Most precise single redshift bound to the variability of the fine-structure constant

Verification of theoretical predictions of an oscillating behavior of the fine-structure constant, α, with cosmic time requires high precision measurements at individual redshifts, while in earlier studies the mean Δα/α values averaged over wide redshift intervals were usually reported. This requirement can be met via the Single Ion Differential α Measurement (SIDAM) procedure. We apply SIDAM to the FeII lines associated with the damped Lyα system observed at z = 1.15 in the spectrum of HE0515-4414. The weighted mean calculated on the base of carefully selected 34 FeII pairs is < Δα/α > = (-0.07 ± 0.84) × 10^-6. The precision of this estimate represents a large improvement over previous measurements of Δα/α.