Evaluating radiation transport errors in merger simulations using a Monte Carlo algorithm

Abstract

Neutrino-matter interactions play an important role in the postmerger evolution of neutron star-neutron star and black hole-neutron star mergers. Most notably, they determine the properties of the bright optical/ infrared transients observable after a merger. Unfortunately, Boltzmann's equations of radiation transport remain too costly to be evolved directly in merger simulations. Simulations rely instead on approximate transport algorithms with unquantified modeling errors. In this paper, we use for the first time a timedependent general relativistic Monte Carlo (MC) algorithm to solve Boltzmann's equations and estimate important properties of the neutrino distribution function ∼10 ms after a neutron star merger that resulted in the formation of a massive neutron star surrounded by an accretion disk. We do not fully couple the MC algorithm to the fluid evolution, but use a short evolution of the merger remnant to critically assess errors in our approximate gray two-moment transport scheme. We demonstrate that the analytical closure used by the moment scheme is highly inaccurate in the polar regions, but performs well elsewhere. While the average energy of polar neutrinos is reasonably well captured by the two-moment scheme, estimates for the neutrino energy become less accurate at lower latitudes. The two-moment formalism also overestimates the density of neutrinos in the polar regions by ∼50%, and underestimates the neutrino pair-annihilation rate at the poles by factors of 2-3. Although the latter is significantly more accurate than one might have expected before this study, our results indicate that predictions for the properties of polar outflows and for the creation of a baryon-free region at the poles are likely to be affected by errors in the two-moment scheme, thus limiting our ability to reliably model kilonovae and gamma-ray bursts.

Neutron star merger remnants

“…In the M1 scheme, the moments representation of the Boltzmann equation is truncated at the second moment and a closure interpolating between the thin and thick regime is imposed to the equations. The compact binaries simulations performed so far with this scheme are performed in the gray regime [122,214,279]. The leakage scheme is an approximation to the transport problem that accounts for changes to the lepton number and for the loss of energy due to the emission of neutrinos.…”

“…The neutrino wind originates on the disc edge, close to the neutrinosphere, and above the remnant where baryon pollution is minimal. Note that a precise prediction of properties of polar ejecta is presently beyond the possibilities of neutrinos schemes employed in ab-initio NR simulations [43,45,184,214].…”

Distinguishing the nature of comparable-mass neutron star binary systems with multimessenger observations: GW170817 case study

“…In all simulations, a small amount of cold, neutron-rich material is dynamically ejected in the equatorial plane by the merger: 0.002 M ⊙ (0.004 M ⊙ ) for NS-NS and <0.001 M ⊙ for NS-BH binaries. Less neutron-rich polar ejecta is observed, but in the absence of magnetic fields its mass is negligible (and not resolved in the simulations); see [44]. Note that none of our simulations produce a relativistic jet, e.g., as observed for GW170817 [45,46], which is unsurprising as our simulations do not include any magnetohydrodynamics (MHD) effects (see [47] for incipient jets in a NS-BH simulation).…”

Improved leakage-equilibration-absorption scheme (ileas) for neutrino physics in compact object mergers

“…ILEAS is not only computationally very efficient but also appears to be competitive concerning its accuracy compared to other forefront developments of neutrino transport treatments for CO mergers and their remnants. The work by Foucart et al (2018) underlines that M1 solutions have their own shortcomings when applied to the highly aspherical environments of merger remnants. For this reason our tests with BH-torus systems, comparing ILEAS to M1 results from the ALCAR code, cannot be considered as finally conclusive regarding the accuracy of ILEAS.…”

“…We refrain from performing a comparison of the rates immediately above the BH and in the close vicinity of the z-axis, because a consistent treatment of general relativistic and special relativistic effects would be needed to describe the influence of the BH or ultrarelativistic GRB jets. Furthermore, Foucart et al (2018), for example, compared Monte Carlo results and M1 results in the context of a HMNS surrounded by a torus and pointed out that the inexact M1 closure strongly overestimates the number density in the polar regions, by ∼50 per cent for νe andνe, which leads to significant boosting of the absorption rates by charged-current reactions and excess heating. Just et al (2015a) also reported similar behaviour when comparing BH-torus calculations with a ray-tracing Boltzmann solver against their M1 results.…”