Paper CCIII: Magnetar X-Ray Polarimetry as a Test of the H+⊕H−Brane Sector Boundary: Three Predictions from G2-Symmetric Brane Re-Entry Radiation for

The Imaging X-ray Polarimetry Explorer (IXPE) has measured a 90◦swing in polarization angle (PA) at ∼4–5 keV for the anomalous X-ray pulsar 4U 0142+61, attributed conventionally to a transition between O-mode (condensed surface) and X-mode (resonant Compton scattering) emission. We show that this transition is the observational signature of the H+/H−sector boundary of the One-Octonion Brane-Bulk Framework: O-mode photons couple to the H+ = {e0, e1, e2, e3} matter sector (electroweak, perceptible), while X-mode photons couple to the H−= {e4, e5, e6, e7} colour sector (closed under the framework's confinement identity [e4, e5, e6] = −2e7). The QED vacuum birefringence that freezes the polarization at the polarization limiting radius rpl ≈100 RNS is identified as the anisotropy induced by the e7 Higgs-Fano condensate. The magnetic T∗ 2 sump radius rT ∗ 2 (mag) = p GM/a0,eff ≈280 RNS for B = 1014 G is the brane scale at which the polarization is created; rpl is the scale at which it is locked. Three zero-free-parameter predictions follow: (P-mag-1) the PA-swing energy scales as (Bsurf/Bq)1/2 across magnetar sources, testable immediately from the existing IXPE catalog; (P-mag-2) the phase-resolved PA curve exhibits 12-fold G2 Weyl-group dihedral symmetry at high precision; (P-mag-3) the polarization degree in the O→X transition zone follows PD(E) ∝1 −sinc2(E/Eref), the same brane-bulk transit formula that governs pion decay kinematics. All three are falsifiable with existing IXPE data and ongoing observations. 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.