Secondary-Mass Features improve Spectral-Siren $H_0$ Constraints

Abstract: Gravitational-wave (GW) signals from compact binary coalescences (CBCs) enable independent measurements of the Hubble constant (H_0) via the spectral siren method, which critically depends on an accurate model of the source-frame mass distribution. While the primary mass function has been extensively studied, the impact of the secondary mass distribution on cosmological inference has been largely overlooked. Here, we perform a joint inference of population and cosmological parameters using 142 confident CBC detections from GWTC-4.0, adopting a new parametric model that flexibly describes features in both the component-mass spectrum and the pairing function, with particular emphasis on the secondary masses. We find (H_0 = 71.4^{+13.8}_{-13.4} \;\mathrm{km\,s^{-1}\,Mpc^{-1}}) (68\% CL) from spectral sirens alone, and (H_0 = 73.5^{+9.2}_{-7.2} \;\mathrm{km\,s^{-1}\,Mpc^{-1}}) when combined with the bright siren GW170817. Compared to the standard LVK Fullpop-4.0 analysis, these constraints represent improvements of (\sim29.8\%) and (\sim22.2\%) in (H_0) uncertainty, respectively. The enhanced precision is driven by previously unmodeled features, including peaks near (18\,M_\odot) and (65\,M_\odot) as well as mass-dependent pairing transitions at (28\,M_\odot) and (52\,M_\odot). Our results demonstrate that the secondary mass function is also a key ingredient for precision standard siren cosmology.