Paper CCLIX: Distinguishing Lensed Binary Neutron Stars from Hierarchical Mergers in the Gravitational-Wave Mass Gap — A Population-Synthesis Refinement of the Ryczanowski et al. (2025) Follow-up Strategy

Ryczanowski, Lyons, Smith & Massey (Phil. Trans. R. Soc. A 383:20240118, 2025) propose an electromagnetic follow-up strategy that enables the discovery of strongly lensed binary neutron stars from LIGO/Virgo/KAGRA mass-gap (2.5–5 M⊙) triggers. Their binary classification — lensed BNS vs. primordial mass-gap BBH — implicitly assumes these two channels exhaust the population. Within the One-Octonion Brane-Bulk framework and consistent with the Antonini PISN hierarchical channel (Nat. Astron. s41550-026-02847-0, 2026), a third channel — hierarchical 2G+1G mergers with first-generation remnant masses M_rem ≈ 0.95(m₁+m₂) and Kerr spin a_rem ≈ 0.69 (Paper CCLVI) — populates the same mass-gap window at non-negligible rate inside dense stellar clusters. These hierarchical events are EM-dark and would be misclassified as primordial mass-gap BBH in the Ryczanowski flow. We propose augmenting the flow with two OOB-predicted, channel-orthogonal observables: gravitational-wave birefringence at the throat (Paper CCXXXII) and throat-threaded magnetic birefringence (Paper CCXLV). The χ_eff distribution provides a third, model-agnostic discriminant: lensed BNS peak near 0; hierarchical 2G+1G peak near 0.35 ≈ χ_eff(a_rem=0.69, m₂/m_total ≈ 0.5). Population-synthesis estimates yield 5–20% hierarchical contamination of any null-EM-follow-up mass-gap subset. Five predictions: (P1) 30-60% of null-EM bucket has χ_eff > 0.2; (P2) bimodal χ_eff distribution; (P3) joint CCXXXII + CCXLV detection on high-χ_eff sub-pop; (P4) lensed-BNS recovery rate consistency check; (P5) 3G+1G stacking required to reach Antonini's χ_eff,max ≈ 0.5. Honest scope: population fractions order-of-magnitude; framework contribution is structural (third channel + two orthogonal observables), not rate-quantitative; requires O5 sensitivity. Part of the One-Octonion Brane-Bulk Framework series. Anchor DOI: 10.5281/zenodo.19120873. Community: one-octonion-brane-bulk. Author: Bharathi Dasan Jagadeesan, M.D., University of Minnesota. ORCID: 0000-0002-1143-941X.