Most precise single redshift bound to

¹ Department of Theoretical Astrophysics, Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia e-mail: lev@astro.ioffe.rssi.ru
² Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
³ Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 14 July 2005
Accepted: 21 November 2005

Abstract

Aims.Verification the theoretical prediction of oscillation of the fine-structure constant over cosmic time requires high precision  measurements at individual redshifts. In earlier studies the mean  values averaged over wide redshift intervals were usually reported. This requirement can be met using the single ion differential α measurement (SIDAM) procedure. We apply SIDAM to the lines associated with the damped Lyα system observed at z_abs = 1.15 in the spectrum of HE 0515–4414. Methods.The spectrum was obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT). Results.The weighted mean calculated using carefully selected 34 pairs (, and 2586 Å) is ( C.L.). The precision of this estimate improves by a factor of 2 the previous one reported for the same system. The obtained result represents an absolute improvement with respect to what has been done in the measurements of .