Publications

Rodrigo Fernández, Sherwood Richers, Nicole Mulyk, Steven Fahlman

[11/2022] Physical Review D

We examine the effect of neutrino flavor transformation by the fast flavor instability (FFI) on long-term mass ejection from accretion disks formed after neutron star mergers. Neutrino emission and absorption in the disk set the composition of the disk ejecta, which subsequently undergoes r -process nucleosynthesis upon expansion and cooling. Here we perform 28 time-dependent, axisymmetric, viscous-hydrodynamic simulations of accretion disks around hypermassive neutron stars (HMNSs) of variable lifetime, using a 3-species neutrino leakage scheme for emission and an annular-lightbulb scheme for absorption. We include neutrino flavor transformation due the FFI in a parametric way, by modifying the absorbed neutrino fluxes and temperatures, allowing for flavor mixing at various levels of flavor equilibration, and also in a way that aims to respect the lepton-number preserving symmetry of the neutrino self-interaction Hamiltonian. We find that for a promptly-formed black hole (BH), the FFI lowers the average electron fraction of the disk outflow due to a decrease in neutrino absorption, driven primarily by a drop in electron neutrino/antineutrino flux upon flavor mixing. For a long-lived HMNS, the disk emits more heavy lepton neutrinos and reabsorbs more electron neutrinos than for a BH, with a smaller drop in flux compensated by a higher neutrino temperature upon flavor mixing. The resulting outflow has a broader electron fraction distribution, a more proton-rich peak, and undergoes stronger radiative driving. Disks with intermediate HMNS lifetimes show results that fall in between these two limits. In most cases, the impact of the FFI on the outflow is moderate, with changes in mass ejection, average velocity, and average electron fraction of order ∼10 %, and changes in the lanthanide/actinide mass fraction of up to a factor ∼2 .

Sherwood Richers

[10/2022] Physical Review D

Neutrinos can rapidly change flavor in the inner dense regions of core-collapse supernovae and neutron star mergers due to the neutrino fast flavor instability. If the amount of flavor transformation is significant, the fast flavor instability (FFI) could significantly affect how supernovae explode and how supernovae and mergers enrich the universe with heavy elements. Since many state of the art supernova and merger simulations rely on neutrino transport algorithms based on angular moments of the radiation field, there is incomplete information with which to determine if the distributions are unstable to the FFI. In this work we test the performance of several proposed moment-based instability tests in the literature. We perform time-independent general relativistic neutrino transport on a snapshot of a 3D neutron star merger simulation to generate reasonable neutrino distributions and check where each of these criteria correctly predict instability. In addition, we offer a new "maximum entropy" instability test that is somewhat more complex, but offers more detailed (though still approximate) estimates of electron lepton number crossing width and depth. We find that this maximum entropy test and the resonant trajectory test are particularly accurate at predicting instability in this snapshot, though all tests predict instability where significant flavor transformation is most likely.

Sherwood Richers, Huaiyu Duan, Meng-Ru Wu, Soumya Bhattacharyya, Masamichi Zaizen, Manu George, Chun-Yu Lin, Zewei Xiong

[08/2022] Physical Review D

The fast flavor instability (FFI) is expected to be ubiquitous in core-collapse supernovae and neutron star mergers. It rapidly shuffles neutrino flavor in a way that could impact the explosion mechanism, neutrino signals, mass outflows, and nucleosynthesis. The variety of initial conditions and simulation methods employed in simulations of the FFI prevent an apples-to-apples comparison of the results. We simulate a standardized test problem using five independent codes and verify that they are all faithfully simulating the underlying quantum kinetic equations under the assumptions of axial symmetry and homogeneity in two directions. We quantify the amount of numerical error in each method and demonstrate that each method is superior in at least one metric of this error. We make the results publicly available to serve as a benchmark. 

Evan, Grohs, Sherwood Richers, Sean Couch, Francois Foucart, James Kneller, Gail McLaughlin

[07/2022] arXiv

The flavor evolution of neutrinos in core collapse supernovae and neutron star mergers is a critically important unsolved problem in astrophysics. Following the electron flavor evolution of the neutrino system is essential for calculating the thermodynamics of compact objects as well as the chemical elements they produce. Accurately accounting for flavor transformation in these environments is challenging for a number of reasons, including the large number of neutrinos involved, the small spatial scale of the oscillation, and the nonlinearity of the system. We take a step in addressing these issues by presenting a method which describes the neutrino fields in terms of angular moments. Our moment method successfully describes the fast flavor neutrino transformation phenomenon which is expected to occur in regions close to the central object. We apply our moment method to neutron star merger conditions and show that we are able to capture the three phases of growth, saturation, and decoherence by comparing with particle-in-cell calculations. We also determine the size of the growing fluctuations in the neutrino field.

