e., sound speed)., Comment: 10 pages, no figs, to be published in Phys. Rev. D

We explore the effect magnetic fields have on self-gravitating accretion disks around spinning black holes via numerical evolutions in full dynamical magnetohydrodynamic spacetimes. The configurations we study are unstable to the Papaloizou-Pringle Instability (PPI). PPI-saturated accretion tori have been shown to produce gravitational waves, detectable to cosmological distances by third-generation gravitational wave (GW) observatories. While the PPI operates strongly for purely hydrodynamic disks, the situation can be different for disks hosting initially small magnetic fields. Evolutions of disks without self-gravity in fixed BH spacetimes have shown that small seed fields can initiate the rapid growth of the magneto-rotational instability (MRI), which then strongly suppresses the PPI. Since realistic astrophysical disks are expected to be magnetized, PPI-generated GW signals may be suppressed as well. However, it is unclear what happens when the disk self-gravity is restored. Here, we study the impact of magnetic fields on the PPI-saturated state of a self-gravitating accretion disk around a spinning BH ($\chi = 0.7$) aligned with the disk angular momentum, as well as one around a non-spinning BH. We find the MRI is effective at reducing the amplitude of PPI modes and their associated GWs, but the systems still generate GWs. Estimating the detectability of these systems accross a wide range of masses, we show that magnetic fields reduce the maximum detection distance by Cosmic Explorer from 300Mpc (in the pure hydrodynamic case) to 45Mpc for a $10 M_{\odot}$ system, by LISA from 11500Mpc to 2700Mpc for a $2 \times 10^{5} M_{\odot}$ system, and by DECIGO from $z \approx 5$ down to $z \approx 2$ for a $1000 M_{\odot}$ system.

This review aims at providing an extensive discussion of modern constraints relevant for dense and hot strongly interacting matter. It includes theoretical first-principle results from lattice and perturbative QCD, as well as chiral effective field theory results. From the experimental side, it includes heavy-ion collision and low-energy nuclear physics results, as well as observations from neutron stars and their mergers. The validity of different constraints, concerning specific conditions and ranges of applicability, is also provided.

We perform general relativistic simulations of magnetized, accreting disks onto spinning binary black holes (BHBHs) with different mass ratios (MRs). The magnitude of the individual BH spins are all $\chi= 0.26$ and lie either along the initial orbital plane or $45^\circ$ above it. We evolve these systems throughout the inspiral, merger and postmerger phases to identify the impact of the BH spins and the MR on any jet and their electromagnetic (EM) signatures. We find that incipient jets are launched from both BHs regardless of the MR and along the spin directions as long as the force-free parameter $B^2/(8\,\pi\rho_0)$ in the funnel and above their poles is larger than one. At large distances the two jets merge into a single one what may prevent the EM detection of individual jets. As the accretion rate reaches a quasistationary state during predecoupling, we observe a sudden amplification of the outgoing Poynting luminosity that depends on the MR. Following merger, the sudden change in the direction of the spin of the BH remnant with respect to the spins of its progenitors causes a reorientation of the jet, which drives a single, high-velocity, outward narrow beam collimated by a tightly wound, helical Bfield which, in turn, boosts the Poynting luminosity. This effect is nearly MR independent. During this process, a kink is produced in the Bfield lines, confining the jet. The kink propagates along the jet but rapidly decays, leaving no memory of the spin-shift. Our results suggest that the merger of misaligned, low-spinning, BHBH mergers in low-mass disks may not provide a viable scenario to explain X-shaped radio galaxies if other features are not taken into account. However, the sudden changes in the outgoing luminosity at merger may help to identify mergers of BHs in active galactic nuclei, shedding light on BH growth mechanisms and the observed co-evolution of their host galaxies., Comment: 14 pages, 7 figures

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 cut-off 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., Comment: 12 pages, 6 figures

We perform general relativistic simulations of self-gravitating black hole-disks in which the spin of the black hole is significantly tilted ($45^\circ$ and $90^\circ$) with respect to the angular momentum of the disk and the disk-to-black hole mass ratio is $16\%-28\%$. The black holes are rapidly spinning with dimensionless spins up to $\sim 0.97$. These are the first self-consistent hydrodynamic simulations of such systems, which can be prime sources for multimessenger astronomy. In particular tilted black hole-disk systems lead to: i) black hole precession; ii) disk precession and warping around the black hole; iii) earlier saturation of the Papaloizou-Pringle instability compared to aligned/antialigned systems, although with a shorter mode growth timescale; iv) acquisition of a small black-hole kick velocity; v) significant gravitational wave emission via various modes beyond, but as strong as, the typical $(2,2)$ mode; and vi) the possibility of a broad alignment of the angular momentum of the disk with the black hole spin. This alignment is not related to the Bardeen-Petterson effect and resembles a solid body rotation. Our simulations suggest that any electromagnetic luminosity from our models may power relativistic jets, such as those characterizing short gamma-ray bursts. Depending on the black hole-disk system scale the gravitational waves may be detected by LIGO/Virgo, LISA and/or other laser interferometers.

