1 Introduction

Since the discovery of the short-period light variations in cool Ap star HD 101065 (Kurtz & Wegner 1979) this phenomenon became an object of interest for many studies. At present the group of rapidly oscillating Ap (roAp) stars consists of 32 members. These stars oscillate with periods in the range of 4-16 min and very low amplitudes ( $\Delta B\leq15$ mmag). $\gamma$ Equ is the second brightest roAp star. The latest extensive analysis of its light variation from multi-site observations (Martinez et al. 1996) provides support for p-modes with four pulsation frequencies corresponding to periods from 11.68 to 12.45 min. Libbrecht (1988) was the first who found radial velocity variations with an amplitude of 42 ms^-1 and with two out of four photometric periods: 12.20 and 11.68 min. Kanaan & Hatzes (1998) performed an extensive study of the radial velocity variations due to pulsations in $\gamma$ Equ based on high-precision echelle spectroscopy with an iodine cell. They found RV amplitudes ranging from 30 to 1000 ms^-1 as measured from individual lines and concluded that the pulsational amplitudes depend on chemical species and are higher for weaker lines of the same element. Practically at the same time Malanushenko et al. (1998) published results of RV analysis of the individual lines in the 6112-72 Å spectral region. They discovered the highest RV amplitudes, up to 800 ms^-1, for the lines of Pr III and Nd III, while most other strong and weak lines in this region did not show amplitudes exceeding the accuracy of the RV measurements ($\approx$100 ms^-1). Malanushenko et al. (1998) found that the main frequency of RV pulsations, 1365 $\mu$Hz (12.21 min), coincides with one of the photometric frequencies. Later Savanov et al. (1999) showed that some lines from the spectral regions for which Kanaan & Hatzes (1998) found large RV variations were not identified properly and in reality belong to singly and doubly ionized Pr and Nd.

Figure 1: A comparison between spectrum synthesis calculations (thin curve) and average spectrum of $\gamma$ Equ (thick curve) is shown in the upper panel. The middle panel displays the difference between individual and average observed spectra. The profiles of the consecutive pulsation phases are shifted in the vertical direction by 0.02; the lower panel shows the standard deviation for each pixel of the observed spectrum.

Figure 1: continued.

$\gamma$ Equ is the slowest known rotator among Ap stars. Its rotational period, $P=77 \pm 10$ yr, was determined from published longitudinal magnetic field measurements by Leroy et al. (1994). Due to its extremely low rotational velocity we cannot observe any line profile variations connected with the surface abundance inhomogeneity typical for Ap stars. However, the short-term spectroscopic variability of $\gamma$ Equ may be caused by non-radial oscillations. We obtained time-series of high-resolution high S/N spectra of $\gamma$ Equ to study in detail the line profile variations due to stellar pulsations.

The observational data and reduction procedure are described in Sect. 2. Line identification and spectrum synthesis details are discussed in Sect. 3. We present the results of RV measurements in Sect. 4, while an analysis of REE line profile variations and an attempt of mode identification are given in Sect. 5. In Sect. 6 we discuss the pulsational broadening of the time-averaged $\gamma$ Equ spectrum.