GRMHD simulations of prompt-collapse neutron star mergers: the absence of jets

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 {\it delayed} collapse to a black hole surrounded by a {\it weighty} magnetized accretion disk can drive magnetically-powered jets. We now perform magnetohydrodynamic simulations in full general relativity of binary neutron stars mergers that undergo {\it prompt} collapse to explore the possibility of jet formation from black hole-{\it light} accretion disk remnants. We find that after $t-t_{\rm BH}\sim 26(M_{\rm NS}/1.8M_\odot)$ms [$M_{\rm NS}$ 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.Comment: 11 pages, 6 figures, matches published versio

Jet launching from binary black hole-neutron star mergers: Dependence on black hole spin, binary mass ratio and magnetic field orientation

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 showed that a magnetically--driven jet can be launched by the remnant if the NS is endowed with a dipole B field extending from the interior into the exterior as in a radio pulsar. These self-consistent studies considered a BHNS system with mass ratio $q=3:1$, BH spin $a/M_{BH}=0.75$ aligned with the total orbital angular momentum (OAM), and a NS that is irrotational, threaded by an aligned B field, and modeled by an $\Gamma$--law equation of state with $\Gamma=2$. 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 BH spin ($a/M_{BH} =-0.5,\,0,\,0.5,\,0.75$), in the mass ratio ($q=3:1$ and $q=5:1$), and in the orientation of the B field (aligned and tilted by $90^\circ$ with respect to the OAM). We find that by $\Delta t\sim 3500M-4000M \sim 88(M_{NS}/1.4M_\odot){\rm ms}-100(M_{NS}/1.4M_\odot)\rm ms$ after the peak gravitational wave signal a jet is launched in the cases where the initial BH spin is $a/M_{BH}= 0.5$ or $0.75$. The lifetime of the jets[$\Delta t\sim 0.5(M_{NS}/1.4M_\odot){\rm s-0.7}(M_{NS}/1.4M_\odot)\rm s$] and their Poynting luminosities [$L_{jet}\sim 10^{51\pm 1}\rm erg/s$] are consistent with sGRBs, as well as with the Blandford--Znajek mechanism. By the time we terminate our simulations, we do not observe either an outflow or a large-scale B 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 B fields.Comment: 17 pages, 14 figures. Added references, matches published versio

Magnetic Braking and Damping of Differential Rotation in Massive Stars

Fragmentation of highly differentially rotating massive stars that undergo collapse has been suggested as a possible channel for binary black hole formation. Such a scenario could explain the formation of the new population of massive black holes detected by the LIGO/VIRGO gravitational wave laser interferometers. We probe that scenario by performing general relativistic magnetohydrodynamic simulations of differentially rotating massive stars supported by thermal radiation pressure plus a gas pressure perturbation. The stars are initially threaded by a dynamically weak, poloidal magnetic field confined to the stellar interior. We find that magnetic braking and turbulent viscous damping via magnetic winding and the magnetorotational instability in the bulk of the star redistribute angular momentum, damp differential rotation and induce the formation of a massive and nearly uniformly rotating inner core surrounded by a Keplerian envelope. The core + disk configuration evolves on a secular timescale and remains in quasi-stationary equilibrium until the termination of our simulations. Our results suggest that the high degree of differential rotation required for $m=2$ seed density perturbations to trigger gas fragmentation and binary black hole formation is likely to be suppressed during the normal lifetime of the star prior to evolving to the point of dynamical instability to collapse. Other cataclysmic events, such as stellar mergers leading to collapse, may therefore be necessary to reestablish sufficient differential rotation and density perturbations to drive nonaxisymmetric modes leading to binary black hole formation.Comment: 11 pages, 5 figures. Minor changes, matches published versio

Simulating the Magnetorotational Collapse of Supermassive Stars: Incorporating Gas Pressure Perturbations and Different Rotation Profiles

Collapsing supermassive stars (SMSs) with masses $M \gtrsim 10^{4-6}M_\odot$ have long been speculated to be the seeds that can grow and become supermassive black holes (SMBHs). We previously performed GRMHD simulations of marginally stable magnetized $\Gamma = 4/3$ polytropes uniformly rotating at the mass-shedding limit to model the direct collapse of SMSs. These configurations are supported entirely by thermal radiation pressure and model SMSs with $M \gtrsim 10^{6}M_\odot$. We found that around $90\%$ of the initial stellar mass forms a spinning black hole (BH) surrounded by a massive, hot, magnetized torus, which eventually launches an incipient jet. Here we perform GRMHD simulations of $\Gamma \gtrsim 4/3$, polytropes to account for the perturbative role of gas pressure in SMSs with $M \lesssim 10^{6}M_\odot$. 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 find that the mass of the BH remnant is $90\%-99\%$ of the initial stellar mass, depending sharply on $\Gamma -4/3$ as well as on the initial stellar rotation profile. After $t\sim 250-550M\approx 1-2\times 10^3(M/10^6M_\odot)$s following the BH formation, a jet is launched and it lasts for $\sim 10^4-10^5(M/10^6M_\odot)$s, consistent with the duration of long gamma-ray bursts. Our results suggest that the Blandford-Znajek mechanism powers the jet. They are also in agreement with our proposed universal model that estimates accretion rates and luminosities that characterize magnetized BH-disk remnant systems that launch a jet. This model helps explain why the outgoing luminosities for vastly different BH-disk formation scenarios all reside within a narrow range ($\sim 10^{52 \pm 1} \rm erg/s$), roughly independent of $M$.Comment: 16 pages, 7 figures. Added references, matches published versio

