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Fast Quasi-Periodic Oscillations (QPOs, frequencies of $\sim 20 - 1840$ Hz)have been recently discovered in the ringing tail of giant flares from SoftGamma Repeaters (SGRs), when the luminosity was of order $10^{41}-10^{41.5}$erg/s. These oscillations persisted for many tens of seconds, remained coherentfor up to hundreds of cycles and were observed over a wide range of rotationalphases of the neutron stars believed to host SGRs. Therefore these QPOs musthave originated from a compact, virtually non-expanding region inside thestar's magnetosphere, emitting with a very moderate degree of beaming (if atall). The fastest QPOs imply a luminosity variation of $\Delta L/\Delta t\simeq 6 \times 10^{43}$ erg s$^{-2}$, the largest luminosity variation everobserved from a compact source. It exceeds by over an order of magnitude theusual Cavallo-Fabian-Rees (CFR) luminosity variability limit for amatter-to-radiation conversion efficiency of 100%. We show that such an extremevariability can be reconciled with the CFR limit if the emitting region isimmersed in a magnetic field $\gtrsim 10^{15}$ G at the star surface, providingindependent evidence for the superstrong magnetic fields of magnetars.Comment: Accepted for publication in the Astrophysical Journal, Part

SGR 1806−20: Evidence for a Superstrong Magnetic Field from Quasi‐Periodic Oscillations