Cyclotron lines in gamma-ray bursts and magnetic field decay

The presence of cyclotron lines in a significant fraction of γ-ray burst spectra suggests a source association with strongly magnetized neutron stars. The burster distance scale is not known but constrained by their angular- and brightness distribution. Using pulsar data we calculate the corresponding age distribution of Galactic neutron stars and apply an exponential field decay model to test whether the observed incidence rate of cyclotro lines is consistent with decay time scales derived for radio pulsars. We find that the statistical properties of γ-ray bursts are inconsistent with the idea of events originating at arbitrary times on neutron stars whose fields decay on time scales shorter than ~109 yrs. Possible interpretations of this inconsistency with radio pulsar field decay models are discussed.