A combined multimessenger analysis: Neutron stars are stellar remnants with densities greater than that of an atomic nucleus. The properties of matter under such extreme conditions are poorly understood and inaccessible to terrestrial laboratories. Dietrich et al. developed a framework to combine multiple constraints on the masses and radii of neutron stars, including data from gravitational waves, electromagnetic observations, and theoretical nuclear physics calculations. They used this analysis to constrain the neutron-star equation of state and also improved the precision on the gravitational wave (standard siren) measurement of the Hubble constant—the expansion rate of the Universe. Science , this issue p. 1450

Interpreting high-energy, astrophysical phenomena, such as supernova explosions or neutron-star collisions, requires a robust understanding of matter at supranuclear densities. However, our knowledge about dense matter explored in the cores of neutron stars remains limited. Fortunately, dense matter is not probed only in astrophysical observations, but also in terrestrial heavy-ion collision experiments. Here we use Bayesian inference to combine data from astrophysical multi-messenger observations of neutron stars^1–9 and from heavy-ion collisions of gold nuclei at relativistic energies^10,11 with microscopic nuclear theory calculations^12–17 to improve our understanding of dense matter. We find that the inclusion of heavy-ion collision data indicates an increase in the pressure in dense matter relative to previous analyses, shifting neutron-star radii towards larger values, consistent with recent observations by the Neutron Star Interior Composition Explorer mission^5–8,¹⁸. Our findings show that constraints from heavy-ion collision experiments show a remarkable consistency with multi-messenger observations and provide complementary information on nuclear matter at intermediate densities. This work combines nuclear theory, nuclear experiment and astrophysical observations, and shows how joint analyses can shed light on the properties of neutron-rich supranuclear matter over the density range probed in neutron stars.

We present possis, a time-dependent three-dimensional Monte Carlo code for modelling radiation transport in supernovae and kilonovae. The code incorporates wavelength- and time-dependent opacities, and predicts viewing-angle dependent spectra, light curves, and polarization for both idealized and hydrodynamical explosion models. We apply the code to a kilonova model with two distinct ejecta components, one including lanthanide elements with relatively high opacities and the other devoid of lanthanides and characterized by lower opacities. We find that a model with total ejecta mass $M_\mathrm{ej}=0.04\, \mathrm{M}_\odot$ and half-opening angle of the lanthanide-rich component Φ = 30° provides a good match to GW 170817/AT 2017gfo for orientations near the polar axis (i.e. for a system viewed close to face-on). We then show how crucial is the use of self-consistent multidimensional models in place of combining one-dimensional models to infer important parameters, such as the ejecta masses. We finally explore the impact of Mej and Φ on the synthetic observables and highlight how the relatively fast computation times of possis make it well-suited to perform parameter-space studies and extract key properties of supernovae and kilonovae. Spectra calculated with possis in this and future studies will be made publicly available.

Spectroscopic detection of narrow emission lines traces the presence of circumstellar mass distributions around massive stars exploding as core-collapse supernovae. Transient emission lines disappearing shortly after the supernova explosion suggest that the material spatial extent is compact and implies an increased mass loss shortly prior to explosion. Here, we present a systematic survey for such transient emission lines (Flash Spectroscopy) among Type II supernovae detected in the first year of the Zwicky Transient Facility survey. We find that at least six out of ten events for which a spectrum was obtained within two days of the estimated explosion time show evidence for such transient flash lines. Our measured flash event fraction (>30% at 95% confidence level) indicates that elevated mass loss is a common process occurring in massive stars that are about to explode as supernovae.