2 Observations and spectra reduction

The observations of $\gamma$ Equ were collected in July 1999 in the backup programme of an observing run devoted to the investigation of magnetic fields on M-dwarf stars. We used the Very Long Camera of the Coude Echelle Spectrograph, fibre-linked with the Cassegrain focus of the ESO 3.6-m telescope. The combination of the third (highest resolution) CES image slicer and ESO CCD#38 provided a resolving power of $\lambda/\Delta\lambda\simeq 170\,000$. The observations of $\gamma$ Equ were carried out for 1.5 hours. During that time we obtained 31 60 ^s exposures of a 26 Å spectral region centred at $\lambda$ 6152 Å. The first spectrum was obtained at the heliocentric Julian date HJD ₁=2451381.783444, while the last one was acquired at HJD ₃₁=2451381.823897. A signal-to-noise ratio of 130 was achieved in each individual exposure. A Th-Ar comparison spectrum was registered immediately before and after the observations of $\gamma$ Equ.

The basic steps of spectra reduction (bias subtraction, flat field correction, extraction of 1D spectra and wavelength calibration) were performed with the set of IDL-based routines, specially adapted for the reduction of CES spectra. We used 12 emission lines in each of the Th-Ar comparison spectra in order to establish the wavelength scale. The positions of the emission lines were determined by fitting a Gaussian to the line profiles. Then a second order polynomial was used to fit the pixel-wavelength relation. This procedure allowed us to establish the wavelength scale with an internal accuracy of $\sim$ $5 \times 10^{-5}$ Å. We found a shift of 22 ms^-1 between Th-Ar spectra taken before and after observations of $\gamma$ Equ. By comparison, during 1.5 hours of observations the heliocentric radial velocity of the observing site changed by 113 ms^-1 in the direction towards the star. However, in order to simplify the spectroscopic analysis, no correction for the drift of the zero velocity point was made at this stage of the reduction, and an average dispersion relation was used for all 31 spectra. Instead we took into account the linear drift of the spectrograph reference frame when fitting radial velocity variations of the individual spectral lines (Sect. 4).

The average instrumental profile was determined from the same 12 Th-Ar emission lines that were used for the construction of the wavelength scale. The instrumental profile is well approximated with a Gaussian, corresponding to a resolving power of $\lambda/\Delta\lambda = 166\,000$. We found no evidence for a temporal variation of the instrumental profile or a systematic change in the dispersion direction.

In the final stage of the spectrum reduction special care was taken in order to achieve a consistent continuum normalization of the individual spectra. With the help of spectrum synthesis we selected a subset of spectral regions, free of strong lines, and then iteratively fitted a cubic polynomial through the continuum points, rejecting points with a large deviation from the provisional continuum level. Then we inspected the difference between each individual and the average spectrum (Fig. 1) and modified the selection of the line-free regions until large-scale deviations of individual spectra from the average were removed.