Bulletin of the American Physical Society
APS April Meeting 2021
Volume 66, Number 5
Saturday–Tuesday, April 17–20, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session G09: Compact Object Mergers: Neutron StarsLive
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Sponsoring Units: DAP Chair: Eric Burns, LSU |
Sunday, April 18, 2021 8:30AM - 8:42AM Live |
G09.00001: Long Term 3D-MHD Simulations of Neutron Star Merger Accretion Tori with Realistic Microphysics Steven Fahlman, Rodrigo Fernandez We examine the long-term evolution of accretion tori around the black hole remnants of compact object mergers involving at least one neutron star, to better understand the role of secular outflows in the creation of kilonovae and the synthesis of r-process elements. We modify the FLASH code to evolve magnetohydrodynamics in non-uniform 3D spherical coordinates, enabling efficient evolution of magnetic fields over large simulation domains. Gravity is implemented as a pseudo-Newtonian potential. We include neutrino evolution via an improved lightbulb/leakage scheme and take into account nuclear recombination of $\alpha$-particles in the equation of state. With this new framework, we evolve post-merger systems of tori around black holes and examine the outflows. We find results broadly consistent with general relativistic simulations. Magnetically driven outflows unbind a significant fraction of torus mass over a few seconds, with velocities $\sim 0.1c$ and average electron fractions favouring lanthanide-rich ejecta. Ejected torus mass is negatively correlated with system compactness. The fraction of mass with $Y_e > 0.25$ is insufficient to explain the blue kilonova of GW170817 based on current kilonovae models. [Preview Abstract] |
Sunday, April 18, 2021 8:42AM - 8:54AM Live |
G09.00002: Igniting weak interactions in neutron-star post-merger accretion disks Soumi De, Daniel Siegel The merger of two neutron stars or of a neutron star and a black hole typically results in the formation of a post-merger accretion disk. Outflows from disks may dominate the overall ejecta from mergers and be a major source of r-process nuclei in our universe. We explore the parameter space of such disks, their outflows, and r-process yields by performing three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations with weak interactions and approximate neutrino transport. We discuss the mapping between initial binary parameters and the parameter space of resulting disks, chiefly characterized by their initial accretion rate. We demonstrate the existence of an ignition threshold for weak interactions in the parameter space separating a neutrino-cooled regime and an advection dominated regime. While neutrino-cooled disks can produce the entire range of r-process elements in good agreement with the observed solar system abundances, advection heated disks show suppressed production of light r-process elements. With gravitational-wave detectors starting to sample the neutron star merger parameter space, our disk realizations provide a suite of templates that can be matched with future observations via their associated kilonova emission. [Preview Abstract] |
Sunday, April 18, 2021 8:54AM - 9:06AM Live |
G09.00003: Fast ejecta as a potential way to distinguish neutron stars from black holes in high-mass mergers Elias Most, Jens Papenfort, Samuel Tootle, Luciano Rezzolla Gravitational wave events involving very massive neutron stars, such as GW190425, have just started to be detected. Although typically classified as binary neutron star mergers, the observed gravitational-wave signal is usually not able to clearly establish a neutron-star nature of the massive primary object in the system. Thus, a black hole--neutron star system cannot be fully ruled out by the gravitational wave detection alone. In this talk, I will show how early fast ejecta -- only produced in binary neutron star mergers -- can potentially resolve this question and shed light on the nature of the binary system. By comparing simulations of binary neutron star and black hole -- neutron star mergers of exactly the same masses and spins, I will show that such fast ejecta are entirely absent, if the primary is a black hole. Because our simulations indicate that the mass ejecta and accretion disks produced in the merger are comparable in both cases, the presence of fast ejecta might be the only distinguishing feature present in the electromagnetic afterglow accompanying such a gravitational wave event. [Preview Abstract] |
Sunday, April 18, 2021 9:06AM - 9:18AM Live |
G09.00004: Resolving Fast Ejecta from Binary Neutron Star Mergers using the Grid-Based Code FLASH Coleman Dean, Rodrigo Fernández We simulate relativistic ejecta from the contact interface of a Binary Neutron Star (BNS) merger at the highest resolution to date. The fastest component of this ejecta is predicted to freeze out the r-process and power an ultraviolet (UV) precursor to the kilonova through the radioactive decay of free neutrons. The amount of this fast ejecta produced in particle-based and grid-based hydrodynamic merger simulations is not consistent between methods, with grid codes producing orders of magnitude less fast outflows than particle codes. Here we investigate whether this deficit of fast ejecta in grid codes is due to insufficient spatial resolution, by simulating BNS mergers in a two dimensional co-rotating cylindrical coordinate system. We use self-gravitating Newtonian hydrodynamics, a gravitational wave emission based inspiral rate, and a piecewise polytropic equation of state with approximate thermal effects. This allows us to achieve a minimum grid spacing of 4 m, which resolves the pressure scale height of the stellar surface regions, where the fastest ejecta is produced upon contact. We discuss the implications of our results for the existence of prompt UV counterparts of kilonovae. [Preview Abstract] |
