Effect Of Black Hole Spin Research Articles

Eccentric black hole-neutron star mergers: Effects of black hole spin and equation of state

There is a high level of interest in black hole-neutron star binaries, not only because their mergers may be detected by gravitational wave observatories in the coming years, but also because of the possibility that they could explain a class of short duration gamma-ray bursts. We study black hole-neutron star mergers that occur with high eccentricity as may arise from dynamical capture in dense stellar regions such as nuclear or globular clusters. We perform general relativistic simulations of binaries with a range of impact parameters, three different initial black hole spins (zero, aligned and antialigned with the orbital angular momentum), and neutron stars with three different equations of state. We find a rich diversity across these parameters in the resulting gravitational wave signals and matter dynamics, which should also be reflected in the consequent electromagnetic emission. Before tidal disruption, the gravitational wave emission is significantly larger than perturbative predictions suggest for periapsis distances close to effective innermost stable separations, exhibiting features reflecting the zoom-whirl dynamics of the orbit there. Guided by the simulations, we develop a simple model for the change in orbital parameters of the binary during close-encounters. Depending upon the initial parameters of the system, we find that mass transfer during nonmerging close-encounters can range from essentially zero to a sizable fraction of the initial neutron star mass. The same holds for the amount of material outside the black hole post-merger, and in some cases roughly half of this material is estimated to be unbound. We also see that nonmerging close-encounters generically excite large oscillations in the neutron star that are qualitatively consistent with $f$-modes.

Accuracy of the post-Newtonian approximation. II. Optimal asymptotic expansion of the energy flux for quasicircular, extreme mass-ratio inspirals into a Kerr black hole

We study the effect of black hole spin on the accuracy of the post-Newtonian approximation. We focus on the gravitational energy flux for the quasicircular, equatorial, extreme mass-ratio inspiral of a compact object into a Kerr black hole of mass $M$ and spin $J$. For a given dimensionless spin $a\ensuremath{\equiv}J/{M}^{2}$ (in geometrical units $G=c=1$), the energy flux depends only on the orbital velocity $v$ or (equivalently) on the Boyer-Lindquist orbital radius $r$. We investigate the formal region of validity of the Taylor post-Newtonian expansion of the energy flux (which is known up to order ${v}^{8}$ beyond the quadrupole formula), generalizing previous work by two of us. The error function used to determine the region of validity of the post-Newtonian expansion can have two qualitatively different kinds of behavior, and we deal with these two cases separately. We find that, at any fixed post-Newtonian order, the edge of the region of validity (as measured by $v/{v}_{\mathrm{ISCO}}$, where ${v}_{\mathrm{ISCO}}$ is the orbital velocity at the innermost stable circular orbit) is only weakly dependent on $a$. Unlike in the nonspinning case, the lack of sufficiently high-order terms does not allow us to determine if there is a convergent to divergent transition at order ${v}^{6}$. Independent of $a$, the inclusion of angular multipoles up to and including $\ensuremath{\ell}=5$ in the numerical flux is necessary to achieve the level of accuracy of the best-known ($N=8$) post-Newtonian expansion of the energy flux.

Effects of Black Hole Spin on the Limit-Cycle Behaviour of Accretion Disks

We present a spatially 1.5-dimensional, time-dependent numerical study of accretion disks around Kerr black holes. Our study focuses on the limit-cycle behavior of thermally unstable accretion disks. We find that maximal luminosity may be a more appropriate probe of black hole spin than the cycle duration and influence radius.

Simulations of magnetized discs around black holes: effects of black hole spin, disc thickness and magnetic field geometry

The standard general relativistic model of a razor-thin accretion disk around a black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at the radius of the innermost stable circular orbit (ISCO) and that, outside the ISCO, the shear stress is produced by an effective turbulent viscosity. However, astrophysical accretion disks are not razor-thin, it is uncertain whether the shear stress necessarily vanishes at the ISCO, and the magnetic field, which is thought to drive turbulence in disks, may contain large-scale structures that do not behave like a simple local scalar viscosity. We describe three-dimensional general relativistic magnetohydrodynamic simulations of accretion disks around black holes with a range of spin parameters, and we use the simulations to assess the validity of the NT model. Our fiducial initial magnetic field consists of multiple (alternating polarity) poloidal field loops whose shape is roughly isotropic in the disk in order to match the isotropic turbulence expected in the poloidal plane. For a thin disk with an aspect ratio |h/r| ~ 0.07 around a non-spinning black hole, we find a decrease in the accreted specific angular momentum of 2.9% relative to the NT model and an excess luminosity from inside the ISCO of 3.5%. The deviations in the case of spinning black holes are of the same order. In addition, the deviations decrease with decreasing |h/r|. We therefore conclude that magnetized thin accretion disks in x-ray binaries in the thermal/high-soft spectral state ought to be well-described by the NT model, especially at luminosities below 30% of Eddington where we expect a very small disk thickness |h/r| <~ 0.05. We also discuss how the stress and the luminosity inside the ISCO depend on the assumed initial magnetic field geometry. (abridged)

Phenomenology of production and decay of spinning extra-dimensional black holes at hadron colliders

We present results of CHARYBDIS2, a new Monte Carlo simulation of black hole production and decay at hadron colliders in theories with large extra dimensions and TeV-scale gravity. The main new feature of CHARYBDIS2 is a full treatment of the spin-down phase of the decay process using the angular and energy distributions of the associated Hawking radiation. Also included are improved modelling of the loss of angular momentum and energy in the production process as well as a wider range of options for the Planck-scale termination of the decay. The new features allow us to study the effects of black hole spin and the feasibility of its observation in such theories. The code and documentation can be found at http://projects.hepforge.org/charybdis2/.

