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Merging of Neutron Stars

    Principal Investigators: Hans-Thomas Janka

Merging binary neutron stars are among the strongest known sources of gravitational waves (GWs) and are widely favored as astrophysical events with properties suitable for explaining gamma-ray bursts (GRBs) of the class of short, hard bursts. This project is on the way of developing improved computational tools and numerical models for the very last stages of the binary evolution, the final plunge and merging of the neutron stars, and the early stages of the remnant formation. The goal is a better understanding of the link between the characteristics of the observable signals -- gravitational waves as well as GRBs --, and the binary and neutron star properties.

Merging binary neutron stars are among the strongest known sources of gravitational waves (GWs) and are widely favored as astrophysical events with properties suitable for explaining gamma-ray bursts (GRBs) of the class of short, hard bursts. This project is on the way of developing improved computational tools and numerical models for the very last stages of the binary evolution, the final plunge and merging of the neutron stars, and the early stages of the remnant formation. The goal is a better understanding of the link between the characteristics of the observable signals --- gravitational waves as well as GRBs ---, and the binary and neutron star properties. To this end we proceed along two lines of investigation, accounting for the facts that the involved numerical and physical problems are too challenging to be solved in one big leap, and the relevant range of temporal and spatial scales is too wide to be manageable with one kind of modeling tool. On the one hand, we use existing computational methods (e.g., a Smoothed Particle Hydrodynamics scheme) which still employ approximations to the treatment of general relativity. Including important microphysics, these codes allow us to study various questions of astrophysical interest in the context of merger events at the forefront of current research, for example the dependence of the GW signature on the nuclear equation of state, the formation and evolution of accretion tori around the postmerger remnant --- a black hole (BH) or a hypermassive neutron star ---, and the launch of ultrarelativistic GRB jets by the energy release from neutrino-antineutrino annilihation in such BH-torus systems. The association of compact object mergers with short GRBs indeed seems to be supported by the recent detections of well localized short GRBs in the outskirts of elliptical galaxies with an old stellar population and no major star formation activity. On the other hand, being aware of the shortcomings of current simulations, we are in the process of constructing new codes for the next generation of models, in which general relativistic aspects in combination with important microphysics (physical dense matter equations of state, neutrino transport) will be treated more rigorously. First progress could be achieved, but ongoing development is necessary, and the final goal will require also the combination of expertise from different projects within this Transregional Collaborative Research Centre.

Merger of a 1.2+1.6Msolar NS binary (click for full size) The first observed and well localized short GRB050509b Jet formation in a BH-torus system

Researchers

Publications

[1] Measuring neutron-star properties via gravitational waves from binary mergers
A. Bauswein, H.-T. Janka, Phys. Rev. Lett. 108, 011101 (2012)

[2] Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers
A. Bauswein, H.-T. Janka, K. Hebeler, A. Schwenk, Phys. Rev. D 86, 063001 (2012)

[3] The r-process in the neutrino-driven wind from a black-hole torus
S. Wanajo, H.-Th. Janka, Astrophys. J. 746, 180 (2012)

[4] The decompression of the outer neutron star crust and r-process nucleosynthesis
S. Goriely, N. Chamel, H.-T. Janka, J. M. Pearson, J. M., Astron. Astrophys. 531, A78 (2011)

[5] r-process Nucleosynthesis in Dynamically Ejected Matter of Neutron Star Mergers
S. Goriely, A. Bauswein, H.-T. Janka, Astrophys. J. Lett. 738, L32 (2011)

[6] Gravitational waves and non-axisymmetric oscillation modes in mergers of compact object binaries
N. Stergioulas, A. Bauswein, K. Zagkouris, H.-T. Janka, Mon. Not. Roy. Astron. Soc. 418, 427 (2011)

[7] Discriminating strange star mergers from neutron star mergers by gravitational-wave measurements
A. Bauswein, R. Oechslin, H.-Th. Janka, Phys. Rev. D 81, 024012 (2010)

[8] Local simulations of the magnetized Kelvin-Helmholtz instability in neutron-star mergers
M. Obergaulinger, M.A. Aloy, E. Mueller, Astron. Astrophys. 515, A30 (2010)

[9] Testing approximations of thermal effects in neutron star merger simulations
A. Bauswein, H.-Th. Janka, R.Oechslin, Phys. Rev. D 82, 084043 (2010)

[10] Relativistic simulations of neutron star and strange star mergers
A. Bauswein, H.-Th. Janka, High Performance Computing in Science and Engineering, Proc. of HLRB and KONWIHR Review, Results and Future Projects Workshop: Preparing for Petascale Systems, at Leibniz Supercomputing Centre in Garching, Springer