GW170817, General Relativistic Magnetohydrodynamic Simulations, and the Neutron Star Maximum Mass

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): ${M_{\rm max}^{\rm sph}}\lesssim 2.74/\beta$, 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 ${M_{\rm max}^{\rm sph}}$ in the range $2.16-2.28 M_\odot$. A minimal set of assumptions based on these simulations distinguishes this analysis from previous ones, but leads to a 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.Comment: 6 pages, 1 figure. Matches published versio

Constant circulation sequences of binary neutron stars and their spin characterization

For isentropic fluids, dynamical evolution of a binary system conserves the baryonic mass and circulation; therefore, sequences of constant rest mass and constant circulation are of particular importance. In this work, we present the extension of our Compact Object CALculator (\cocal{}) code to compute such quasiequilibria and compare them with the well-known corotating and irrotational sequences, the latter being the simplest, zero-circulation case. The circulation as a measure of the spin for a neutron star in a binary system has the advantage of being exactly calculable since it is a local quantity. To assess the different measures of spin, such as the angular velocity of the star, the quasilocal, dimensionless spin parameter $J/M^2$, or the circulation $\mathcal{C}$, we first compute sequences of single, uniformly rotating stars and describe how the different spin diagnostics are related to each other. The connection to spinning binary systems is accomplished through the concept of circulation and the use of the constant rotational velocity formulation. Finally, we explore a modification of the latter formulation that naturally leads to differentially rotating binary systems.Comment: 9 pages, 7 figures, matches published versio

Magnetorotational Collapse of Supermassive Stars: Black Hole Formation, Gravitational Waves and Jets

We perform MHD simulations in full GR of uniformly rotating stars that are marginally unstable to collapse. Our simulations model the direct collapse of supermassive stars (SMSs) to seed black holes (BHs) that can grow to become the supermassive BHs at the centers of quasars and AGNs. They also crudely model the collapse of massive Pop III stars to BHs, which could power a fraction of distant, long gamma-ray bursts (GRBs). The initial stellar models we adopt are $\Gamma = 4/3$ polytropes seeded with a dynamically unimportant dipole magnetic field (B field). We treat initial B-field configurations either confined to the stellar interior or extending out from the interior into the stellar exterior. The BH formed following collapse has mass $M_{BH} \simeq 0.9M$ (where $M$ is the mass of the initial star) and spin $a_{BH}/M_{BH}\simeq 0.7$. A massive, hot, magnetized torus surrounds the remnant BH. At $\Delta t\sim 400-550M\approx 2000-2700(M/10^6M_\odot)$s following the gravitational wave (GW) peak amplitude, an incipient jet is launched. The disk lifetime is $\Delta t\sim 10^5(M/10^6M_\odot)$s, and the jet luminosity is $L_{EM}\sim 10^{51-52}$ ergs/s. If $\gtrsim 1-10\%$ of this power is converted into gamma rays, SWIFT and FERMI could potentially detect these events out to large redshifts $z\sim 20$. Thus, SMSs could be sources of ultra-long GRBs and massive Pop III stars could be the progenitors that power a fraction of the long GRBs observed at redshift $z \sim 5-8$. GWs are copiously emitted during the collapse, and peak at $\sim 15(10^6 M_{\odot}/M)\rm mHz$ ($\sim 0.15(10^4 M_{\odot}/M)\rm Hz$), i.e., in the LISA (DECIGO/BBO) band; optimally oriented SMSs could be detectable by LISA (DECIGO/BBO) at $z \lesssim 3$ ($z \lesssim 11$). Hence $10^4 M_{\odot}$ SMSs collapsing at $z\sim 10$ are promising multimessenger sources of coincident gravitational and electromagnetic waves.Comment: 14 pages, 9 figures, replaced with the published versio

Disks Around Merging Binary Black Holes: From GW150914 to Supermassive Black Holes

We perform magnetohydrodynamic simulations in full general relativity of disk accretion onto nonspinning black hole binaries with mass ratio 36:29. 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 0.4s 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 post-merger, 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.Comment: 14 pages, 6 figures, 5 tables, added discussion and references, matches published versio

Binary neutron star mergers: a jet engine for short gamma-ray bursts

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of quasi-circular, equal-mass, binary neutron stars that undergo merger. The initial stars are irrotational, $n=1$ polytropes and are magnetized. We explore two types of magnetic-field geometries: one where each star is endowed with a dipole magnetic field extending from the interior into the exterior, as in a pulsar, and the other where the dipole field is initially confined to the interior. In both cases the adopted magnetic fields are initially dynamically unimportant. The merger outcome is a hypermassive neutron star that undergoes delayed collapse to a black hole (spin parameter $a/M_{\rm BH} \sim 0.74$) immersed in a magnetized accretion disk. About $4000M \sim 60(M_{\rm NS}/1.625M_\odot)$ ms following merger, the region above the black hole poles becomes strongly magnetized, and a collimated, mildly relativistic outflow --- an incipient jet --- is launched. The lifetime of the accretion disk, which likely equals the lifetime of the jet, is $\Delta t \sim 0.1 (M_{\rm NS}/1.625M_\odot)$ s. In contrast to black hole--neutron star mergers, we find that incipient jets are launched even when the initial magnetic field is confined to the interior of the stars.Comment: 6 pages, 3 figures, 1 table, matches published versio

Gravitational wave content and stability of uniformly, rotating, triaxial neutron stars in general relativity

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 analogues 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 towards axisymmetry, maintaining their uniform rotation, kinetic energy, and angular momentum profiles while losing their triaxiality.Comment: 12 pages, 5 figure