Abstract
New viscous neutrino-radiation hydrodynamics simulations are performed for accretion disks surrounding a spinning black hole with low mass $3M_\odot$ and dimensionless spin 0.8 or 0.6 in full general relativity, aiming at modeling the evolution of a merger remnant of massive binary neutron stars or low-mass black hole-neutron star binaries. We reconfirm the following results found by previous studies of other groups: 15-30% of the disk mass is ejected from the system with the average velocity of $\sim $5-10% of the speed of light for the plausible profile of the disk as merger remnants. In addition, we find that for the not extremely high viscous coefficient case, the neutron richness of the ejecta does not become very high, because weak interaction processes enhance the electron fraction during the viscous expansion of the disk before the onset of the mass ejection, resulting in the suppression of the lanthanide synthesis. For high-mass disks, the viscous expansion timescale is increased by a longer-term neutrino emission, and hence, the electron fraction of the ejecta becomes even higher. We also confirm that the mass distribution of the electron fraction depends strongly on the magnitude of the given viscous coefficient. This demonstrates that a first-principle magnetohydrodynamics simulation is necessary for black hole-disk systems with sufficient grid resolution and with sufficiently long timescale (longer than seconds) to clarify the nucleosynthesis and electromagnetic signals from them.
Highlights
The first direct detection of gravitational waves from the final stage of an inspiraling binary neutron star system (GW170817) by advanced LIGO and advanced VIRGO [1] was accompanied with a wide variety of the follow-up observations of electromagnetic counterparts [2]
Numerical computations are performed for the black hole–disk systems summarized in the previous section
Our present results suggest that even for the ejecta from a black hole–disk system, the lanthanide fraction could be minor, in contrast to the previous understanding (e.g., Refs. [10,11,14]), if the mass ejection sets in for ≳0.3 s after the merger: A kilonova associated with the ejecta from black hole–disk systems may shine in an optical band for the early time after the merger in contrast to the previous belief [58]
Summary
The first direct detection of gravitational waves from the final stage of an inspiraling binary neutron star system (GW170817) by advanced LIGO and advanced VIRGO [1] was accompanied with a wide variety of the follow-up observations of electromagnetic counterparts [2]. This event heralded the opening of the era of multimessenger astronomy composed of gravitational-wave and electromagnetic-counterpart observations, and it demonstrated. That the observation of electromagnetic signals plays a key role for understanding the merger and subsequent mass ejection processes of neutron-star binaries, which cannot be understood only from gravitational-wave observation. A neutron-rich material, which can synthesize heavy r-process elements, was ejected in the dynamical process, while in
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