2D End-to-End Modeling of Kilonovae from Binary Neutron Star Merger Remnants

TL;DR

This work develops an end-to-end kilonova modeling pipeline that connects 3D GRMHD simulations of HMNS winds to 2D FLASH hydrodynamics and Sedona radiative transfer to predict viewing-angle dependent light curves and spectra. By testing three remnant lifetimes (12 ms, 240 ms, continuous) and two $r$-process heating rates (H3, H4) with $Y_e$-based compositions, the study shows longer-lived remnants yield more massive ejecta and stronger angular dependencies, with blue kilonova emission possible when the HMNS survives long enough. The results underscore the importance of self-consistent, long-timescale modeling and angle-dependent observables for interpreting kilonova observations, and demonstrate a computationally efficient path toward linking high-fidelity merger simulations with radiative transfer. The pipeline lays groundwork for future 3D extensions and inclusion of additional ejecta components (tidal and disk winds) to more fully reproduce events like AT2017gfo.

Abstract

We investigate the kilonova emission resulting from outflows produced in a three-dimensional (3D) general-relativistic magnetohydrodynamic (GRMHD) simulation of a hypermassive neutron star (HMNS) remnant. We map the outflows into the FLASH hydrodynamics code to model their expansion in axisymmetry, and study the effects of employing different $r$-process heating rates. Except for the highest heating rate prescription, we find no significant differences with respect to overall ejecta dynamics and morphology compared to the simulation without heating. Once homologous expansion is attained, typically after $\sim$ 2s for these ejecta, we map the outflows to the Sedona radiative transfer code and compute the spectral evolution of the kilonova and broadband light curves in various Legacy Survey of Space and Time (LSST) bands. The kilonova properties depend on the remnant lifetime, with peak luminosities and peak timescales increasing for longer-lived remnants that produce more massive ejecta. For all models, there is a strong dependence of both the bolometric and broadband light curves on the viewing angle. While the short-lived (12ms) remnant produces higher luminosities when viewed from angles closer to the pole, longer-lived remnants (240ms and 2.5s) are more luminous when viewed from angles closer to the equator. Our results highlight the importance of self-consistent, long-term modeling of merger ejecta, and taking viewing-angle dependence into account when interpreting observed kilonova light curves. We find that magnetized outflows from a HMNS -- if it survives long enough -- could explain blue kilonovae, such as the blue emission seen in AT2017gfo.

Figures (18)

• Figure 1: Sketch of the merger remnant, showing outflows (left) and their associated (non-)thermal emission components (right), including red and blue kilonova emission.
• Figure 2: Schematic of the simulation pipeline. A 3D GRMHD simulation models the binary neutron star merger from the hypermassive remnant phase up to black hole formation at 12ms post-mapping. The resulting outflow is angle-averaged to 2D and used as input for the FLASH hydrodynamics code under three ejecta injection scenarios: 12ms cutoff, 240ms injection, and continuous inflow. Outputs are post-processed with the radiative transfer code Sedona to compute light curves and spectra.
• Figure 3: Evolution of the ejecta in the 12ms injection scenario with heating mode H3, shown at three different times: 12ms (left column), 0.5 s (middle), and 2.5 s (right). Each row corresponds to a different quantity: mass density (top), radial velocity (middle), and electron fraction $Y_e$ (bottom). The $v_r$ colorbar ranges from $-0.1c$ to $0.5c$, with negative values indicating fallback, though these are not apparent here because they occur close to the remnant and are obscured due to the large spatial domain used to highlight the ejecta evolution. The spatial domain increases across columns to ensure that key features remain visible, with the final column (2.5 s) covering the full FLASH domain.
• Figure 4: Evolution of the ejecta in the 240ms injection scenario with heating mode H3, shown at three different times: 12ms (left column), 0.5 s (middle), and 2.5 s (right). Each row corresponds to a different quantity: mass density (top), radial velocity (middle), and electron fraction $Y_e$ (bottom). The $v_r$ colorbar ranges from $-0.1c$ to $0.5c$, with negative values indicating fallback, though these are not apparent here because they occur close to the remnant and are obscured due to the large spatial domain used to highlight the ejecta evolution. The spatial domain increases across columns to ensure that key features remain visible, with the final column (2.5 s) covering the full FLASH domain.
• Figure 5: Evolution of the ejecta in the continuous injection scenario with heating mode H3, shown at three different times: 12ms (left column), 0.5 s (middle), and 2.5 s (right). Each row corresponds to a different quantity: mass density (top), radial velocity (middle), and electron fraction $Y_e$ (bottom). The $v_r$ colorbar ranges from $-0.1c$ to $0.5c$, with negative values indicating fallback, though these are not apparent here because they occur close to the remnant and are obscured due to the large spatial domain used to highlight the ejecta evolution. The spatial domain increases across columns to ensure that key features remain visible, with the final column (2.5 s) covering the full FLASH domain.