Fast Quasi-Periodic Oscillations ( QPOs , frequencies of \sim 20 - 1840 Â Hz ) have been recently discovered in the ringing tail of giant flares from Soft Gamma Repeaters ( SGRs ) , when the luminosity was of order 10 ^ { 41 } -10 ^ { 41.5 } Â erg/s .
These oscillations persisted for many tens of seconds , remained coherent for up to hundreds of cycles and were observed over a wide range of rotational phases of the neutron stars believed to host SGRs .
Therefore these QPOs must have originated from a compact , virtually non-expanding region inside the star ’ s magnetosphere , emitting with a very moderate degree of beaming ( if at all ) .
The fastest QPOs imply a luminosity variation of \Delta L / \Delta t \simeq 6 \times 10 ^ { 43 }  erg s ^ { -2 } , the largest luminosity variation ever observed from a compact source .
It exceeds by over an order of magnitude the usual Cavallo-Fabian-Rees ( CFR ) luminosity variability limit for a matter-to-radiation conversion efficiency of 100 % .
We show that such an extreme variability can be reconciled with the CFR limit if the emitting region is immersed in a magnetic field \gtrsim 10 ^ { 15 } Â G at the star surface , providing independent evidence for the superstrong magnetic fields of magnetars .