Delta Scuti Network observations of XX Pyx: detection of 22 pulsation modes and of short-term amplitude and frequency variations

We report multisite observations devoted to the main-sequence delta Scuti s tar XX Pyx, conducted as the l7th run of the Delta Scuti Network. Over 125 nights a total of 550 h of usable time-series photometric B- and V-filter d ata were acquired involving both photoelectric and CCD measurements at eigh t observatories spread around the world, which represents the most extensiv e single time-series for any pulsating star other than the Sun obtained so far. We describe our observations and reduction methods, and present the frequen cy analysis of our new data. First, we detect six new pulsation and five ne w combination frequencies in the star's light curves. We also discover evid ence for amplitude and/or frequency variations of some of the modes during the observations. These can occur on time-scales as short as 20 d and show quite diverse behaviour. To take them into account in the frequency analysi s, a so-called non-linear frequency analysis method was developed, allowing us to quantify the temporal variability of the modes and to compensate for it. Following that we continue the frequency search and we also incorporat e published multisite observations. In this way, we reveal three more pulsa tion and two more combination frequencies. In the end, we report a total of 30 significant frequencies - 22 of which correspond to independent pulsati on modes. This is the largest number of independent modes ever detected in the light curves of a delta Scuti star.