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of GWs can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas., Comment: Accepted in: Living Reviews in Relativity

The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultracompact stellar-mass binaries, massive black hole binaries, and extreme or intermediate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

We perform general relativistic, magnetohydrodynamic (GRMHD) 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 (M1) formalism. The binaries consist of two identical, irrotational stars modeled by the SLy nuclear equation of state (EOS). 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 $(\nu_e, \bar{\nu}_e, \nu_{\rm x})$ and related processes. We trace the evolution until the system reaches a quasiequilibrium state after BH formation. We find that the BH + disk remnant eventually launches an incipient jet. The electromagnetic Poynting luminosity is $\sim 10^{53} \rm \, erg\, s^{-1}$, consistent with that of typical short gamma-ray bursts (sGRBs). The effect of neutrino cooling shortens the lifetime of the HMNS, and lowers the amplitude of the major peak of the gravitational wave (GW) 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 $\sim 10^{52-53} \rm \,erg\,s^{-1}$ 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., Comment: 32 page, 16 figures

We assess whether gravity darkening, induced by a tidal interaction during a stellar fly-by, might be sufficient to explain the Great Dimming of Betelgeuse. Adopting several simple approximations, we calculate the tidal deformation and associated gravity darkening in a close tidal encounter, as well as the reduction in the radiation flux as seen by a distant observer. We show that, in principle, the duration and degree of the resulting stellar dimming can be used to estimate the minimum pericenter separation and mass of a fly-by object, which, even if it remains undetected otherwise, might be a black hole, neutron star, or white dwarf. Our estimates show that, while such fly-by events may occur in other astrophysical scenarios, where our analysis should be applicable, they likely are not large enough to explain the Great Dimming of Betelgeuse by themselves., Comment: 7 pages, 4 figures; version published in MNRAS

We perform the first magnetohydrodynamic simulations in full general relativity of self-consistent rotating neutron stars (NSs) with ultrastrong mixed poloidal and toroidal magnetic fields. The initial uniformly rotating NS models are computed assuming perfect conductivity, stationarity, and axisymmetry. Although the specific geometry of the mixed field configuration can delay or accelerate the development of various instabilities known from analytic perturbative studies, all our models finally succumb to them. Differential rotation is developed spontaneously in the cores of our magnetars which, after sufficient time, is converted back to uniform rotation. The rapidly rotating magnetars show a significant amount of ejecta, which can be responsible for transient kilonova signatures. However no highly collimated, helical magnetic fields or incipient jets, which are necessary for gamma-ray bursts, arise at the poles of these magnetars by the time our simulations are terminated., Comment: 8 pages

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 BH + 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 $\sim 25\,\rm ms$ later than those with pulsar-like magnetic fields. The lifetime of the jet [$\Delta t\lesssim 150\,\rm ms$] and its outgoing Poynting luminosity [$L_{\rm EM}\sim 10^{52\pm 1}\rm erg/s$] 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., Comment: 21 pages, 12 figures. Matches published version

We consider spherically symmetric accretion onto a small, possibly primordial, black hole residing at the center of a neutron star governed by a cold nuclear equation of state (EOS). We generalize the relativistic Bondi solution for such EOSs, approximated by piecewise polytropes, and thereby obtain analytical expressions for the steady-state matter profiles and accretion rates. We compare these rates with those found by time-dependent, general relativistic hydrodynamical simulations upon relaxation and find excellent agreement. We consider several different candidate EOSs, neutron star masses and central densities and find that the accretion rates vary only little, resulting in an accretion rate that depends primarily on the black hole mass, and only weakly on the properties of the neutron star., Comment: 10 pages, 3 figures; version accepted for publication in Phys. Rev. D

We revisit the system consisting of a neutron star that harbors a small, possibly primordial, black hole at its center, focusing on a nonspinning black hole embedded in a nonrotating neutron star. Extending earlier treatments, we provide an analytical treatment describing the rate of secular accretion of the neutron star matter onto the black hole, adopting the relativistic Bondi accretion formalism for stiff equations of state that we presented elsewhere. We use these accretion rates to sketch the evolution of the system analytically until the neutron star is completely consumed. We also perform numerical simulations in full general relativity for black holes with masses up to nine orders of magnitude smaller than the neutron star mass, including a simulation of the entire evolution through collapse for the largest black hole mass. We construct relativistic initial data for these simulations by generalizing the black hole puncture method to allow for the presence of matter, and evolve these data with a code that is optimally designed to resolve the vastly different length scales present in this problem. We compare our analytic and numerical results, and provide expressions for the lifetime of neutron stars harboring such endoparasitic black holes., Comment: 20 pages, 9 figures; version published in PRD