McKenzie Myers, Theo Cooper, MacKenzie Warren, Jim Kneller, Gail McLaughlin, Sherwood Richers, Evan Grohs, Carla Froehlich

[06/2022] Physical Review D

The successful transition from core-collapse supernova simulations using classical neutrino transport to simulations using quantum neutrino transport will require the development of methods for calculating neutrino flavor transformations that mitigate the computational expense. One potential approach is the use of angular moments of the neutrino field, which has the added appeal that there already exist simulation codes which make use of moments for classical neutrino transport. Evolution equations for quantum moments based on the quantum kinetic equations can be straightforwardly generalized from the evolution of classical moments based on the Boltzmann equation. We present an efficient implementation of neutrino transformation using quantum angular moments in the free streaming, spherically symmetric bulb model. We compare the results against analytic solutions and the results from more exact multi-angle neutrino flavor evolution calculations. We find that our moment-based methods employing scalar closures predict, with good accuracy, the onset of collective flavor transformations seen in the multi-angle results. However, they over-estimate the coherence between neutrinos traveling along different trajectories in tests that neglect neutrino-neutrino interactions. More sophisticated quantum closures may improve the agreement between the inexpensive moment-based methods and the multi-angle approach.

Sherwood A. Richers, Donald E. Willcox, Nicole M. Ford

[11/2021] Physical Review D

Neutrino flavor instabilities have the potential to shuffle neutrinos between electron, mu, and tau flavor states, possibly modifying the core-collapse supernova mechanism and the heavy elements ejected from neutron star mergers. Analytic methods indicate the presence of so-called fast flavor transformation instabilities, and numerical simulations can be used to probe the nonlinear evolution of the neutrinos. Simulations of the fast flavor instability to date have been performed assuming imposed symmetries. We perform simulations of the fast flavor instability that include all three spatial dimensions and all relevant momentum dimensions in order to probe the validity of these approximations. If the fastest growing mode has a wave number along a direction of imposed symmetry, then the instability can be suppressed. The late-time equilibrium distribution of flavor, however, seems to be little affected by the number of spatial dimensions. This is a promising hint that the results of lower-dimensionality simulations to date have predictions that are robust against their the number of spatial dimensions, though simulations of a wider variety of neutrino distributions need to be carried out to support this claim more generally. 

Sherwood A. Richers, Donald E. Willcox, Nicole M. Ford, Andrew T. Myers

[4/2021] Physical Review D

Neutrinos drive core-collapse supernovae, launch outflows from neutron star merger accretion disks, and set the ratio of protons to neutrons in ejecta from both systems that generate heavy elements in the universe. Neutrinos of different flavors interact with matter differently, and much recent work has suggested that fast flavor instabilities are likely ubiquitous in both systems, but the final flavor content after the instability saturates has not been well understood. In this work we present particle-in-cell calculations which follow the evolution of all flavors of neutrinos and antineutrinos through saturation and kinematic decoherence. We conduct one-dimensional three-flavor simulations of neutrino quantum kinetics to demonstrate the outcome of this instability in a few example cases. We demonstrate the growth of both axially symmetric and asymmetric modes whose wavelength and growth rate match predictions from linear stability analysis. Finally, we vary the number density, flux magnitude, and flux direction of the neutrinos and antineutrinos and demonstrate that these factors modify both the growth rate and post-saturation neutrino flavor abundances. Weak electron lepton number (ELN) crossings in these simulations produce both slow growth of the instability and little difference between the flavor abundances in the initial and final states. In all of these calculations the same number of neutrinos and antineutrinos change flavor, making the least abundant between them the limiting factor for post-saturation flavor change. Many more simulations and multi-dimensional simulations are needed to fully probe the parameter space of the initial conditions.

Sherwood A. Richers

[10/2020] Physical Review D

Many modern simulations of accretion disks use moment-based methods for radiation transport to determine the thermal evolution of the disk and the properties of the ejected matter. The popular M1 scheme that evolves the rank-0 and rank-1 moments requires an analytic approximation for the rank-2 and higher tensors. We present the open-source Monte Carlo steady-state general-relativistic neutrino transport code SedonuGR, which we use to assess fundamental analytic closure assumptions, quantify proposed closure errors, and test an extension of the maximum entropy Fermi-Dirac (MEFD) closure to the rank-3 moment. We demonstrate that the fundamental assumptions employed in all analytic closures are strongly violated. This violation is most evident at the interface between the equatorial disk and the evacuated polar regions. Finally, we calculate the neutrino momentum and energy deposition rate from neutrino pair annihilation, and demonstrate that a moment-based annihilation power calculation is accurate to at most ∼20% if the rank-2 and higher moments are neglected. Out of a selection of eight closures in the literature, we demonstrate that different closures reproduce different aspects of the radiation field (pressure tensor, rank-3 tensor, pair annihilation rate), though the MEFD, Levermore, and Janka 2 closures are all reasonable. The extra information from the neutrino degeneracy used in the MEFD closure is unable to account for the diversity in the rank-2 and rank-3 moments.

