Gamma-Ray bursts from binary neutron star mergers Roland Oechslin MPA Garching, SFB/TR 7

1. SFB/TR7 Gamma-Ray bursts from binary neutron star mergers Roland Oechslin MPA Garching, SFB/TR 7 Albert Einstein‘s Century, Paris, 21.07.2005

2. SFB/TR7 Outline of this talk: Gamma-Ray bursts: Basic Properties GRB central engine: The Neutron star merger model Observations from GRB050509b

3. SFB/TR7 Gamma-Ray bursts: Basic features • GRBs: Short and intese bursts of gamma-rays • Discovered accidentally in the late 1960s (Vela satellites) • Duration: 0.01s . T .100s, bimodal distribution: short (T.2s) and long bursts (T&2s) • Non-thermal spectrum, peak energy at some 100keV with high energy tail

4. SFB/TR7 Gamma-Ray bursts: Basic features • Energy release: ~ 1053 erg (assuming isotropic emission, ~1051 erg with beaming, ~100 times less for short GRBs) • Rapid variability on timescales Dt. 10ms

5. SFB/TR7 Gamma-Ray bursts: Observational constraints and implications • Timescales:Dt¿T: • Dt = O(ms): ) GRB must involve a compact object • T=O(s): • ) Central Source is active much longer than the dynamical • timescale of the compact object) Cannot produce a GRB with one single energy release (e.g. an explosion)) Natural model: accretion onto a compact object • ) To account for the bimodal distribution: Two preferred • models:

6. SFB/TR7 Gamma-Ray bursts: Central engine Long bursts: Collapsar model Collapse of a massive star into a BH BH mass: ~10M¯ Accretion disk mass: some M¯ Accretion rate: ~0.1M¯ s-1 (MacFadyen et al.) Short bursts: Neutron star merger model Merger of a binary neutron star and formation of a BH-disk system BH mass: ~3M¯ Accretion disk mass: ~0.1M¯ Accretion rate: ~1M¯

7. SFB/TR7 Short Gamma-Ray bursts: Merger model Merger of a BNS and formation of a NS/BH-disk system on a dynamical timescale of O(ms). BH mass: ~3M¯ Disk mass: ~0.01M¯-0.1M¯ • Emission of n´s in the hot accretion disk • Deposition of energy through nnbar-annilihation in • the baryon-poor funnel around the rotation axis driving a baryonic jet.- Emission of g´s in internal shocks (S. Rosswog et al.)

8. SFB/TR7 The merger model: Energetics • generic discmass: 0.05M¯ ' 1053erg • gravitational energy !n´s ~10% • nnbar! e+e-! 2g ! Ekin,ouflow ~ 0.1%-1% • Ekin,outflow! GRB-g´s · 100% • EGRB! EGRBiso£10-100 • EGRBiso' 1050-1052erg: compatible with observations!

9. SFB/TR7 The merger model: Numerical results from NSM simulations with neutrinos nnbar-annilihation and energy deposition above the baryon-poor rotation axis. (M. Ruffert et al., 2001) (S. Rosswog et al., 2003)

10. SFB/TR7 The merger model: The evolution of the outflow (Aloy et al., 2004) • Start out from a post-merger BH (implemented as a SS-background)-disk system and mimic the annilihation by deposing energy in a cone around the rotation axis. • Relativistic outflows with G>100 are possible • Assumed discmass: 0.13M¯, energy deposition ¸ 1049erg • Typical jet opening angle 5°-10°, determined by the accretion disk • Typical isotropized energies ~1051 ergs • But: a sufficiently low baryon density in the outflow funnel is crucial!

11. SFB/TR7 Can we model the required BH-disc system ? (RO & H.-T. Janka, 2005, submitted to MNRAS) • Simulate the merger phase numerically with: • GR hydro with SPH, ~400‘000 particles • approximate GR treatment (conformally flat approximation) • non-zero temperature EoS (Shen et al.,1999) • no neutrinos • Initial model: irrotational, NSs on circular orbit in equilibrium • grav. NS masses varied from 1.2 – 1.6 M¯ • Mass ratio varied from q=0.75 – 1 • Disk matter defined as matter with j>jISCO,central remnant

12. SFB/TR7 Evolution and disk formation: green/blue: star 1/2 red: particles ending up in the disk yellow: particles that currently fulfill the disk criterion.

13. SFB/TR7 Disk formation: Angular momentum is transferred along spiral arms from the center to larger radii. Asymmetric case (q<1): Large primary spiral arm from tidally disrupted lighter companion (green). Secondary spiral arms from surface material of the post-merger remnant. Symmetric case (q'1): Only secondary spiral arms present. Overall discmass considerably smaller. ! If the central remnant collapses immediately to a BH, no secondary spiral arms will develop!

14. SFB/TR7 Disk formation: Asymmetric cases Main contribution to the future disk from primary spiral arm. Symmetric cases Main contribution from the secondary (smaller) spiral arms

15. SFB/TR7 Discmasses: Dependence on binary parameters ! Strong dependence on q, approx. linear, with a flattening near q=1. ! Weak dependence on the total mass.

16. SFB/TR7 Observational constraints from GRB050509b: GRB050509b: First well-localized, short-hard GRB. (Gehrels et al., 2005; Bloom et al., 2005; Hjorth et al., 2005) • likely to be associated with a nearby elliptical galaxy (G1) at z'0.225. If so: • no indication for recent star formation in G1) compact object merger scenario favoured (inspiral timescale » O(Myrs)-O(Gyrs)). • Eg,iso'1048-49 erg • T' 30ms, optical upper limits in afterglow • ) compatible with a small accretion disk, i.e. with a symmetric binary. • ) suggesting a small amount of ejected radiating • material.

17. SFB/TR7 Observational constraints from GRB050509b: • T' 30ms • ) suggests a collapse time of the remnant to a BH tcoll.200ms. (tcoll+T)*vn-driven wind<Tc, vn-driven wind'0.1c

18. SFB/TR7 Conclusions The merger of two NSs is a possible progenitor for the short GRB central engine. The merger outcome, a hot accretion disk, emits n‘s which then deposit energy via nnbar-annilihation along the polar axis to drive a baryonic outflow. We have investigated merger dynamics and disk formation depending on the initial NS masses and mass ratios and find diskmass values between ~0.01M¯ and ~0.15M¯. The observations from GRB050509b are compatible with the merger model and with our results. They suggest a small disk and a remnant collapse within ~200ms.