An Axion-Photon Coupling Bound Conditional on a Field Excursion: What Birefringence Does and Does Not Supply

This paper examines what coupling between an axion-like field and the photon follows from the reported rotation, once the field excursion is stated, and what follows when no excursion is supplied at all. The calculation is performed from stated assumptions and published polarisation measurements, using the deposited script so that every quoted number can be reproduced. The headline outputs are: beta deg = 0.30; g at 1e18 = 1.05e-20; g at Mpl = 8.58e-22; unconditional bound = none - excursion not supplied. The treatment separates what the published measurements supply from what a theoretical framework must additionally provide; where a required quantity is not supplied, that absence is reported plainly as a negative result rather than replaced by an assumed value. The study is computed from stated assumptions and published inputs; it is not a new measurement. SIMULATED, NOT MEASURED in the sense that all derived quantities are calculated, not observed.