Delayed outflows from black hole accretion tori following neutron star binary coalescence

Abstract: Expulsion of neutron-rich matter following the merger of neutron star (NS) binaries is crucial to the radioactively-powered electromagnetic counterparts of these events and to their relevance as sources of r-process nucleosynthesis. Here we explore the long-term (viscous) evolution of remnant black hole accretion disks formed in such mergers by means of two-dimensional, time-dependent hydrodynamical simulations. The evolution of the electron fraction due to charged-current weak interactions is included, and neutrino self-irradiation is modeled as a lightbulb that accounts for the disk geometry and moderate optical depth effects. Over several viscous times (~1s), a fraction ~10% of the initial disk mass is ejected as a moderately neutron-rich wind (Y_e ~ 0.2) powered by viscous heating and nuclear recombination, with neutrino self-irradiation playing a sub-dominant role. Although the properties of the outflow vary in time and direction, their mean values in the heavy-element production region are relatively robust to variations in the initial conditions of the disk and the magnitude of its viscosity. The outflow is sufficiently neutron-rich that most of the ejecta forms heavy r-process elements with mass number A >130, thus representing a new astrophysical source of r-process nucleosynthesis, distinct from that produced in the dynamical ejecta. Due to its moderately high entropy, disk outflows contain a small residual fraction ~1% of helium, which could produce a unique spectroscopic signature.

• The paper demonstrates that viscous heating and nuclear recombination drive outflows ejecting about 10% of the disk mass with an electron fraction around 0.2.
• It employs 2D hydrodynamical simulations integrating weak reactions and neutrino self-irradiation to capture the evolution of post-merger accretion disks.
• The findings refine kilonova models by showing that the expelled, neutron-rich material produces distinct infrared emissions and spectroscopic signatures.

Analyzing the Delayed Outflows from Black Hole Accretion Tori Post Neutron Star Binary Coalescence

The paper, authored by Rodrigo Fernandez and Brian D. Metzger, investigates the long-term evolution of black hole (BH) accretion disks resulting from neutron star (NS) binary mergers. The primary focus is on the conditions conducive to outflows from these accretion tori and the implications for kilonova emissions and $r$-process nucleosynthesis. Using two-dimensional hydrodynamical simulations, the authors integrate weak reactions and model neutrino self-irradiation, while evaluating the impact of viscous heating and nuclear recombination on the ejection of neutron-rich material.

Key Findings

1. Outflow Characteristics: The study highlights that approximately 10% of the initial accretion disk mass is ejected as moderately neutron-rich wind, characterized by an electron fraction ($Y_e$) of about 0.2. This neutron-rich composition supports the formation of heavy $r$-process elements, with mass numbers largely over $A \gtrsim 130$, therefore proposing a distinct, astrophysical source for heavy element synthesis beyond those produced in the merger's dynamical ejecta.
2. Role of Heating Mechanisms: The analysis reveals that the disk outflows are primarily driven by viscous heating and nuclear recombination rather than neutrino-driven winds, indicating dominance of these processes in unbinding matter from the disk. Neutrino irradiation, while integrated as a subdominant factor, plays an insignificant energetic role in most disk conditions explored.
3. Implications for Electromagnetic Counterparts: The expelled disk materials have implications for electromagnetic emissions, notably kilonovae. The ejected matter's composition can affect opacity values, suggesting possible emission in the infrared spectrum, contrary to previous optical-focused models. Additionally, the presence of helium, although minimal, could introduce unique spectroscopic signatures, aiding potential observational separation from purely dynamical ejecta scenarios.

Scientific and Practical Implications

The findings underscore the significance of previously underestimated post-merger conditions in understanding electromagnetic transients and nucleosynthesis in the universe. The robustness of outflow compositions suggests their relatively consistent contribution as an $r$-process nucleosynthesis site, thus supporting binary NS mergers (including BH-NS cases) as key, though not singular, contributors to the galactic chemical evolution concerning heavy elements.

These results are pivotal in refining the models for electromagnetic counterparts following gravitational wave signals, such as those expected from Advanced LIGO/Virgo observatories. Understanding these detailed outflow mechanics allows astronomers to predict better and identify neutron-star merger events through multi-messenger astronomy, by combining gravitational wave detections with electromagnetic signals.

Future Prospects

Looking forward, the research sets a foundational basis for more advanced studies, urging further exploration into the intricate role of neutrino interactions and magnetic fields, which were beyond the scope of the current hydrodynamic model. Additionally, relating the outflow properties directly to observable phenomena like spectral lines will require enhancing the fidelity of current models, perhaps integrating three-dimensional frameworks that account for more complex accretion disk dynamics.

In conclusion, the paper enriches our comprehension of the interaction dynamics following neutron star mergers, significantly advancing the predictive models for nuclear synthesis and kilonova emissions. These insights not only augment our theoretical understanding but also inform ongoing and future observational strategies in the rapidly evolving field of astrophysics.