Standardizing kilonovae and their use as standard candles to measure the Hubble constant

The detection of GW170817 is revolutionizing many areas of astrophysics with the joint observation of gravitational waves and electromagnetic emissions. These multimessenger events provide a new approach to determine the Hubble constant, thus, they are a promising candidate for mitigating the tension between measurements of type-Ia supernovae via the local distance ladder and the cosmic microwave background. In addition to the "standard siren" provided by the gravitational-wave measurement, the kilonova itself has characteristics that allow one to improve existing measurements or to perform yet another, independent measurement of the Hubble constant without gravitational-wave information. Here, we employ standardization techniques borrowed from the type-Ia community and apply them to kilonovae, not using any information from the gravitational-wave signal. We use two versions of this technique, one derived from direct observables measured from the light curve, and the other based on inferred ejecta parameters, e.g., mass, velocity, and composition, for two different models. These lead to Hubble Constant measurements of H₀ = 109⁺⁴⁹₋₃₅ km s⁻¹ Mpc⁻¹ for the measured analysis, and H₀ = 85⁺²²₋₁₇ km ⁻¹ Mpc⁻¹ and H₀ = 79⁺²³₋₁₅ km s⁻¹ Mpc⁻¹ for the inferred analyses. This measurement has error bars within ~2 to the gravitational-wave measurements (H₀ = 74⁺¹⁶₋₈ km s⁻¹ Mpc⁻¹), showing its promise as an independent constraint on H₀.