Dr Stephan Rosswog
Professor of Astrophysics
Jacobs University Bremen
Campus Ring 1
D-28759 Bremen, Germany
Tel: +49 421 200-3226
email: s.rosswog@jacobs-university.de



Mergers of double neutron star binaries

Background

Neutron star:

  • corpse of a massive star, produced in a supernova explosion
  • typical neutron star mass: 1.4 solar masses
  • radius: about 10 km
  • densities are even higher than in an atomic nucleus


    Neutron star binaries:

  • are observed in our Galaxy, to date eight such systems known.
  • emit gravitational waves and therefore slowly spiral towards each other
  • this decay is in accurate agreement with the predictions from Einstein's Theory of General Relativity (Nobelprize for Physics 1993: R. Hulse and J. Taylor)
  • the last minutes of before the coalescence could be detected by ground-based Gravitational wave detectors such as the American LIGO project or the German British GEO600
  • the final coalescence releases a tremendous amount of energy, and is thought to produce a short Gamma-ray Burst, one of the brightest explosions in the Universe since the Big Bang


    Short Gamma-ray bursts:

  • Gamma-ray bursts come in two flavours:
    i) long bursts with a duration of about 30 s that are related to a rare type of supernova explosion and
    ii) short bursts with a duration of about 0.3 s that are thought to be caused by mergers of either two neutron stars or a neutron star with a stellar-mass black hole
  • A review: "A very good year for explosions", Robert Irion, Science 2006
  • the presented somputer simulations are the very first that take magnetic fields into account



    In the following we will briefly illustrate

    i) magnetic field evolution and

    ii) the neutrino-driven wind from a merger remnant



    The following images and Movies are produced from the simulations that are described in Price & Rosswog, Science, 312, 5774 (2006)


    Images

    Click for higher resolution versions. Use of any images is on the condition that they be accompanied by the line: Credit: Daniel Price and Stephan Rosswog

    press release image 1 press release image 2 press release image 3 press release image 4




    Movies

    Movies are in quicktime format, playable with the free Apple Quicktime Player (download here).
    Alternatively some movies are also given in .flc format which can be played on unix systems using xanim.
    To see a directory listing of all of the movies, click here.


    movie showing collision of two neutron stars (with top chopped off)
    Full length Quicktime (51Mb)    		 Description: Animation of the coalescence of two magnetised neutron stars, with the colours showing magnetic field strengths in the material at and below the orbital plane. The animation shows the first 12 milliseconds of the merger and has dimensions of approximately 140 km from left to right. The stars move gradually towards each other and then merge in a ``plunging phase'' within about one orbital period. This object sheds mass into spiral arms that are subsequently wrapped around the central object to form a hot torus. The magnetic field is amplified in the shear instability between the stars and subsequently advected with the matter to cover the surface of the central merger remnant.

    Formats:


    Full length 800x534 half-stars animation: 51Mb Quicktime .mov , 103Mb .flc format
    Shorter length 800x534 half-stars animation: 21Mb Quicktime .mov , 39Mb .flc format
    High resolution 1600x1068 half-stars animation: 132Mb Quicktime .mov
    movie showing collision of two neutron stars (full stars)Quicktime (28Mb) Description: Animation of the coalescence of two magnetised neutron stars, with the colours showing magnetic field strengths at the surface. The animation shows the first 8 milliseconds of the merger and has dimensions of approximately 140 km from left to right. The stars move gradually towards each other and then merge in a ``plunging phase'' within about one orbital period. This object sheds mass into spiral arms that are subsequently wrapped around the central object to form a hot torus. The magnetic field is rapidly amplified and strong field pockets (coloured yellow-white) can be seen to quickly cover the surface of the central merger remnant.

    Formats:

    Shorter length 800x534 full stars animation: 28Mb Quicktime .mov, 59Mb .flc format
    Quicktime (146Mb) Description: The evolution of the magnetic field is shown as a cross section through the midplane. Fluid instabilities cause the interface to curl up into vortices. It is in these vortices that the field is strongly amplified. The strength of the magnetic field may be compared to the colour bar on the right, where field strengths > 10^15G represent those stronger than any previously known (i.e. in Magnetars).

    Formats:

    Cross section slice animation: 146Mb Quicktime .mov
    Quicktime (13Mb) Description: In this animation, two stars of different initial masses were used (star 1 has 1.1 times the mass of the sun, whilst star 2 has 1.6 x M_sun - in the previous simulations both stars had mass 1.4 M_sun). The magnetic field is similarly amplified although the geometry of the collision and subsequent torus formation is quite different.

    Formats:

    Unequal mass ratio animation: 13Mb Quicktime .mov, 22Mb .flc format

    Neutrino signatures and neutrino-driven winds from neutron star merger remnants

    Results are taken from Neutrino signatures and the neutrino-driven wind in binary neutron star mergers
    L. Dessart, C. Ott, A. Burrows, S. Rosswog, E. Livne, 23 pages, submitted to ApJ (2008)

    Background

    Once a neutron star binary has merged, it consists of three components (see movies above): i) a "central object", initially an extremely massive neutron star that will most likely, at some point, collapse into a black hole, ii) a disk that is geometrically thick and (partially) opaque even to neutrinos and iii)"tidal tails" that are either bound and fall back or dynamically ejected and a possible source of neutron-rich isotopes.
    The extreme densities (>1014 g/ccm) and high temperatures (> 1010 K) favour copious neutrino emission, typical neutrino luminosities (sum of all flavours) are around 1053 erg/s. The neutrino emission properties from 3D neutron star merger simulations can be found in Rosswog and Liebendoerfer (2003). At these luminosities, neutrinos ablate a large amount of baryonic material from the remnant, which poses a strong danger for launching the ultra-relativistic jets (Γ > 100) that are required to explain the short variation and still non-thermal spectra in gamma-ray bursts. This potential "baryonic thread" from neutrino-driven winds was known since a few years, however, whether such a jet can form depends crucially on the wind geometry which up to very recently completely unknown.

    In the following we describe the first ever study of the neutrino-driven wind from a neutron star merger.

  • In a first step neutron star mergers are simulated with the 3D-magnetohydrodynamics code MAGMA (Rosswog and Price 2007).
  • The results o these calculations are mapped onto rotationally symmetric, 2-dimensional grid that can be used as initial conditions for the 2D-neutrino-radiation hydrodynamics code VULCAN/2D (Dessart et al. 2006a; Burrows et al. 2007b; Ott et al. 2008). Examples of such initial conditions are shown in the following:
  • In the next step these initial conditions are evolved further in VULCAN/2D (a detailed description is provided in our paper Dessart et al. 2008). In particular, these are to our knowledge, the calculations ever that address the crucial question of neutrino-driven wind from a merger remnant. The regions in which energy is gained or lost via neutrinos is shown in the following figure (both with the multi-group flux-limited diffusion and the Sn-method).

    The neutrino-heating leads to a very strong baryonic wind along to original binary rotation axis. The mass loss (per time and solid angle) is shown in the following figures:


  • A movie of the emerging wind (courtesy Luc Dessart) can found here (click image):


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