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.

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 Γ ≥ 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 ≲ ≲ 10 to the dark-matter content of the Universe.

We present fully general-relativistic numerical evolutions of self-gravitating tori around spinning black holes with dimensionless spin / = 0.7 parallel or antiparallel 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 nonspinning 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 a large range of masses. We find that systems of 10 -relevant for black hole-neutron star mergers-are detectable by Cosmic Explorer out to ~300 Mpc, while DECIGO (LISA) will be able to detect systems of 1000 (10 )-relevant for disks forming in collapsing supermassive stars-out to cosmological redshift of ~ 5 ( ~ 1). Computing the accretion rate of these systems we find that these systems may also be promising sources of coincident electromagnetic signals.

An excess -ray signal toward the outer halo of M31 has recently been reported. Although other explanations are plausible, the possibility that it arises from dark matter (DM) is valid. In this work we interpret the excess in the framework of DM annihilation, using as our representative case WIMP DM annihilating to bottom quarks, and we perform a detailed study of the systematic uncertainty in the -factor for the M31 field. We find that the signal favors a DM particle with a mass of ~45-72 GeV. While the mass is well constrained, the systematic uncertainty in the cross section spans 3 orders of magnitude, ranging from ~5 × 10-5 × 10 cm s. This high uncertainty is due to two main factors, namely, an uncertainty in the substructure nature and geometry of the DM halos for both M31 and the Milky Way (MW), and correspondingly, an uncertainty in the contribution to the signal from the MW's DM halo along the line of sight. However, under the conditions that the minimum subhalo mass is ≲10 and the actual contribution from the MW's DM halo along the line of sight is at least ~30% of its total value, we show that there is a large overlap with the DM interpretations of both the Galactic center (GC) excess and the antiproton excess, while also being compatible with the limits for the MW dwarf spheroidals. More generally, we summarize the results from numerous complementary DM searches in the energy range 10 GeV-300 GeV corresponding to the GC excess and identify a region in parameter space that still remains viable for discovery of the DM particle.

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 ( / = 0 and 0.75), the NS spin ( / = -0.17, 0, 0.23, and 0.33), and the binary mass ratio ( = : = 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 Δ ~ 3500-5500 ~ 88-138( /1.4 ) ms after the peak gravitational-wave amplitude, a magnetically driven jet is launched only for = 3:1 regardless of the initial NS spin. The lifetime of the jets [Δ ~ 0.5-0.8( /1.4 ) s] and their outgoing Poynting luminosity [ ~ 10 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 consider. The mass range of dynamically ejected matter is 10-10( /1.4 ) , which can power kilonovas with peak bolometric luminosities ~ 10-10 erg/s with rise times ≲6.5 h and potentially detectable by the LSST.

Binary neutron star mergers can be sources of gravitational waves coincident with electromagnetic counterpart emission across the spectrum. To solidify their role as multimessenger sources, we present fully 3D, general relativistic, magnetohydrodynamic simulations of highly spinning binary neutrons stars initially on quasicircular orbits that merge and undergo delayed collapse to a black hole. The binaries consist of two identical stars modeled as Γ = 2 polytropes with spin = 0.36 aligned along the direction of the total orbital angular momentum . Each star is initially threaded by a dynamical unimportant interior dipole magnetic field. The field is extended into the exterior where a nearly force-free magnetosphere resembles that of a pulsar. The magnetic dipole moment is either aligned or perpendicular to and has the same initial magnitude for each orientation. For comparison, we also impose symmetry across the orbital plane in one case where in both stars is aligned along . We find that the lifetime of the transient hypermassive neutron star remnant, the jet launching time, and the ejecta (which can give rise to a detectable kilonova) are very sensitive to the magnetic field orientation. By contrast, the physical properties of the black hole + disk remnant, such as the mass and spin of the black hole, the accretion rate, and the electromagnetic (Poynting) luminosity, are roughly independent of the initial magnetic field orientation. In addition, we find imposing symmetry across the orbital plane does not play a significant role in the final outcome of the mergers. Our results suggest that, as in the black hole-neutron star merger scenario, an incipient jet emerges only when the seed magnetic field has a sufficiently large-scale poloidal component aligned to the initial orbital angular momentum. The lifetime [Δ ≳ 140( /1.625 ) ms] and Poynting luminosities [ ≃ 10 erg/s] of the jet, when it forms, are consistent with typical short gamma-ray bursts, as well as with the Blandford-Znajek mechanism for launching jets.