General relativistic simulations of black-hole–neutron-star mergers: Effects of black-hole spin

Black-hole--neutron-star (BHNS) binary mergers are candidate engines for generating both short-hard gamma-ray bursts and detectable gravitational waves. Using our most recent conformal thin-sandwich BHNS initial data and our fully general relativistic hydrodynamics code, which is now adaptive mesh refinement capable, we are able to efficiently and accurately simulate these binaries from large separations through inspiral, merger, and ringdown. We evolve the metric using the Baumgarte-Shapiro-Shibata-Nakamura formulation with the standard moving puncture gauge conditions, and handle the hydrodynamics with a high-resolution shock-capturing scheme. We explore the effects of BH spin (aligned and antialigned with the orbital angular momentum) by evolving three sets of initial data with BH:NS mass ratio $q=3$: the data sets are nearly identical, except the BH spin is varied between $a/{M}_{\mathrm{BH}}=\ensuremath{-}0.5$ (antialigned), 0.0, and 0.75. The number of orbits before merger increases with $a/{M}_{\mathrm{BH}}$, as expected. We also study the nonspinning BH case in more detail, varying $q$ between 1, 3, and 5. We calculate gravitational waveforms for the cases we simulate and compare them to binary black-hole waveforms. Only a small disk ($&lt;0.01{M}_{\ensuremath{\bigodot}}$) forms for the antialigned spin case ($a/{M}_{\mathrm{BH}}=\ensuremath{-}0.5$) and for the most extreme-mass-ratio case ($q=5$). By contrast, a massive (${M}_{\mathrm{disk}}\ensuremath{\approx}0.2{M}_{\ensuremath{\bigodot}}$) hot disk forms in the rapidly spinning ($a/{M}_{\mathrm{BH}}=0.75$) aligned BH case. Such a disk could drive a short-hard gamma-ray burst, possibly by, e.g., producing a copious flux of neutrino-antineutrino pairs.

Disk‐dominated States of 4U 1957+11:Chandra,XMM‐Newton, andRXTEObservations of Ostensibly the Most Rapidly Spinning Galactic Black Hole

We present simultaneous Chandra High-Energy Transmission Gratings (HETG) and Rossi X-ray Timing Explorer (RXTE) observations of a soft state of the black hole candidate 4U 1957+11. These spectra, having limited hard X-ray excess, are an excellent test of disk atmosphere models that include effects of black hole spin. The HETG data show, by modeling the broadband continuum and direct fitting of absorption edges, that the disk spectrum is only very mildly absorbed, with NH = (1–2) × 1021 cm−2. These data additionally reveal λλ13.449 Ne IX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies an inclined disk around a low-mass black hole rotating with normalized spin a* ≈ 1. We show, however, that pure Schwarzschild models describe the data extremely well, albeit with large disk atmosphere ``color-correction'' factors. Standard correction factors can be attained if one incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin, even with this otherwise extremely well-measured, nearly pure disk spectrum. XMM-Newton RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and distance of 4U 1957+11; however, it is also driven by the limited Chandra and XMM-Newton bandpasses. We therefore present a series of 48 RXTE observations taken at different brightness/hardness levels. These data prefer a spin of a* ≈ 1, even when including a mild Comptonization component; however, they also show evolution of the color-correction factors. If the rapid-spin models with standard correction factors are to be believed, then the RXTE observations predict that 4U 1957+11 can range from a 3 M☉ black hole at 10 kpc with a* ≈ 0.83 to a 16 M☉ black hole at 22 kpc with a* ≈ 1.

On the Duration of Long GRBs: Effects of Black Hole Spin

In the frame of the collapsar model for long gamma-ray bursts (GRBs), we investigate the formation of a torus around a spinning black hole (BH), and we check what rotational properties a progenitor star must have in order to sustain torus accretion over relatively long activity periods. We also study the time evolution of the BH spin parameter. We take into account the coupling between BH mass, its spin parameter, and the critical specific angular momentum of accreting gas needed for the torus to form. The large BH spin reduces the critical angular momentum which in turn can increase the GRB duration with respect to the Schwarzschild BH case. We quantify this effect and estimate the GRB durations in three cases: when a hyperaccreting torus operates or a BH spins very fast or both. We show under what conditions a given progenitor star produces a burst that can last as short as several seconds and as long as several hundred of seconds. Our models indicate that it is possible for a single collapse to produce three kinds of jets: (1) a very short, lasting between a fraction of a second and a few seconds, precursor jet, powered only by a hyperaccreting torus before the BH spins up, (2) an early jet, lasting several tens of seconds and powered by both hyperaccretion and BH rotation, and (3) a late jet, powered only by the spinning BH.