arXiv subjects

Hirotada Okawa

Publications and source records attributed to Hirotada Okawa.

58 records · Page 4Linked to original sources

arXiv2011

Gravitational waves from spinning black hole-neutron star binaries: dependence on black hole spins and on neutron star equations of state

We study the merger of black hole (BH)-neutron star (NS) binaries with a variety of BH spins aligned or anti-aligned with the orbital angular momentum, and with the mass ratio in the range MBH/MNS = 2--5, where MBH and MNS are the mass of the BH and NS, respectively. We model NS matter by systematically parametrized piecewise polytropic equations of state. The initial condition is computed in the puncture framework adopting an isolated horizon framework to estimate the BH spin and assuming an irrotational velocity field for the fluid inside the NS. Dynamical simulations are performed in full general relativity by an adaptive mesh refinement code, SACRA. The treatment of hydrodynamic equations and estimation of the disk mass are improved. We find that the NS is tidally disrupted irrespective of the mass ratio when the BH has a moderately large prograde spin, whereas only binaries with low mass ratios, MBH/MNS <~ 3 or small compactnesses of the NSs, bring the tidal disruption when the BH spin is zero or retrograde. The mass of the remnant disk is accordingly large as >~ 0.1 Msun, which is required by central engines of short gamma-ray bursts, if the BH spin is prograde. Information of the tidal disruption is reflected in a clear relation between the compactness of the NS and an appropriately defined "cutoff frequency" in the gravitational-wave spectrum, above which the spectrum damps exponentially. We find that the tidal disruption of the NS and excitation of the quasinormal mode of the remnant BH occur in a compatible manner in high mass-ratio binaries with the prograde BH spin. The correlation between the compactness and the cutoff frequency still holds for such cases. It is also suggested by extrapolation that the merger of an extremely spinning BH and an irrotational NS binary does not lead to formation of an overspinning BH.

astro-ph.HE↗

arXiv2011

Binary Neutron Star Mergers: Dependence on the Nuclear Equation of State

We perform a numerical-relativity simulation for the merger of binary neutron stars with 6 nuclear-theory-based equations of state (EOSs) described by piecewise polytropes. Our purpose is to explore the dependence of the dynamical behavior of the binary neutron star merger and resulting gravitational waveforms on the EOS of the supernuclear-density matter. The numerical results show that the merger process and the first outcome are classified into three types; (i) a black hole is promptly formed, (ii) a short-lived hypermassive neutron star (HMNS) is formed, (iii) a long-lived HMNS is formed. The type of the merger depends strongly on the EOS and on the total mass of the binaries. For the EOS with which the maximum mass is larger than 2Msun, the lifetime of the HMNS is longer than 10 ms for a total mass m_0=2.7Msun. A recent radio observation suggests that the maximum mass of spherical neutron stars is M_max \geq 1.97\pm 0.04Msun in one σlevel. This fact and our results support the possible existence of a HMNS soon after the onset of the merger for a typical binary neutron star with m_0=2.7Msun. We also show that the torus mass surrounding the remnant black hole is correlated with the type of the merger process; the torus mass could be large, \geq 0.1Msun, in the case that a long-lived HMNS is formed. We also show that gravitational waves carry information of the merger process, the remnant, and the torus mass surrounding a black hole.

astro-ph.HE↗

arXiv2011

Is super-Planckian physics visible? -- Scattering of black holes in 5 dimensions

It may be widely believed that probing short-distance physics is limited by the presence of the Planck energy scale above which scale any information is cloaked behind a horizon. If this hypothesis is correct, we could observe quantum behavior of gravity only through a black hole of Planck mass. We numerically show that in a scattering of two black holes in the 5-dimensional spacetime, a visible domain, whose curvature radius is much shorter than the Planck length, can be formed. Our result indicates that super-Planckian phenomena may be observed without an obstruction by horizon formation in particle accelerators.

gr-qc↗

arXiv2008

High-velocity collision of two black holes

We study nonaxisymmetric collision of two black holes (BHs) with a high velocity $v=|dx^i/dx^0|=0.6$--$0.9c$ at infinity, where $x^μ$ denotes four-dimensional coordinates. We prepare two boosted BHs for the initial condition which is different from that computed by a simple moving-puncture approach. By extrapolation of the numerical results, we find that the impact parameter has to be smaller that $\approx 2.5GM_0/c^2$ for formation of a BH in the collision for $v \to c$, where $M_0 c^2$ is the initial total ADM mass energy of the system. For the critical value of the impact parameter, 20--30% of mass energy and 60--70% of angular momentum are dissipated by gravitational radiation for $v=0.6$--$0.9c$.

gr-qc↗