News and Updates

To detect and characterize gravitational waves from neutron star binaries, LIGO needs good models of all possible signals. Numerical relativity can’t practically be used for every case, but it is needed to test and calibrate the simpler models that LIGO can use. Inspiral waveforms from binaries with neutron stars differ from binary black hole waveforms by the presence of tidal forces. In a recent paper, Tanja Hinderer and collaborators use SXS black hole-neutron star simulations to validate a new model of these tidal forces. They find that tidal effects can be stronger than previously expected when they come close to resonance with a neutron star’s preferred ways of ringing (its normal modes of oscillation).

Matter flung out into space during black hole-neutron star mergers may well be one of the major contributors to the “r-process” heavy-elements in the universe such as gold and lead. To test this idea, we calculated the nuclear reactions taking place in the ejected matter from our black hole-neutron star mergers using the “SkyNet” nucleosynthesis code written by SXS member Jonas Lippuner. Our first studies found that it is very easy for our ejecta to produce the high-mass r-process elements (in abundance ratios not terribly different from what’s in the sun), but very little of the low-mass r-process elements are made, meaning they would have to come from a different source. We then found that this underproduction can be ameliorated (but not removed) by the effect of neutrinos given off by the merger remnant being absorbed by the outgoing matter, which changes its composition. Read the paper here [arXiv:1601.07942].

The SXS collaboration has produced a catalog of about 90 simulations of binary black holes with spins aligned with the orbital angular momentum. We sample systems with both spins co-rotating, one co-rotating and one counter-rotating, or both counter-rotating. We compare these simulations with several waveform models in use by LIGO, and find generally excellent agreement. The papers can be accessed [arXiv:1512.06800] and [arXiv:1601.05396].

CITA Professor Harald Pfeiffer has been awarded a Wilhelm Bessel Research Award of the Alexander von Humboldt Foundation. The award honors Prof. Pfeiffer’s outstanding research record, and invites him to a long-term research stay in Germany. Pfeiffer, who holds the Canada Research Chair in Numerical Relativity and Gravitational Wave Astrophysics, performs research on black holes and Neutron stars. He uses Canadian supercomputers to investigate what happens when such objects collide with each other. Of particular importance is the emission of gravitational waves, ripples in space and time itself, emitted by such collisions. Special-purpose gravitational wave detectors in the U.S., Europe and Japan are searching for these waves, to gain new insights into black holes and Neutron stars that emit gravitational waves, and into any other astrophysical processes that emit gravitational waves. Pfeiffer is also a member of the CITA research group that contributes to analyzing the data of the LIGO, Virgo and GEO gravitational wave detectors, located in the U.S., Italy and Germany, respectively.

The inspiral and merger of binaries composed of two neutron stars are excellent gravitational wave sources and strong candidate explanations for short duration gamma ray bursts. In a series of recent papers, SXS researchers have joined the effort to model these violent events. Nick Tacik and collaborators have been studying the usually-overlooked effect of the spins of the two neutron stars on their final orbits of inspiral. Read the paper here: [arXiv:1508.06986]. Roland Haas, working with many on the SXS team, has carried our first binary neutron star simulation all the way through the collision of the two stars and the collapse of the giant merged neutron star into a black hole. A paper on this simulation is still in the works, but it’s already been used to test a recipe for generating binary neutron star waveforms without full numerical simulations of these events. The paper, by Barkett et al., is available at [arXiv:1509.05782]. The idea is that binary black hole simulations are cheaper than binary neutron star simulations, and one can roughly convert the former into the latter by adding an analytic approximation of the neutron star tidal effects. Lastly, SXS researcher Francois Foucart has carried out merger simulations that study in detail what happens right after two neutron stars merge into a single more-massive neutron star. These post-merger neutron stars ring and give off possibly-detectable gravitational waves. Studying the post-merger mess required us to incorporate realistic nuclear physics and neutrino emission effects into simulations. Read the paper at [arXiv:1510.06398].