arXiv subjects

Stuart L. Shapiro

Publications and source records attributed to Stuart L. Shapiro.

At least 19 recordsLinked to original sources

arXiv2026

Multimessenger Signatures of Tilted, Self-Gravitating, Black Hole Disks

We perform fully relativistic GRMHD simulations of magnetized, self-gravitating black hole-disk (BHD) systems in which the black hole spin is misaligned with the disk angular momentum. Massive disks (disk to BH mass ratios of $16-28\%$) around rapidly rotating black holes ($\chi\lesssim 0.97$) develop a nonaxisymmetric instability for tilt angles from $0^\circ$ to $180^\circ$. Magnetic stresses damp, but do not completely suppress, the nonaxisymmetric instability, and corresponding gravitational wave (GW) emission, in aligned systems, while they enhance it in antialigned BHDs: MRI-driven turbulence enhances angular momentum transport and accelerates nonlinear instability evolution in misaligned configurations. All models launch magnetically driven jets consistent with the Blandford-Znajek (BZ) mechanism, with collimation depending on spin orientation. The GWs reflect strong nonaxisymmetric structure from a persistent $m=1$ mode. The coupling between fast MRI and the slower nonaxisymmetric instability growth governs the outcome, with tilt controlling how MRI modifies the global mode. These simulations provide the first self-consistent GRMHD treatment of tilted, self-gravitating BHD systems and support their role as multimessenger sources.

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arXiv2026

Subsolar mass black holes from stellar collapse induced by primordial black holes

While no gravitational-wave detection of subsolar mass black holes has been confirmed to date, a number of candidate detections invite us to speculate on the origin of such black holes should a detection be confirmed. It is generally assumed that the observation of a black hole with subsolar mass $M_{\rm obs}$ would provide strong evidence for primordial black holes (PBHs). The mass $M_{\rm PBH}$ of the PBH, however, does not necessarily have to be equal to $M_{\rm obs}$, as it would in what we term a ``direct PBH scenario". Instead, a black hole of mass $M_{\rm obs}$ may form in a capture of a much smaller primordial black hole, $M_{\rm PBH} \ll M_{\rm obs}$, by a dwarf star of mass $M_* \simeq M_{\rm obs}$, followed by the total consumption of the star by the PBH. We provide some rough estimates and demonstrate that such an ``indirect PBH scenario" may also lead to significant populations of black holes with mass $M_{\rm obs}$, especially in dwarf galaxies, and may be able to explain rare subsolar mass events.

astro-ph.HE↗

arXiv2025

Impact of Magnetic Field Topology on Electromagnetic and Gravitational Waves from Binary Neutron Star Merger Remnants

We perform general relativistic magnetohydrodynamic (GRMHD) simulations of binary neutron star (BNS) mergers with four distinct magnetic field topologies: (i) a dipole pulsar-like configuration, (ii) a mixed linear superposition of poloidal and toroidal components inside the star, and (iii-iv) two topologies featuring a smooth transition from a confined mixed core to a pulsar-like structure at radii $0.95\,R_{\rm NS}$ and $0.5\,R_{\rm NS}$, with $R_{\rm NS}$ the radius of the star. The latter topologies are explored in BNS merger studies for the first time. We evolve systems with two equations of state (EoS), SLy and WFF1, with ADM masses 2.7 and 2.6, respectively, and include an additional lower-mass SLy binary to probe the behavior of long-lived remnants. We perform an extensive analysis of the emission properties of the systems, both electromagnetic and gravitational waves, and of the properties of the remnants, namely their frequency modes, density eigenfunctions, rotation, temperature, and convective stability. We report three key results: (1) for the first time, we assess the convective stability of magnetized remnants, extending previous unmagnetized analyses; (2) we identify a clear secondary peak in the gravitational-wave spectrum of pulsar-like configurations, consistent with the nonlinear coupling of the $m=0$ and $m=2$ modes, which is absent in other topologies; and (3) the magnetic field topology strongly influences the gravitational wave emission properties to the extent that nearby ($<50\,{\rm Mpc}$) events could allow one to observationally distinguish between different field structures with future gravitational-wave detectors. Across all models, we obtain luminosities compatible with short gamma-ray bursts (sGRBs), with purely poloidal configurations being the most efficient in driving possible relativistic jets.