Sunday, April 18, 2021 9:18AM - 9:30AM Live |
G09.00005: Interpolating Detailed Simulations of Kilonovae Marko Ristic, Benjamin Champion, Richard O'Shaughnessy, Ryan Wollaeger, Oleg Korokbin, Eve Chase, Christopher Fryer, Aimee Hungerford, Christopher Fontes Starting with a grid of 2D anisotropic simulations of kilonova light curves covering a wide range of ejecta properties, we apply adaptive-learning techniques to iteratively choose new simulations and produce high-fidelity surrogate models for those simulations. These surrogate models allow for continuous evaluation across model parameters while retaining the microphysical details about the ejecta. We demonstrate how to use our interpolated models to infer kilonova parameters. With our model, we estimate the ejecta responsible for the emission associated with GW170817. In the future, we plan to apply our methods to more physically complex kilonova simulations for a deeper understanding of neutron star merger ejecta properties. [Preview Abstract] |
Sunday, April 18, 2021 9:30AM - 9:42AM Live |
G09.00006: Multidimensional Radiative Transfer Models of Kilonovae Oleg Korobkin, Ryan Wollaeger, Christopher Fryer, Aimee Hungerford, Stephan Rosswog, Christopher Fontes, Matthew Mumpower, Eve Chase, Wesley Even, Jonah Miller, Wendell Misch, Jonas Lippuner Observations of neutron star merger kilonovae such as the one associated with GW1708917 could be fit by a number of simulations that qualitatively agree, but can quantitatively differ (e.g. in total r-process mass) by an order of magnitude. Here we categorize kilonova ejecta into several typical morphologies motivated by numerical simulations, and apply a radiative transfer Monte Carlo code to study how the shape of the ejecta affects the emission. We find major impacts on both spectra and light curves. The peak bolometric luminosity can vary by two orders of magnitude and the timing of its peak by a factor of five. Moreover, different combinations of two-component morphologies lead to a highly diverse set of light curves. We find various radiative multicomponent effects influencing the spectra of kilonovae, such as blocking, flux redirection and double reprocessing. We identify geometry-dependent P Cygni features in late spectra that directly map to strong lines in the simulated opacity of Nd, which not only constrains the ejecta geometry but also allows the direct probing of the r-process abundances. [Preview Abstract] |
Sunday, April 18, 2021 9:42AM - 9:54AM Live |
G09.00007: Constraining the Properties of Kilonovae based on the Zwicky Transient Facility Searches for 13 Neutron Star Mergers Priyadarshini Rajkumar, Michael Coughlin, Shreya Anand, Siddharth Mohite In their third observing run (O3), LIGO and Virgo detected gravitational-wave (GW) candidates from several neutron star-black hole (NSBH) and binary neutron star (BNS) mergers. The Zwicky Transient Facility (ZTF), an optical time-domain survey telescope, followed-up thirteen of these GW events in search of kilonovae (KNe; electromagnetic counterparts to GW events). However, no KNe were found. To assess the implications on potential KN emission based on the upper limits, we determined empirical limits on the KN peak magnitude and evolution rate. In this work, we present a novel Bayesian statistical framework to derive more realistic constraints on kilonova parameters. To exclude improbable regions of the parameter space, we weight these constraints with priors informed by radiative-transport based KN models, parameterized by ejecta mass and inclination angle. Using ZTF observations of GW190425, we compare the Bayesian non-detection constraints with our previous empirical limits. Finally, we close with an application of this methodology to derive constraints on KN ejecta masses. [Preview Abstract] |
Sunday, April 18, 2021 9:54AM - 10:06AM Live |
G09.00008: 3D Simulations of the Fast Flavor Instability Sherwood Richers, Don Willcox, Nicole Ford, Andrew Myers Neutrinos transport energy, drive outflows, and determine the ratio of electrons to protons in the ejecta from neutron star mergers that enriches the surrounding universe with heavy elements. Only electron flavor neutrinos and antineutrinos modify the composition of the ejecta and thus directly affect the synthesis of heavy elements. Instabilities that cause rapid mixing of neutrino flavor are likely ubiquitous in mergers, but are poorly understood and are absent from global simulations. I will present the first (local) three-dimensional simulations of this instability using the new open-source particle-in-cell neutrino quantum kinetics code Emu. I will demonstrate that the growth phase of the instability matches theoretical predictions, describe abundances of each neutrino species after the instability saturates, and discuss the implications for nucleosynthesis in neutron star mergers. [Preview Abstract] |
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