[11] Mass ejection by strange star mergers and observational implications
A. Bauswein, H.-Th. Janka, R. Oechslin, G. Pagliara, I. Sagert, J. Schaffner-Bielich, M. M. Hohle, R. Neuhaeuser, Phys. Rev. Lett. 103, 011101 (2009)

[12] Polytropic neutron star - black hole merger simulations with a Paczyski-Wiita potential
M. Ruffert, H.-Th. Janka, Astron. Astrophys. 514, A66 (2009)

[13] Neutrino-antineutrino annihilation in BH-torus systems
R. Birkl, H.-T. Janka, M.-A. Aloy, and E. Mueller, Astron. Astrophys. 463, 51-67 (2007)

[14] Relativistic neutron star merger simulations with non-zero temperature euqations of state I. Variations of binary parameters ans equations of state
R. Oechslin, H.-T. Janka, and A. Marek, Astron. Astrophys. 467, 395-409 (2007)

[15] Gravitational Waves from Relativistic Neutron Star Mergers with nonzero-temperature Equations of State
R. Oechslin and H.-Th. Janka, Phys. Rev. Lett. 99, 121102 (2007)

[16] Relativistic outfows in gamma-ray bursts
M.-A. Aloy, M. Obergaulinger, Circumstellar Media and Late Stages of Massive Stellar Evolution, Revista Mexicana de Astronomia y Astro?sica Conference Series, Vol. 30, pp. 96-103 (2007)

[17] Torus formation in neutron star mergers and well-localized short gamma-ray bursts
R. Oechslin and H.-T. Janka, Mon.Not.Roy.Astron.Soc. 368 (2006) 1489-1499

[18] Off-axis properties of short gamma-ray bursts
H.-T. Janka, P. Mazzali, M.-A. Aloy and E. Pian, Astrophys.J. 645 (2006) 1305-1314

[19] Deconfinement Phase Transition in Relativistic Neutron Star Mergers,
G. Poghosyan, R. Oechslin, K. Uryu, and F.K. Thielemann, in Superdense QCD Matter and Compact Stars, Proceedings of the NATO Advanced Research Workshop on Superdense QCD Matter and Compact Stars, Yerevan, Armenia, Sept. 27 - Oct. 4, 2003, Eds. D. Blaschke and D. Sedrakian, NATO Science Series, Vol. 197 (2006).

[20] Short Gamma-Ray Bursts from Binary Neutron Star Mergers
R. Oechslin and H.-Th. Janka, Proceedings of the Albert Einstein Century International Conference, Paris, France, July 18-23, 2005, Eds. J.-M. Alimi and A. Fuezfa (2006).

[21] High-resolution shock capturing scheme for ideal hydrodynamics in general relativity optimized for quasistationary solutions
W. Kastaun, Physical Review D

[22] Relativistic outflows from remnants of compact object mergers and their viability for short gamma-ray bursts
M.-A. Aloy, H.-T. Janka and E. Mueller, Astron. Astrophys. 436, 273 (2005)

[23] Three-dimensional simulations of non-stationary accretion by remnant black holes of compact object mergers
S. Setiawan, M. Ruffert, and H.-T. Janka, Astron. Astrophys. 458, 553-567 (2006)

[24] The influence of quark matter at high densitites on binary neutron star mergers
R. Oechslin, K. Uryu, G. Proghosyan and F.-K. Thielemann, Mon. Not. R. Astron. Soc. 349, 1469 (2004)

[25] Non-stationary hyperaccretion of stellar-mass black holes in three dimension: Torus evolution and neutrion emission
S. Setiawan, M. Ruffert and H.-T. Janka, Mon. Not. R. Astron. Soc. 352, 753 (2004)

[26] The first steps in the life of a short GRB
M.-A. Aloy, H.-T. Janka, and E. Mueller, in: Gamma-Ray Bursts: 30 Years of Discovery, Proceedings of a Gamma-Ray Burst Symposium, Santa Fe, New Mexico, Sept. 8-12, 2003, Eds. E.E. Fenimore and M. Galassi, AIP Conference Proceedings, Vol. 727, Melville, NY: American Institute of Physics, p. 380 (2004)

Theses

[27] R. Birkl, Diploma thesis, TU Muenchen (2006)

[28] Developing a code for general relativistic hydrodynamics with application to neutron star oscillations
W. Kastaun, PhD thesis (2007)

[29] Relativistic simulations of compact object mergers for nucleonic matter and strange quark matter
A. Bauswein, PhD thesis (2010)

[30] Astrophysical magnetohydrodynamics and radiative transfer: numerical methods and applications
M. Obergaulinger, PhD thesis (2008)