Neutron stars (NSs) are extraordinary not only because they are the densest form of matter in the visible Universe but also because they can generate B-fields ten orders of magnitude larger than those currently constructed on Earth. The combination of extreme gravity with the enormous electromagnetic (EM) fields gives rise to spectacular phenomena like those observed on August 2017 with the merger of a binary neutron star (NSNS) system, an event that generated a gravitational wave (GW) signal, a short $\gamma$-ray burst (sGRB), and a kilonova. This event serves as the highlight so far of the era of multimessenger astronomy. In this review, we present the current state of our theoretical understanding of compact binary mergers containing NSs as gleaned from the latest general relativistic magnetohydrodynamic simulations. Such mergers can lead to events like the one on August 2017, GW170817, and its EM counterparts, GRB 170817 and AT 2017gfo. In addition to exploring the GW emission from binary black hole-neutron star and NSNS mergers, we also focus on their counterpart EM signals. In particular, we are interested in identifying the conditions under which a relativistic jet can be launched following these mergers. Such a jet is an essential feature of most sGRB models and provides the main conduit of energy from the central object to the outer radiation regions. Jet properties, including their lifetimes and Poynting luminosities, the effects of the initial B-field geometries and spins of the coalescing NSs, as well as their governing equation of state, are discussed. Lastly, we present our current understanding of how the Blandford-Znajek mechanism arises from merger remnants as the trigger for launching jets, if, when and how a horizon is necessary for this mechanism, and the possibility that it can turn on in magnetized neutron ergostars, which contain ergoregions, but no horizons., Comment: Invited review for the Research Topic: "Neutron Star Physics in the Multi-Messenger Discourse", Frontiers Astronomy and Space Sciences. 22pages, 12 figures. Matches published version

We explore in general relativity the survival time of neutron stars that host an endoparasitic, possibly primordial, black hole at their center. Corresponding to the minimum steady-state Bondi accretion rate for adiabatic flow that we found earlier for stiff nuclear equations of state (EOSs), we derive analytically the maximum survival time after which the entire star will be consumed by the black hole. We also show that this maximum survival time depends only weakly on the stiffness for polytropic EOSs with $\Gamma \geq 5/3$, so that this survival time assumes a nearly universal value that depends on the initial black hole mass alone. Establishing such a value is important for constraining the contribution of primordial black holes in the mass range $10^{-16} M_\odot \lesssim M \lesssim 10^{-10} M_\odot$ to the dark-matter content of the Universe., Comment: 5 pages

We revisit Bondi accretion - steady-state, adiabatic, spherical gas flow onto a Schwarzschild black hole at rest in an asymptotically homogeneous medium - for stiff polytropic equations of state (EOSs) with adiabatic indices $\Gamma > 5/3$. A general relativistic treatment is required to determine their accretion rates, for which we provide exact expressions. We discuss several qualitative differences between results for soft and stiff EOSs - including the appearance of a minimum steady-state accretion rate for EOSs with $\Gamma \geq 5/3$ - and explore limiting cases in order to examine these differences. As an example we highlight results for $\Gamma = 2$, which is often used in numerical simulations to model the EOS of neutron stars. We also discuss a special case with this index, the ultra-relativistic `causal' EOS, $P = \rho$. The latter serves as a useful limit for the still undetermined neutron-star EOS above nuclear density. The results are useful, for example, to estimate the accretion rate onto a mini-black hole residing at the center of a neutron star., Comment: 9 pages, 8 figures; accepted for publication in the Monthly Notices of the Royal Astronomical Society; corrected mistake in equation (43)

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 kilonovae, 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 ($a_{\rm BH}/M_{\rm BH}=0$ and $0.75$), the NS spin ($a_{\rm NS}/M_{\rm NS}=-0.17,\,0,\,0.23$ and $0.33$), and the binary mass ratio ($q\equiv M_{\rm BH}:M_{\rm NS}=3:1$ 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~$\Delta t\sim 3500-5500M\sim 88-138(M_{\rm NS}/1.4M_\odot)\,\rm ms$ 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 [$\Delta t\sim 0.5-0.8(M_{\rm NS}/1.4 M_\odot)\,\rm s$] and their outgoing Poynting luminosity [$L_{\rm Poyn}\sim 10^{51.5\pm 0.5}\,\rm erg/s$] 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 considered. The mass range of dynamically ejected matter is $10^{-4.5}-10^{-2}~(M_{\rm NS}/1.4M_\odot)M_\odot$, which can power kilonovae with peak bolometric luminosities $L_{\rm knova}\sim 10^{40}-10^{41.4}$ erg/s with rise times $\lesssim 6.5\,\rm h$ and potentially detectable by the LSST., Comment: 15 pages, 10 figures. Matches published version

