A Magnetic-Field Bound from the Synchrotron Self-Absorption Turnover in Compact Jet Cores

This paper addresses a single accounting question in the physics of relativistic jets: what magnetic field follows, with almost no free parameters, from the frequency at which a compact jet core becomes opaque to its own synchrotron radiation. Using published turnover frequencies, flux densities and angular sizes for five compact cores, the calculation derives new bounds and budgets from standard synchrotron and special-relativistic relations, with every number produced by the deposited compute script. The headline results are: max B = 1.28, min B = 0.000625. 3C 273 core: 0.000625, 3C 279 core: 0.000961, BL Lac core: 0.000932, M87 core: 0.0018, Centaurus A core: 1.28. The inferred fields fall in the range expected for parsec-scale jet bases, from well below a gauss to a few gauss depending on source, with the ordering driven mainly by turnover frequency. The study is a computation from stated assumptions and published inputs; it is simulated, not measured, and its conclusions are stated at the precision those inputs support.