In this paper, we present an exact solution for (3 + 1)-dimensional Einstein-scalar-Gauss-Bonnet theory (EsGB) in electrovacuum. The solution is characterized by only one parameter, , which in general can be associated with the electromagnetic field and the scalar field. We show that the solution corresponds to a charged wormhole with a throat at the region = || and is also supported by a real scalar field having a positive kinetic term. We show that the solution belongs to the most general class of solutions known as Ellis wormholes but without the need for "exotic matter" or a phantom scalar field.

We perform a new, detailed calculation of the flux and energy spectrum of Earth-emerging leptons generated from the interactions of tau neutrinos and antineutrinos in the Earth. A layered model of the Earth is used to describe the variable density profile of the Earth. Different assumptions regarding the neutrino charged- and neutral-current cross sections as well as the -lepton energy loss models are used to quantify their contributions to the systematic uncertainty. A baseline simulation is then used to generate the optical Cherenkov signal from upward-moving extensive air showers generated by the -lepton decay in the atmosphere, applicable to a range of space-based instruments. We use this simulation to determine the neutrino sensitivity for ≳ 10 PeV for a space-based experiment with performance similar to that for the Probe of Extreme Multi-Messenger Astrophysics (POEMMA) mission currently under study.

We consider here some popular () models generally viewed as possible alternatives to the existence of dark energy in General Relativity. For each of these, we compute the redshift at which the angular diameter distance () is expected to reach its maximum value. This turning point in () was recently measured in a model-independent way using compact quasar cores and was found to occur at = 1.70 ± 0.20. We compare the predictions of for the () models with this observed value to test their viability at a deeper level than has been attempted thus far, thereby quantifying an important observational difference between such modified gravity scenarios and standard Lambda Cold Dark Matter (ΛCDM) cosmology. Our results show that, while the most popular () models today are consistent with this measurement to within 1, the turning point will allow us to prioritize these alternative gravity theories as the measurement precision continues to improve, particularly with regard to how well they mitigate the tension between the predictions of ΛCDM and the observations. For example, while the Hu-Sawicki version of () increases this tension, the Starobinky model reduces it.

Indirect searches for dark matter through Standard Model products of its annihilation generally assume a cross-section which is dominated by a term independent of velocity (-wave annihilation). However, in many DM models an -wave annihilation cross-section is absent or helicity suppressed. To reproduce the correct DM relic density in these models, the leading term in the cross section is proportional to the DM velocity squared (-wave annihilation). Indirect detection of such -wave DM is difficult because the average velocities of DM in galaxies today are orders of magnitude slower than the DM velocity at the time of decoupling from the primordial thermal plasma, thus suppressing the annihilation cross-section today by some five orders of magnitude relative to its value at freeze out. Thus -wave DM is out of reach of traditional searches for DM annihilations in the Galactic halo. Near the region of influence of a central supermassive black hole, such as Sgr A*, however, DM can form a localized over-density known as a "spike". In such spikes the DM is predicted to be both concentrated in space and accelerated to higher velocities, thereby allowing the -ray signature from its annihilation to potentially be detectable above the background. We use the Large Area Telescope to search for the -ray signature of -wave annihilating DM from a spike around Sgr A* in the energy range 10 GeV-600 GeV. Such a signal would appear as a point source and would have a sharp line or box-like spectral features difficult to mimic with standard astrophysical processes, indicating a DM origin. We find no significant excess of rays in this range, and we place upper limits on the flux in -ray boxes originating from the Galactic Center. This result, the first of its kind, is interpreted in the context of different models of the DM density near Sgr A*.