We present fully general-relativistic numerical evolutions of self-gravitating tori around spinning black holes with dimensionless spin $a/M = 0.7$ parallel or anti-parallel to the disk angular momentum. The initial disks are unstable to the hydrodynamic Papaloizou-Pringle Instability which causes them to grow persistent orbiting matter clumps. The effect of black hole spin on the growth and saturation of the instability is assessed. We find that the instability behaves similarly to prior simulations with non-spinning black holes, with a shift in frequency due to spin-induced changes in disk orbital period. Copious gravitational waves are generated by these systems, and we analyze their detectability by current and future gravitational wave observatories for large range of masses. We find that systems of $10 M_\odot$ - relevant for black hole-neutron star mergers - are detectable by Cosmic Explorer out to $\sim300$ Mpc, while DECIGO (LISA) will be able to detect systems of $1000 M_\odot$ ($10^5M_\odot$) - relevant for disks forming in collapsing supermassive stars - out to cosmological redshift of $z\sim5$ ($z\sim 1$). Computing the accretion rate of these systems we find that these systems may also be promising sources of coincident electromagnetic signals., Comment: 15 pages, 12 figures

We perform the first fully general relativistic, magnetohydrodynamic simulations of dynamically stable hypermassive neutron stars with and without ergoregions to assess the impact of ergoregions on launching magnetically--driven outflows. The hypermassive neutron stars are modeled by a compressible and causal equation of state and are initially endowed with a dipolar magnetic field extending from the stellar interior into its exterior. We find that, after a few Alfv\'en times, magnetic field lines in the ergostar (star that contains ergoregions) and the normal star have been tightly wound in both cases into a helical funnel within which matter begins to flow outward. The maximum Lorentz factor in the outflow is $\Gamma_L \sim 2.5$, while the force-free parameter holds at $B^2/8\pi\rho_0\lesssim 10$. These values are incompatible with highly relativistic, magnetically-driven outflows (jets) and short $\gamma$-ray bursts. We compare these results with those of a spinning black hole surrounded by a magnetized, massless accretion disk that launches a bona fide jet. Our simulations suggest that the Blandford-Znajek mechanism for launching relativistic jets only operates when a black hole is present, though the Poynting luminosity in all cases is comparable. Therefore, one cannot distinguish a magnetized, accreting black hole from a magnetized hypermassive neutron star in the so-called mass-gap based solely on the value of the observed Poynting luminosity. These results complement our previous studies of supramassive remnants and suggest that it would be challenging for either normal neutron stars or ergostars in a hypermassive state to be the progenitors of short $\gamma$-ray bursts., Comment: 7 pages, 3 figures. Matches published version

We adopt a two-moment formalism, together with a reference-metric approach, to express the equations of relativistic radiation hydrodynamics in a form that is well-suited for numerical implementations in curvilinear coordinates. We illustrate the approach by employing a gray opacity in an optically thick medium. As numerical demonstrations we present results for two test problems, namely stationary, slab-symmetric solutions in flat spacetimes, including shocks, and heated Oppenheimer-Snyder collapse to a black hole. For the latter, we carefully analyze the transition from an initial transient to a post-transient phase that is well described by an analytically-known diffusion solution. We discuss the properties of the numerical solution when rendered in moving-puncture coordinates., Comment: 15 pages, 7 figures

The recent discovery by LIGO/Virgo of a merging binary having a $\sim 23 M_\odot$ black hole and a $\sim 2.6 M_\odot$ compact companion has triggered a debate regarding the nature of the secondary, which falls into the so-called mass gap. Here we explore some consequences of the assumption that the secondary was a neutron star (NS). We show with concrete examples of heretofore viable equations of state (EOSs) that rapid uniform rotation may neither be necessary for some EOSs nor sufficient for others to explain the presence of a NS. Absolute upper limits for the maximum mass of a spherical NS derived from GW170817 already suggest that this unknown compact companion might be a slowly or even a nonrotating NS. However several soft NS EOSs favored by GW170817 with maximum spherical masses $\lesssim 2.1 M_\odot$ cannot be invoked to explain this object, even allowing for maximum uniform rotation. By contrast, sufficiently stiff EOSs that yield $2.6 M_\odot$ NSs which are slowly rotating or, in some cases, nonrotating, and are compatible with GW170817 and the results of NICER, can account for the black hole companion., Comment: 7 pages, 2 figures. Matches published version