While we would fully expect the Time Series Analysis component of ASTERIX to function as described in this guide, the User should be aware that the use this software has now been superseded, and that there are no further plans for the development of Time Series software within ASTERIX.
ROSAT flies a near circular low earth orbit. The drawback of such an orbit is that periodically the satellite must stop observing while it passes through regions of high particle background such as the South Atlantic Anomaly. A ROSAT time series of greater than 30 minutes duration may have gaps in it due to this effect; this causes difficulties for traditional time series analysis routines. The ASTERIX document, USER_006, provides a summary of the routines within ASTERIX which have been designed to overcome this problem.
Analysis of PSPC data is further complicated by the window support structure. The detector is continually wobbled while observing to ensure that sources are not continuously hidden behind the fine wire mesh of the support structure. This means that a source will continually pass under wires and its flux will be modulated at the wobble period, (about six minutes). Sources outside the central ring are less prone to this effect because of the wide PSF at high off-axis angles.
A time series may be background subtracted if required in a similar manner to the spectral datasets mentioned earlier. It is likely that several of the time bins in the file will be only partly exposed and so it is important that the data is exposure corrected with xrtcorr before being analysed.
Due to the nature of ROSAT observations there can be very long gaps between data in a time series. To save disk space and processing time, the gaps in the time series may be removed with the program streamline.
The program lombscar provides a good technique for searching for peaks in the power spectrum of 'gappy' data.