2012A&A...541A..69W

The variation of the dimensionless fundamental physical constant µ=m_p/m_e - the proton to electron mass ratio - can be constrained via observation of Lyman and Werner lines of molecular hydrogen in the spectra of damped Lyman alpha systems (DLAs) in the line of sight to distant QSOs. Our intention is to maximize the possible precision of quasar absorption spectroscopy with regard to the investigation of the variation of the proton-to-electron mass-ratio µ. The demand for precision requires an understanding of the errors involved and effective techniques to handle present systematic errors. An analysis based on UVES high resolution data sets of QSO 0347-383 and its DLA is put forward and new approaches to some of the steps involved in the data analysis are introduced. We apply corrections for the observed offsets between discrete spectra and for the first time we find indications for inter-order distortions. Drawing on VLT-UVES observations of QSO 0347-383 in 2009 our analysis yields Δµ/µ=(4.3±7.2)x10^–6 at z_abs=3.025. Current analyzes tend to underestimate the impact of systematic errors. Based on the scatter of the measured redshifts and the corresponding low significance of the redshift-sensitivity correlation we estimate the limit of accuracy of line position measurements to ∼220m/s, consisting of roughly 150m/s due to the uncertainty of the absorption line fit and about 150m/s allocated to systematics related to instrumentation and calibration.