astro-ph.HE↗

arXiv2025

The Gravitational Wave Memory from Binary Neutron Star Mergers

The gravitational wave signal produced by the merger of two compact objects includes both an oscillatory transient and a non-oscillatory part, the so-called memory effect. This produces a permanent displacement of test masses and has not yet been measured. We use general relativistic magnetohydrodynamic simulations, including neutrinos, with several representative viable equations of state, to quantify--for the first time--the effects of the neutron star magnetic field, neutrino emission, and the ejected mass on the linear and nonlinear displacement memory in binary neutron star mergers. We find that the additional contributions due to the emission of electromagnetic radiation, neutrinos and baryonic ejecta can be ~15% of the total memory for moderate magnetic fields and up to ~50% for extreme magnetic fields. The memory is most affected by changes in the equation of state, the binary mass, and the magnetic field. In particular, for moderate premerger field strengths, the dominant impact of the electromagnetic field is the change in the gravitational wave luminosity, and the associated gravitational wave null memory, due to the unstable growth of the magnetic field and the resulting redistribution of angular momentum it induces in the remnant. While the direct electromagnetic contribution to the null memory is additive, the change in the gravitational wave null memory can--in some cases--result in the total memory being smaller than that from the corresponding nonmagnetized binary. Furthermore, in contrast to binary black hole mergers, the growth of the memory in binary neutron star mergers is extended due to the long emission timescale of electromagnetic fields, neutrinos, and ejecta. These results necessitate the consideration of the magnetic field, as well as the equation of state, for accurate parameter estimation in future analyses of gravitational wave memory data.

gr-qc↗

arXiv2025

Can "premature collapse" form black holes in the upper and lower mass gaps?

Observations of gravitational waves from binary black hole mergers, including the recent signals GW231123 and GW230529, have revealed multiple progenitor black holes in the so-called upper and lower mass gaps, respectively. It is generally assumed that massive stars cannot form black holes in the upper mass gap because pair instabilities in the late stage of stellar evolution disrupt the stars, whereas the lower mass gap refers to the gap between the maximum allowed neutron star mass and the smallest black hole mass expected to form in supernova explosions. Here we explore a "premature collapse" scenario in which upper mass gap stars collapse and form black holes before they reach the late stage of stellar evolution. The mechanism for triggering a premature collapse is the capture of a smaller black hole, possibly primordial in nature. A similar capture scenario can occur to produce black holes in the lower mass gap. At least for massive stars, typical stellar rotation rates would likely result in rapidly rotating black holes in such a scenario, naturally explaining the rapid spins inferred from GW231123. Even though our estimates hinge on several parameters with rather large uncertainties, they suggest that, at least in galactic disks, the likelihood of such a capture is small for stars in the upper mass gap, but may lead to a significant population of black holes in the lower mass gap and, in fact, even below the lower mass gap.

astro-ph.HE↗

arXiv2025

Dynamical Evolutions in Globular Clusters and Dwarf Galaxies: Conduction Fluid Simulations

We present a new two-fluid conduction scheme to simulate the evolution of an isolated, self-gravitating, equilibrium cluster of stars and collisionless dark matter on secular (gravothermal) timescales. We integrate the equations in Lagrangian coordinates via a second-order, semi-implicit algorithm, which is unconditionally stable when the mass of the lighter species is much less than that of the heavier species. The method can be straightforwardly generalized to handle a multi-species system with a population of stars or components beyond collisionless dark matter and stars. We apply the method to simulate the dynamical evolution of stellar-dark matter systems, exploring the consequences of mass segregation and gravothermal core collapse, and assessing those effects for observed globular clusters and dwarf galaxies in the Local Volume.

