Multi-periodicity of the Be star eta Centauri from spectroscopic and photometric observations

We present photometric data and 209 high-resolution, high signal-to-noise r atio optical spectra of the Be star eta Centauri obtained in 1993 and 1995, respectively. Time series analysis of these and other data, performed usin g the CLEAN, CLEANEST and Fourier Doppler Imaging techniques, show the pres ence of multiperiodic variations. They are interpreted in terms of low and high order non-radial pulsation modes. A strong frequency of 1.29 c/d is de tected in line profile and photometric variations. It is attributed to a mo de with l = 2. Other high amplitude signals present in spectroscopic data a re 1.78 c/d, 3.82 c/d and 4.51 c/d. A frequency of 1.48 c/d appearing in sp ectroscopic data is compatible with the 1.56 c/d periodicity largely domina nt in observations taken prior to 1993. The corresponding "superperiods" fo r four of the frequencies are commensurate at the 8% level. The star showed period and non-radial pulsation degree variations in timescales as short a s similar to 1.5 hour. The dominant mode apparently alternates between l = 2 and l = 4 every other day during the eight day time span. This could be a modulation linked to the superperiod or to the stellar rotation. The main periodicities detected in our analysis are compatible with theoretical unst able g modes in SPB variables. Short time scale variability (1 - 3 hr), typ ical of high-order p modes is also systematically present.