= Dark-matter annihilation J-factor
{title2=$J=\int_{\Delta\Omega}d\Omega\int\rho_\chi^2\,dl$}
The line-of-sight, solid-angle integral of squared dark-matter density used for <annihilation> fluxes with a position-independent rate coefficient. It separates an astrophysical density model from particle yields and <annihilation> rates. A decay signal instead scales with a single power of density. Substructure or spatially varying velocity-dependent rates can prevent a simple smooth-density factorization.
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