Most precise single redshift bound to Delta alpha/alpha

Verification of theoretical predictions of an oscillating behavior of the fine-structure constant alpha with cosmic time requires high precision Delta alpha/alpha measurements at individual redshifts, while in earlier studies the mean Delta alpha/alpha values averaged over wide redshift intervals were usually reported. This requirement can be met via the single ion differential alpha measurement (SIDAM) procedure proposed in Levshakov et al. (2005). We apply the SIDAM to the FeII lines associated with the damped Ly-alpha system observed at z=1.15 in the spectrum of HE0515-4414. The weighted mean <Delta alpha/alpha> calculated on base of carefully selected 34 FeII pairs {1608,X} (X = 2344, 2374, and 2586 A) is <Delta alpha/alpha> = (-0.07+/-0.84) 10^{-6} (1sigma C.L.). The precision of this estimate improves by a factor 2 the previous one reported for the same system by Quast et al. (2004). The obtained result represents an absolute improvement with respect to what has been done in the measurements of Delta alpha/alpha.