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

Abstract

Fast quasi-periodic oscillations (QPOs; frequencies of ~20-1840 Hz) have recently been discovered in the ringing tail of giant flares from soft gamma repeaters (SGRs) when the luminosity is of order 1041-1041.5 ergs s-1. 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 nonexpanding 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 ΔL/Δt~=61043 ergs 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 >~1015 G at the star surface, providing independent evidence for the superstrong magnetic fields of magnetars.