Gravitational waves (...and GRB central engines...) from neutron star mergers

Transcription

1 Gravitational waves (...and GRB central engines...) from neutron star mergers Roland Oechslin MPA Garching, SFB/TR 7 Ringberg Workshop,

2 In this talk: -Intro: -Overview & Motivation -Neutron star mergers as gamma-ray burst engines and as GW emitters -Results from hydrodynamic simulations of NSM: -A parameter study -How do GW pattern and postmerger configuration depend on the EoS and on the NS mass & spin?

3 Merging neutron stars: Why are they interesting? large, time-dependent quadrupole strong gravitational wave source hot, n-emitting accretion torus around compact merger remnant Potential central engine for short gamma-ray bursts neutron rich ejecta r-process nucleosynthesis

4 Merging neutron stars: Schematic timetable adiabatic inspiral due to GW backreaction merger postmerger O(Myrs-Gyrs) O(ms) O(ms to s) time Hulse-Taylor: PSR f orb ~ Hz t merger ~240Mio. yr Tightest DNS: J f orb ~ Hz t merger ~85Mio. yr NSs are ripped apart by tidal forces: f orb ~500Hz d merger» 3R NS Formation of a dense merger remnant/bh surrounded by a hot accretion torus: M torus» M M remnant» 3M (M. Kramer, Jodrell Bank Obs.)

5 Short Gamma-Ray bursts: The Merger model Merger of a NS/NS or NS/BH binary and formation of a BH - torus system M BH '3M, M disc '0.05M Viscous heating in the torus, emission of neutrions (S. Rosswog, 2003) Energy deposition through nn bar -annilihation in the baryon-poor funnel around the rotation axis Baryonic jet along the rotation axis, collimated by the torus (Other possibility: MHD pinching) Internal shocks between the jet shells lead to emission of g-photons (Aloy et al., 2004)

6 The merger model: Energetics generic torus mass: 0.05M ' erg gravitational energy! n s ~10% nn bar! e + e -! 2g! E kin,ouflow ~ 0.1%-1% E kin,outflow! GRB-g s 10% E GRB! E GRB iso E GRB iso ' erg: compatible with observations!! The torus- and BH-masses are of crucial importance to power a GRB!

7 Generic GW waveform and spectrum Chirp-like inspiral part Spectrum: Broadband contribution. 1 khz Quasi-periodic postmerger part Spectrum: Large peak(s) around 2-4kHz (& 5 khz in case of prompt BH formation)

8 How do the unknown parameter influence the torus und GW signal? Clean system, but several free parameters: - NS spins: irrotating, corotating, counterrotating, tilted spins - gravitational NS masses: from 1.07M 1.6 M - Mass ratio: from q= EoS: Shen, Lattimer-Swesty, ideal gas Mass ratio, total mass Reference model: M, no NS spin, Shen EoS NS spins EoS

9 General relativity Hydrodynamical simulations of merging NS Shibata et al. (Miller at al.) (Shibata & Taniguchi) Conformally flat (Wilson et al.) RO et al, Faber et al. RO & Janka 1PN Ayal et al., Faber et al. Oohara & Nakamura Newtonian Polytropic EoS Oohara & Nakamura (1989), Rasio et al, Centrella et al. Zhuge et al. Physical EoS Ruffert et al. Rosswog et al. Neutrino physics Price & Rosswog Magnetic fields Microphysics

10 Physical and numerical ingredients - Fully relativistic hydrodynamics using SPH, ~ particles - Einstein eqns. are solved approximately: - assume for the spatial metric g ij =y 4 d ij (conformally flat condition)! EE reduce to 5 nonlinear, coupled, elliptic PDEs! Metric is not evolved independently, but is coupled to the matter. -Physical, non-zero temperature EoS (Shen et al.,1999; Lattimer & Swesty, 1991) / ideal gas EoS / zero temperature EoS (Akmal, Pandharipande & Ravenhall, 1998) with thermal extension -No neutrinos, no MHD, dynamically unimportant on this timescales. - Initial data: Binary in equilibrium near ISCO, T=0, n-less b-equilibirum

11 Finite temperature dense matter EoSs: L&S: Lattimer & Swesty, 1991, with K=188MeV Shen: Shen et al., 1998 Available as 3D tables, EoS as function of (r, T, Y e ) Y e =0.05

12 Merger dynamics & torus formation: green/blue: star 1/2 red: particles ending up in the disk yellow: particles that currently fulfill the torus criterion Torus:=matter with j>j LSO where LSO=last stable orbit around a BH with M BH =M remnant & a BH =a remnant

13 Varying the binary parameters: corotating Lattimer-Swesty- EoS Asymmetric, Mass ratio q=0.75

14 Torus masses: Dependence on binary parameters! Strong dependence on the mass ratio q. Saturation at about M torus =0.2M! Weak dependence on the total mass! corot.! NS spin influence via total angular momentum. Detailed merger dynamics seems to be of minor importance. counterrot. irrot.

15 Torus masses: Dependence on the EoS Two possibilities to transfer ang.mom. out to torus: At premerger/merger if the NSs are large enough:! Shen EoS At postmerger by gravitational torques. Effective for very compact and non-axisymmetric remnants:! APR, (LS)

16 How does the GW signal depend on the free parameters? Try to identify characteristic quantities: f peak D E in D E pm f max =f@h max f max : Frequency at the amplitude maximum of the inpiral chirp f peak : Frequency of the dominating oscillation in the postmerger wavetrain D E in, D E pm : Radiated energies during 3ms just before and during 5ms just after merging

17 Dependence of the GW signal/spectrum ±: Shen-EoS (stiff) *: LS-EoS (soft) D,r: APR-EoS (soft, very r ) NS spin +: Counterrotating x: Corotating total mass nuclear EoS mass ratio

18 Conclusions Neutron star mergers are a strong gravitational wave source and thus a prime candidate for detection by one of the ground based interferometers. Currently, they are also the preferred model for the central engine of short GRBs. Using hydrodynamic simulations, we have investigated the merger dynamics and the following torus formation depending on the initial NS mass ratio, spin and the EoS. We found torus masses between ~0.03M (q=1, no spin, LSEoS) and ~0.30M (q=0.55, no spin, Shen EoS). These results are compatible with estimates inferred from observations of the first 4 short GRBs. The merger and postmerger GW signal and spectrum depend very sensitively on the nuclear EoS. A measurement of a merger signal may help to further restrict the EoS parameter space.