Collapsing supermassive stars (SMSs) with masses ≳ 10 have long been speculated to be the seeds that can grow and become supermassive black holes (SMBHs). We previously performed general relativistic magnetohydrodynamic (GRMHD) simulations of marginally stable Γ = 4/3 polytropes uniformly rotating at the mass-shedding limit and endowed initially with a dynamically unimportant dipole magnetic field to model the direct collapse of SMSs. These configurations are supported entirely by thermal radiation pressure and reliably model SMSs with ≳ 10 . We found that around 90% of the initial stellar mass forms a spinning black hole (BH) remnant surrounded by a massive, hot, magnetized torus, which eventually launches a magnetically-driven jet. SMSs could be therefore sources of ultra-long gamma-ray bursts (ULGRBs). Here we perform GRMHD simulations of Γ ≳ 4/3, polytropes to account for the perturbative role of gas pressure in SMSs with ≲ 10 . We also consider different initial stellar rotation profiles. The stars are initially seeded with a dynamically weak dipole magnetic field that is either confined to the stellar interior or extended from its interior into the stellar exterior. We calculate the gravitational wave burst signal for the different cases. We find that the mass of the black hole remnant is 90%-99% of the initial stellar mass, depending sharply on Γ - 4/3 as well as on the initial stellar rotation profile. After ~ 250-550 ≈ 1 - 2 × 10(/10 ) s following the appearance of the BH horizon, an incipient jet is launched and it lasts for ~10-10(/10 ) s, consistent with the duration of long gamma-ray bursts. Our numerical results suggest that the Blandford-Znajek mechanism powers the incipient jet. They are also in rough agreement with our recently proposed universal model that estimates accretion rates and electromagnetic (Poynting) luminosities that characterize magnetized BH-disk remnant systems that launch a jet. This model helps explain why the outgoing electromagnetic luminosities computed for vastly different BH-disk formation scenarios all reside within a narrow range (~10 erg s), roughly independent of .

Identification of the cosmic-ray (CR) "PeVatrons," which are sources capable of accelerating particles to ~10 eV energies and higher, may lead to resolving the long-standing question of the origin of the spectral feature in the all-particle CR spectrum known as the "knee." Because CRs with these energies are deflected by interstellar magnetic fields identification of individual sources and determination of their spectral characteristics is more likely via very high energy -ray emissions, which provide the necessary directional information. However, pair production on the interstellar radiation field (ISRF) and cosmic microwave background (CMB) leads to steepening of the high energy tails of -ray spectra, and should be corrected for to enable true properties of the spectrum at the source to be recovered. Employing recently developed three-dimensional ISRF models this paper quantifies the pair-absorption effect on spectra for sources in the Galactic center (GC) direction at 8.5 and 23.5 kpc distances, with the latter corresponding to the far side of the Galactic stellar disc where it is expected that discrimination of spectral features >10 TeV is possible by the forthcoming Cherenkov Telescope Array (CTA). The estimates made suggest spectral cutoffs could be underestimated by factors of a few in the energy range so far sampled by TeV -ray telescopes. As an example to illustrate this, the recent HESS measurements of diffuse -ray emissions possibly associated with injection of CRs nearby Sgr A* are ISRF corrected, and estimates of the spectral cutoff are reevaluated. It is found that it could be higher by up to a factor of ~2, indicating that these emissions may be consistent with a CR accelerator with a spectral cutoff of at least 1 PeV at the 95% confidence level.

We adopt the fluid conduction approximation to study the evolution of spherical star clusters and self-interacting dark matter (SIDM) halos. We also explore the formation and dynamical impact of density cusps that arise in both systems due to the presence of a massive, central black hole. The large N-body, self-gravitating systems we treat are "weakly collisional": the mean free time between star or SIDM particle collisions is much longer than their characteristic crossing (dynamical) time scale, but shorter than the system lifetime. The fluid conduction model reliably tracks the "gravothermal catastrophe" in star clusters and SIDM halos without black holes. For a star cluster with a massive, central black hole, this approximation reproduces the familiar Bahcall-Wolf quasistatic density cusp for the stars bound to the black hole and shows how the cusp halts the "gravothermal catastrophe" and causes the cluster to re-expand. An SIDM halo with an initial black hole central density spike that matches onto to an exterior NFW profile relaxes to a core-halo structure with a central density cusp determined by the velocity dependence of the SIDM interaction cross section. The success and relative simplicity of the fluid conduction approach in evolving such "weakly collisional," quasiequilibrium Newtonian systems motivates its extension to relativistic systems. We present a general relativistic extension here.

