  Binary black hole (BBH) systems detected via gravitational-wave (GW) emission
are a recently opened astrophysical frontier with many unknowns and
uncertainties. Accurate reconstruction of the binary distribution with as few
assumptions as possible is desirable for inference on formation channels and
environments. Most population analyses have, though, assumed a power law in
binary mass ratio $q$, and/or assumed a universal $q$ distribution regardless
of primary mass. Kernel density estimation (KDE)-based methods allow us to
dispense with such assumptions and directly estimate the joint binary mass
distribution. We deploy a self-consistent iterative method to estimate this
full BBH mass distribution, finding local maxima in primary mass consistent
with previous investigations and a secondary mass distribution with a partly
independent structure, inconsistent with both power laws and with a constant
function of $q$. We find a weaker preference for near-equal mass binaries than
in most previous investigations; instead, the secondary mass has its own
"spectral lines" at slightly lower values than the primary, and we observe an
anti-correlation between primary and secondary masses around the ~$10M_\odot$
peak.
