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): $M_{\max }^{\mathrm{sph}}\lesssim 2.74/\beta$, where $\beta$ 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 $\beta$ to be as high as 1.27, while most realistic candidate equations of state predict $\beta$ to be closer to 1.2, yielding $M_{\max }^{\mathrm{sph}}$ in the range $2.16–2.28M_{\odot }$. 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.

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 early inspirals of stellar-mass black holes that will ultimately 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.

The recent discovery by LIGO/Virgo of a merging binary having a black hole and a 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 an 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 cannot be invoked to explain this object, even allowing for maximum uniform rotation. By contrast, sufficiently stiff EOSs that yield NSs that are slowly rotating or, in some cases, nonrotating, and are compatible with GW170817 and the results of the Neutron Star Interior Composition Explorer (NICER), can account for the black hole companion.

Multi-messenger astrophysics is a fast-growing, interdisciplinary field that combines data, which vary in volume and speed of data processing, from many different instruments that probe the Universe using different cosmic messengers: electromagnetic waves, cosmic rays, gravitational waves and neutrinos. In this Expert Recommendation, we review the key challenges of real-time observations of gravitational wave sources and their electromagnetic and astroparticle counterparts, and make a number of recommendations to maximize their potential for scientific discovery. These recommendations refer to the design of scalable and computationally efficient machine learning algorithms; the cyber-infrastructure to numerically simulate astrophysical sources, and to process and interpret multi-messenger astrophysics data; the management of gravitational wave detections to trigger real-time alerts for electromagnetic and astroparticle follow-ups; a vision to harness future developments of machine learning and cyber-infrastructure resources to cope with the big-data requirements; and the need to build a community of experts to realize the goals of multi-messenger astrophysics.

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 $\mathrm{\Gamma }=2$ polytropes with spin ${\chi }_{\mathrm{NS}}=0.36$ aligned along the direction of the total orbital angular momentum $L$. 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 $\mu$ is either aligned or perpendicular to $L$ and has the same initial magnitude for each orientation. For comparison, we also impose symmetry across the orbital plane in one case where $\mu$ in both stars is aligned along $L$. 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 $\mathrm{hole}+\text{disk remnant}$, 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 [$\mathrm{\Delta }t\gtrsim 140(M_{\mathrm{NS}}/1.625M_{\odot })\mathrm{ms}$] and Poynting luminosities [$L_{\mathrm{EM}}\simeq {10}^{52}\mathrm{erg}/\mathrm{s}$] 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.

Black hole-neutron star (BHNS) mergers are one of the most promising targets for multimessenger astronomy. Using general relativistic magnetohydrodynamic simulations of BHNS undergoing merger we previously showed that a magnetically driven jet can be launched by the $\mathrm{disk}+\text{spinning}$ black hole remnant if the neutron star is endowed with a dipole magnetic field extending from the interior into the exterior as in a radio pulsar. These self-consistent studies considered a BHNS system with mass ratio $q=3:1$, black hole spin $a/M_{\mathrm{BH}}=0.75$ aligned with the total orbital angular momentum, and a neutron star that is irrotational, threaded by an aligned magnetic field and modeled by an $\mathrm{\Gamma }$-law equation of state with $\mathrm{\Gamma }=2$. Here, as a crucial step in establishing BHNS systems as viable progenitors of central engines that power short gamma-ray bursts (sGRBs) and thereby solidify their role as multimessenger sources, we survey different BHNS configurations that differ in the spin of the BH companion ($a/M_{\mathrm{BH}}=-0.5$, 0, 0.5, 0.75), in the mass ratio ($q=3:1$ and $q=5:1$) and in the orientation of the magnetic field (aligned and tilted by 90° with respect to the orbital angular momentum). We find that by $\mathrm{\Delta }t\sim 3500M-4000M\sim 88(M_{\mathrm{NS}}/1.4M_{\odot })\mathrm{ms}-100(M_{\mathrm{NS}}/1.4M_{\odot })\mathrm{ms}$ after the peak gravitational wave signal a magnetically driven jet is launched in the cases where the initial spin of the BH companion is $a/M_{\mathrm{BH}}=0.5$ or 0.75. The lifetime of the jets [$\mathrm{\Delta }t\sim 0.5(M_{\mathrm{NS}}/1.4M_{\odot })\mathrm{s}-0.7(M_{\mathrm{NS}}/1.4M_{\odot })\mathrm{s}$] and their outgoing Poynting luminosities [$L_{\mathrm{jet}}\sim 10^{51\pm 1}\mathrm{erg}/\mathrm{s}$] are consistent with typical sGRBs, as well as with the Blandford-Znajek mechanism for launching jets and their associated Poynting luminosities. By the time we terminate our simulations, we do not observe either an outflow or a large-scale magnetic field collimation in the other configurations we simulate. These results suggest that future multimessenger detections from BHNSs are more likely produced by binaries with highly spinning BH companions and small tilt-angle magnetic fields, though other physical processes not considered here, such as neutrino annihilation, may help to power jets in general cases.