We perform magnetohydrodynamic simulations in full general relativity of disk accretion onto nonspinning black hole binaries with mass ratio = 29/36. We survey different disk models which differ in their scale height, total size and magnetic field to quantify the robustness of previous simulations on the initial disk model. Scaling our simulations to LIGO GW150914 we find that such systems could explain possible gravitational wave and electromagnetic counterparts such as the Fermi GBM hard x-ray signal reported 0.4 s after GW150915 ended. Scaling our simulations to supermassive binary black holes, we find that observable flow properties such as accretion rate periodicities, the emergence of jets throughout inspiral, merger and postmerger, disk temperatures, thermal frequencies, and the time delay between merger and the boost in jet outflows that we reported in earlier studies display only modest dependence on the initial disk model we consider here.

Recent numerical simulations in general relativistic magnetohydrodynamics (GRMHD) provide useful constraints for the interpretation of the GW170817 discovery. Combining the observed data with these simulations leads to a bound on the maximum mass of a cold, spherical neutron star (the TOV limit): , where is the ratio of the maximum mass of a uniformly rotating neutron star (the supramassive limit) over the maximum mass of a nonrotating star. Causality arguments allow to be as high as 1.27, while most realistic candidate equations of state predict to be closer to 1.2, yielding in the range 2.16-2.28. A minimal set of assumptions based on these simulations distinguishes this analysis from previous ones, but leads a to similar estimate. There are caveats, however, and they are enumerated and discussed. The caveats can be removed by further simulations and analysis to firm up the basic argument.

Binary black hole (BBH) mergers provide a prime source for current and future interferometric GW observatories. Massive BBH mergers may often take place in plasma-rich environments, leading to the exciting possibility of a concurrent electromagnetic (EM) signal observable by traditional astronomical facilities. However, many critical questions about the generation of such counterparts remain unanswered. We explore mechanisms that may drive EM counterparts with magnetohydro-dynamic simulations treating a range of scenarios involving equal-mass black-hole binaries immersed in an initially homogeneous fluid with uniform, orbitally aligned magnetic fields. We find that the time development of Poynting luminosity, which may drive jet-like emissions, is relatively insensitive to aspects of the initial configuration. In particular, over a significant range of initial values, the central magnetic field strength is effectively regulated by the gas flow to yield a Poynting luminosity of 10 - 10 ergs, with BBH mass scaled to ≡ /(10 ) and ambient density ≡ (10 g cm). We also calculate the direct plasma synchrotron emissions processed through geodesic ray-tracing. Despite lensing effects and dynamics, we find the observed synchrotron flux varies little leading up to merger.

Inspiraling and merging binary neutron stars are not only important source of gravitational waves, but also promising candidates for coincident electromagnetic counterparts. These systems are thought to be progenitors of short gamma-ray bursts (sGRBs). We have shown previously that binary neutron star mergers that undergo collapse to a black hole surrounded by a magnetized accretion disk can drive magnetically powered jets. We now perform magnetohydrodynamic simulations in full general relativity of binary neutron stars mergers that undergo collapse to explore the possibility of jet formation from black hole- accretion disk remnants. We find that after - ~26(/1.8 ) ms ( is the ADM mass) following prompt black hole formation, there is no evidence of mass outflow or magnetic field collimation. The rapid formation of the black hole following merger prevents magnetic energy from approaching force-free values above the magnetic poles, which is required for the launching of a jet by the usual Blandford-Znajek mechanism. Detection of gravitational waves in coincidence with sGRBs may provide constraints on the nuclear equation of state (EOS): the fate of an NSNS merger-delayed or prompt collapse, and hence the appearance or nonappearance of an sGRB-depends on a critical value of the total mass of the binary, and this value is sensitive to the EOS.

