Three Parts in a Thousand: The Mass Drift a Binary-Neutron-Star Spectral-Siren Measurement Can Afford

Converts the BNS spectral-siren bias bound of Roy et al. (2411.02494) into drift currency: with source-frame mass feature m(z) = m0(1+sz), the inferred-rate bias at z = 0.4 is ~165 s (s = 0.01 gives +1.65%), so the target's <0.5% bound is exactly s <= 0.0030; at z = 1 a drift of s = 0.0081 alone spends the advertised 2% precision. Control: zero drift returns 0.000% bias.