astro-ph.GA↗

arXiv2025

Evolution of a black hole cluster in full general relativity

We evolve for the first time in full general relativity a small, collisional N-body black hole cluster of arbitrary total mass M. The bound cluster is initially compact (radius R/M~10), stable, and consists of 25 equal-mass, nonspinning black holes. The dynamical interactions of compact objects in N-body clusters is of great interest for the formation of black holes in the upper mass gap as well as intermediate and supermassive black holes. These are potential sources of gravitational waves that may be detected by both current and future observatories. Unlike previous N-body Newtonian and post-Newtonian simulations, no "subgrid physics" is required to handle collisions and mergers. We can therefore confirm in full general relativity several predictions from these simulations and analytic estimates: the runaway growth of a large black hole via repeated mergers; spindown of the central black hole with increasing captures; the ejection of a black hole with a large asymptotic velocity due to a several-body interaction; and a regime where mergers occur primarily via direct collisions on highly eccentric orbits instead of quasicircular inspirals. We extract the gravitational wave signal and find it has several distinct features associated with the compact cluster regime. Our results suggest the signal is sufficiently loud that next generation observatories would likely be able to detect similar events across most of the observable universe. This work is a preliminary proof-of-principle study that we hope will open up a new arena for numerical relativity and the study of N-body compact systems.

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arXiv2025

Boosting the growth of intermediate-mass black holes: collisions with massive stars

We perform fully relativistic simulations of the head-on collisions between intermediate-mass black holes and very massive stars. Such collisions are expected to occur in dense stellar clusters and may play an important role in growing the mass of the seed black hole. For the cases considered here, for which the masses of the black holes and stars are comparable, the vast majority of the stellar material is accreted onto the black hole within a stellar dynamical timescale, as expected from analytical estimates, and leads to a rapid growth of the black hole. A small amount of mass, which is shock-heated in the wake of the black hole, is ejected from the collision and will contribute to the interstellar material in the cluster.

astro-ph.HE↗

arXiv2025

Extracting the Temperature Analytically In Hydrodynamics Simulations with Gas and Radiation Pressure

Numerical hydrodynamics simulations of gases dominated by ideal, nondegenerate matter pressure and thermal radiation pressure in equilibrium entail finding the temperature as part of the evolution. Since the temperature is not typically a variable that is evolved independently, it must be extracted from the the evolved variables (e.g. the rest-mass density and specific internal energy). This extraction requires solving a quartic equation, which, in many applications, is done numerically using an iterative root-finding method. Here we show instead how the equation can be solved analytically and provide explicit expressions for the solution. We also derive Taylor expansions in limiting regimes and discuss the respective advantages and disadvantages of the iterative versus analytic approaches to solving the quartic.

astro-ph.HE↗

arXiv2024

Masking equation of state effects in binary neutron star mergers

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.

gr-qc↗

arXiv2024

Postmerger multimessenger analysis of binary neutron stars: Effect of the magnetic field strength and topology

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 magnetic field strengths with maximum values ranging from from $5.5\times 10^{15}G$ to $2.2\times 10^{17}G$ at the time of insertion. We find that magnetic braking and magnetic effective turbulent viscosity drives the merger remnants towards uniform rotation and increases their overall angular momentum loss. As a result, the $f_2$ frequency of the dominant postmerger GW mode shifts upwards over time. The overall shift is up to $\sim 200$Hz for the strongest magnetic field we consider and $\sim 50$Hz for the median case and is therefore detectable in principle by future GW observatories, which should include the magnetic field in their analyses. We also explore the impact of the magnetic field on the postmerger electromagnetic emission, and demonstrate that an extremely large magnetic field, or alternatively a significant shear viscosity mechanism, can cause a supramassive NS remnant to collapse to a BH in less than 100ms and lead to jet formation, although we do not expect the conditions for such an outcome to be realistic.

gr-qc↗

arXiv2024

Constraining the nuclear equation of state from orbits of primordial black holes inside neutron stars

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.

astro-ph.HE↗

arXiv2024

Primordial black holes captured by neutron stars: simulations in general relativity

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.

gr-qc↗

arXiv2024

Jetlike structures in low-mass binary neutron star merger remnants

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.

astro-ph.HE↗

arXiv2024

Primordial black holes captured by neutron stars: relativistic point-mass treatment

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.

gr-qc↗

arXiv2024

Could long-period transients be powered by primordial black hole capture?

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.

astro-ph.HE↗

arXiv2024

Primordial black hole capture, gravitational wave beats, and the nuclear equation of state

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.

gr-qc↗

arXiv2023

Waveform Modelling for the Laser Interferometer Space Antenna

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.

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