We perform magnetohydrodynamic simulations in full general relativity of uniformly rotating stars that are marginally unstable to collapse. These simulations model the direct collapse of supermassive stars (SMSs) to seed black holes that can grow to become the supermassive black holes at the centers of quasars and active galactic nuclei. They also crudely model the collapse of massive Population III stars to black holes, which could power a fraction of distant, long gamma-ray bursts. The initial stellar models we adopt are Γ = 4/3 polytropes initially with a dynamically unimportant dipole magnetic field. We treat initial magnetic-field configurations either confined to the stellar interior or extending out from the stellar interior into the exterior. We find that the black hole formed following collapse has mass ≃ 0.9 (where is the mass of the initial star) and dimensionless spin parameter / ≃ 0.7. A massive, hot, magnetized torus surrounds the remnant black hole. At Δ ~ 400-550 ≈ 2000 - 2700(/10)s following the gravitational wave peak amplitude, an incipient jet is launched. The disk lifetime is Δ ~ 10(/10)s, and the outgoing Poynting luminosity is ~ 10 ergs/s. If of this power is converted into gamma rays, Swift and Fermi could potentially detect these events out to large redshifts ~ 20. Thus, SMSs could be sources of ultra-long gamma-ray bursts (ULGRBs), and massive Population III stars could be the progenitors that power a fraction of the long GRBs observed at redshift ~ 5-8. Gravitational waves are copiously emitted during the collapse and peak at ~15(10/) mHz [~0.15(10 M/M) Hz], i.e., in the LISA (DECIGO/BBO) band; optimally oriented SMSs could be detectable by LISA (DECIGO/BBO) at .Hence, 10 SMSs collapsing at ~ 10 are promising multimessenger sources of coincident gravitational and electromagnetic waves.

Targets for ground-based gravitational wave interferometers include continuous, quasiperiodic sources of gravitational radiation, such as isolated, spinning neutron stars. In this work, we perform evolution simulations of uniformly rotating, triaxially deformed stars, the compressible analogs in general relativity of incompressible, Newtonian Jacobi ellipsoids. We investigate their stability and gravitational wave emission. We employ five models, both normal and supramassive, and track their evolution with different grid setups and resolutions, as well as with two different evolution codes. We find that all models are dynamically stable and produce a strain that is approximately one-tenth the average value of a merging binary system. We track their secular evolution and find that all our stars evolve toward axisymmetry, maintaining their uniform rotation, rotational kinetic energy, and angular momentum profiles while losing their triaxiality.

We have performed magnetohydrodynamic simulations in general relativity of binary neutron star and binary black hole-neutron star mergers, as well as the magnetorotational collapse of supermassive stars. In many cases the outcome is a spinnng black hole (BH) immersed in a magnetized disk, with a jet emanating from the poles of the BH. While their formation scenarios differ and their BH masses, as well as their disk masses, densities, and magnetic field strengths, vary by orders of magnitude, these features conspire to generate jet Poynting luminosities that all lie in the same, narrow range of ~10 erg s. A similar result applies to their BH accretion rates upon jet launch, which is ~0.1-10 s. We provide a simple model that explains these unanticipated findings. Interestingly, these luminosities reside in the same narrow range characterizing the observed luminosity distributions of over 400 short and long GRBs with distances inferred from spectroscopic redshifts or host galaxies. This result, together with the GRB lifetimes predicted by the model, supports the belief that a compact binary merger is the progenitor of an SGRB, while a massive, stellar magnetorotational collapse is the progenitor of an LGRB.

We describe a quantum limit to measurement of classical spacetimes. Specifically, we formulate a quantum Cramér-Rao lower bound for estimating the single parameter in any one-parameter family of spacetime metrics. We employ the locally covariant formulation of quantum field theory in curved spacetime, which allows for a manifestly background-independent derivation. The result is an uncertainty relation that applies to all globally hyperbolic spacetimes. Among other examples, we apply our method to detection of gravitational waves with the electromagnetic field as a probe, as in laser-interferometric gravitational-wave detectors. Other applications are discussed, from terrestrial gravimetry to cosmology.