Gamma-Ray bursts from binary neutron star mergers Roland Oechslin MPA Garching, SFB/TR 7 SFB/TR7...
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Transcript of Gamma-Ray bursts from binary neutron star mergers Roland Oechslin MPA Garching, SFB/TR 7 SFB/TR7...
Gamma-Ray bursts from binary neutron star mergers
Roland OechslinMPA Garching, SFB/TR 7
SFB/TR7
Albert Einstein‘s Century, Paris, 21.07.2005
SFB/TR7
Outline of this talk:
Gamma-Ray bursts: Basic Properties
GRB central engine: The Neutron star merger model
Observations from GRB050509b
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
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 t. 10ms
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:
SFB/TR7
Gamma-Ray bursts: Central engine
Long bursts: Collapsar modelCollapse of a massive star into a BHBH mass: ~10M¯
Accretion disk mass: some M¯
Accretion rate: ~0.1M¯ s-1
Short bursts: Neutron star merger modelMerger of a binary neutron star and formation of a BH-disk systemBH mass: ~3M¯
Accretion disk mass: ~0.1M¯
Accretion rate: ~1M¯
(MacFadyen et al.)
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
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The merger model: Energetics
• generic discmass: 0.05M¯ ' 1053erg• gravitational energy ! ´s ~10%• bar ! e+e- ! 2!kin,ouflow ~ 0.1%-1%• Ekin,outflow ! GRB-´s · 100%• EGRB ! EGRB
iso £10-100• EGRB
iso' 1050-1052erg: compatible with observations!
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)
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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 ¸ 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!
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
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Evolution and disk formation:
green/blue: star 1/2red: particles ending up in the diskyellow: particles that currently fulfill the disk criterion.
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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!
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Disk formation:
Asymmetric casesMain contribution to the futuredisk from primary spiral arm.
Symmetric casesMain contribution from the secondary (smaller) spiral arms
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Discmasses: Dependence on binary parameters
! Strong dependence on q, approx. linear, with a flattening near q=1.
! Weak dependence on the total mass.
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'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.
SFB/TR7
Observational constraints from GRB050509b:
-T' 30ms ) suggests a collapse time of the remnant to a BH
tcoll.200ms.(tcoll+T)*v-driven wind<Tc, v-driven wind'0.1c
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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.