Sherwood A. Richers, Gail C. McLaughlin, James P. Kneller, Alexey Vlasenko

[6/2019] Physical Review D (Errata)

Neutrinos play a critical role of transporting energy and changing the lepton density within core-collapse supernovae and neutron star mergers. The quantum kinetic equations (QKEs) combine the effects of neutrino-matter interactions treated in classical Boltzmann transport with the neutrino flavor-changing effects treated in neutrino oscillation calculations. We present a method for extending existing neutrino interaction rates to full QKE source terms for use in numerical calculations. We demonstrate the effects of absorption and emission by nucleons and nuclei, electron scattering, electron-positron pair annihilation, nucleon-nucleon bremsstrahlung, neutrino-neutrino scattering. For the first time, we include all these collision terms self-consistently in a simulation of the full isotropic QKEs in conditions relevant to core-collapse supernovae and neutron star mergers. For our choice of parameters, the long-term evolution of the neutrino distribution function proceeds similarly with and without the oscillation term, though with measurable differences. We demonstrate that electron scattering, nucleon-nucleon bremsstrahlung processes, and four-neutrino processes dominate flavor decoherence in the protoneutron star (PNS), absorption dominates near the shock, and all of the considered processes except elastic nucleon scattering are relevant in the decoupling region. Finally, we propose an effective decoherence opacity that at most energies predicts decoherence rates to within a factor of 10 in our model PNS and within 20% outside of the PNS.

We perform an extensive study of the influence of nuclear weak interactions on core-collapse supernovae, paying particular attention to consistency between nuclear abundances in the equation of state (EOS) and nuclear weak interactions. To quantify the impact of a consistent treatment of nuclear abundances on CCSN dynamics, we carry out spherically symmetric CCSN simulations with full Boltzmann neutrino transport, systematically changing the treatment of weak interactions, EOSs, and progenitor models. We find that the inconsistent treatment of nuclear abundances between the EOS and weak interaction rates weakens the EOS dependence of both the dynamics and neutrino signals.

The mechanism driving core-collapse supernovae is sensitive to the interplay between matter and neutrino radiation. However, neutrino radiation transport is very difficult to simulate, and several radiation transport methods of varying levels of approximation are available. We carefully compare for the first time in multiple spatial dimensions the discrete ordinates (DO) code of Nagakura, Yamada, and Sumiyoshi and the Monte Carlo (MC) code Sedonu, under the assumptions of a static fluid background, flat spacetime, elastic scattering, and full special relativity. We find remarkably good agreement in all spectral, angular, and fluid interaction quantities, lending confidence to both methods. The DO method excels in determining the heating and cooling rates in the optically thick region. The MC method predicts sharper angular features due to the effectively infinite angular resolution, but struggles to drive down noise in quantities where subtractive cancellation is prevalent, such as the net gain in the protoneutron star and off-diagonal components of the Eddington tensor.

We determine the effects neutrinos have on the accretion disks that form after two neutron tars merge using Monte Carlo methods, and release the source code, too! We see neutrinos being guided out of the system along the upper and lower edges of the disk. We also show that the disk cools and changes the ratio of protons to neutrons much more quickly than was previously calculated. Finally, we calculate the amount of energy deposited by neutrino pair annihilation to be at most 2e48 erg, which is likely not enough to drive a GRB jet. 

We simulate the collapse of a 25 solar mass rapidly rotating, strongly magnetized star using general relativistic magnetohydrodynamics (GRMHD), a realistic nuclear equation of state, and approximate neutrino effects in three dimensions (3D). In the past, there have been quite a few 2D simulations that seem to say this kind of system will generate two jets that just blast through either side of the star. We were able to show, for the first time, that the jets in these 2D simulation actually fail and wrinkle up into lobes filled with magnetic flux tubes when simulated in 3D. But a picture is worth a thousand words, and these movies are made from thousands of pictures, so check them out! 

• (YouTube) - volume rendering of entropy 

• (YouTube) - volume rendering of the plasma beta parameter 

• (YouTube) - entropy slices comparing 2D to 3D 

We perform simulations of accretion disks around black holes using the ATHENA code to try to determine how much resolution is required to attain a form of numerical convergence, and how other algorithmic differences affect the convergence properties. 

We use two-frequency VLBI observations to get an extremely high-resolution spectral index map of the active galactic nucleus in Centaurus A, which allows us to localize possible sources of the high-energy radiation.