1. Gamma-Ray bursts from binary neutron star mergers Roland Oechslin MPA Garching, SFB/TR 7 SFB/TR7 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: ~ 10 53 erg (assuming isotropic emission, ~10 51 erg with beaming, ~100 times less for short GRBs) Rapid variability on timescales  t. 10ms

5. SFB/TR7 Gamma-Ray bursts: Observational constraints and implications Timescales:  t ¿ T:  t = 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 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 ¯ (MacFadyen et al.)

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 ¯ (S. Rosswog et al.) - Emission of ´s in the hot accretion disk - Deposition of energy through bar -annilihation in the baryon-poor funnel around the rotation axis driving a baryonic jet. - Emission of  ´s in internal shocks

8. SFB/TR7 The merger model: Energetics generic discmass: 0.05M ¯ ' 10 53 erg gravitational energy ! ´s~10% bar ! e + e - ! 2  !  kin,ouflow ~ 0.1%-1% E kin,outflow ! GRB-  ´s · 100% E GRB ! E GRB iso £ 10-100 E GRB iso ' 10 50 -10 52 erg: compatible with observations!

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

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  >100 are possible - Assumed discmass: 0.13M ¯, energy deposition ¸ 10 49 erg - Typical jet opening angle 5°-10°, determined by the accretion disk - Typical isotropized energies ~10 51 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>j ISCO,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)). - E ,iso ' 10 48-49 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 t coll. 200ms. (t coll +T)*v -driven wind <Tc, v -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 ‘s which then deposit energy via bar -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.