Francois Foucart

Neutron star-black hole mergers with a nuclear equation of state and neutrino cooling: Dependence in the binary parameters

We present a first exploration of the results of neutron star-black hole mergers using black hole masses in the most likely range of 7M_⊙ –10M_⊙, a neutrino leakage scheme, and a modeling of the neutron star material through a finite-temperature nuclear-theory based equation of state. In the range of black hole spins in which the neutron star is tidally disrupted (χ BH ≳0.7), we show that the merger consistently produces large amounts of cool (T≲1  MeV), unbound, neutron-rich material (M_(ej) ∼ 0.05M_⊙ –0.20M_⊙). A comparable amount of bound matter is initially divided between a hot disk (T_(max) ∼15  MeV) with typical neutrino luminosity of L_ ν ∼10^(53)   erg/s, and a cooler tidal tail. After a short period of rapid protonization of the disk lasting ∼10  ms, the accretion disk cools down under the combined effects of the fall-back of cool material from the tail, continued accretion of the hottest material onto the black hole, and neutrino emission. As the temperature decreases, the disk progressively becomes more neutron rich, with dimmer neutrino emission. This cooling process should stop once the viscous heating in the disk (not included in our simulations) balances the cooling. These mergers of neutron star-black hole binaries with black hole masses of M_(BH) ∼7M_⊙ –10M_⊙, and black hole spins high enough for the neutron star to disrupt provide promising candidates for the production of short gamma-ray bursts, of bright infrared postmerger signals due to the radioactive decay of unbound material, and of large amounts of r-process nuclei.

Black hole-neutron star mergers: Effects of the orientation of the black hole spin

The spin of black holes in black hole-neutron star binaries can have a strong influence on the merger dynamics and the post-merger state; a wide variety of spin magnitudes and orientations are expected to occur in nature. In this paper, we report the first simulations in full general relativity of black hole-neutron star mergers with misaligned black hole spin. We vary the spin magnitude from a_(BH)/M_(BH) = 0 to a_(BH)/M_(BH) = 0.9 for aligned cases, and we vary the misalignment angle from 0 to 80° for a_(BH)/M_(BH) = 0.5. We restrict our study to 3:1 mass-ratio systems and use a simple Γ-law equation of state. We find that the misalignment angle has a strong effect on the mass of the post-merger accretion disk, but only for angles greater than ≈ 40°. Although the disk mass varies significantly with spin magnitude and misalignment angle, we find that all disks have very similar lifetimes ≈ 100  ms. Their thermal and rotational profiles are also very similar. For a misaligned merger, the disk is tilted with respect to the final black hole’s spin axis. This will cause the disk to precess, but on a time scale longer than the accretion time. In all cases, we find promising setups for gamma-ray burst production: the disks are hot, thick, and hyperaccreting, and a baryon-clear region exists above the black hole.

Black Hole-Neutron Star Mergers: Disk Mass Predictions

Determining the final result of black-hole\char21{}neutron-star mergers, and, in particular, the amount of matter remaining outside the black hole at late times and its properties, has been one of the main motivations behind the numerical simulation of these systems. Black-hole\char21{}neutron-star binaries are among the most likely progenitors of short gamma-ray bursts\char22{}as long as massive (probably a few percents of a solar mass), hot accretion disks are formed around the black hole. Whether this actually happens strongly depends on the physical characteristics of the system, and, in particular, on the mass ratio, the spin of the black hole, and the radius of the neutron star. We present here a simple two-parameter model, fitted to existing numerical results, for the determination of the mass remaining outside the black hole a few milliseconds after a black-hole\char21{}neutron-star merger (i.e., the combined mass of the accretion disk, the tidal tail, and the potential ejecta). This model predicts the remnant mass within a few percents of the mass of the neutron star, at least for remnant masses up to 20% of the neutron star mass. Results across the range of parameters deemed to be the most likely astrophysically are presented here. We find that, for

Black hole-neutron star mergers at realistic mass ratios: Equation of state and spin orientation effects

10{M}_{\ensuremath{\bigodot}}

ASSEMBLY OF PROTOPLANETARY DISKS AND INCLINATIONS OF CIRCUMBINARY PLANETS

black holes, massive disks are only possible for large neutron stars (

BLACK HOLE-NEUTRON STAR MERGERS WITH A HOT NUCLEAR EQUATION OF STATE: OUTFLOW AND NEUTRINO-COOLED DISK FOR A LOW-MASS, HIGH-SPIN CASE

{R}_{\mathrm{NS}}\ensuremath{\gtrsim}12\text{ }\text{ }\mathrm{km}

Massive disc formation in the tidal disruption of a neutron star by a nearly extremal black hole

), or quasiextremal black hole spins (

Black hole-neutron star mergers for 10 solar mass black holes

{a}_{\mathrm{BH}}/{M}_{\mathrm{BH}}\ensuremath{\gtrsim}0.9

Evolution of linear warps in accretion discs and applications to protoplanetary discs in binaries

). We also use our model to discuss how the equation of state of the neutron star affects the final remnant, and the strong influence that this can have on the rate of short gamma-ray bursts produced by black-hole\char21{}neutron-star mergers.

First direct comparison of nondisrupting neutron star-black hole and binary black hole merger simulations

Black-hole–neutron-star mergers resulting in the disruption of the neutron star and the formation of an accretion disk and/or the ejection of unbound material are prime candidates for the joint detection of gravitational-wave and electromagnetic signals when the next generation of gravitational-wave detectors comes online. However, the disruption of the neutron star and the properties of the postmerger remnant are very sensitive to the parameters of the binary (mass ratio, black-hole spin, neutron star radius). In this paper, we study the impact of the radius of the neutron star and the alignment of the black-hole spin on black-hole–neutron-star mergers within the range of mass ratio currently deemed most likely for field binaries (M_BH∼7M_NS) and for black-hole spins large enough for the neutron star to disrupt (J_BH/M^(2)_(BH)=0.9). We find that (i) In this regime, the merger is particularly sensitive to the radius of the neutron star, with remnant masses varying from 0.3M_NS to 0.1M_NS for changes of only 2 km in the NS radius; (ii) 0.01M_(⊙)–0.05M_(⊙) of unbound material can be ejected with kinetic energy ≳10^(51)  ergs, a significant increase compared to low mass ratio, low spin binaries. This ejecta could power detectable postmerger optical and radio afterglows. (iii) Only a small fraction of the Advanced LIGO events in this parameter range have gravitational-wave signals which could offer constraints on the equation of state of the neutron star (at best ∼3% of the events for a single detector at design sensitivity). (iv) A misaligned black-hole spin works against disk formation, with less neutron-star material remaining outside of the black hole after merger, and a larger fraction of that material remaining in the tidal tail instead of the forming accretion disk. (v) Large kicks v_kick≳300  km/s can be given to the final black hole as a result of a precessing black-hole–neutron-star merger, when the disruption of the neutron star occurs just outside or within the innermost stable spherical orbit.