Gamma-Ray bursts from binary neutron star mergers

Roland OechslinMPA Garching, SFB/TR 7

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Albert Einstein‘s Century, Paris, 21.07.2005

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Outline of this talk:

Gamma-Ray bursts: Basic Properties

GRB central engine: The Neutron star merger model

Observations from GRB050509b

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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

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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

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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:

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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.)

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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!

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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!

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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.

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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.

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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.