In case of the XRT, the basic data sets for scientific data evaluation are produced through an automated Standard Processing - the Standard Analysis Software System (SASS) - [Voges et al.1992] applied to all data from the XRT instrument. During that process the basic event information first undergoes various corrections and normalizations (e.g. detector linearization and pulse height calibration). At the end of this step each event in the PSPC detector is represented by a set of seven parameters. In addition to the original x, y detector coordinates, the measured pulse height and the arrival time, the corresponding x, y sky position (where the photon is most likely originating from) and a calibrated pulse height (representing the most likely photon energy) are added. Photon Event Files containing this information form the basic input to all further analysis.
As the next step in the Standard Processing, images of the observed sky regions are formed. Different techniques (sliding window and maximum likelihood methods) are used to search the images for point-like sources above a specified significance level. The most probable values for the positions of detected sources, the sizes of the corresponding error boxes, the intensities in different energy bands and a source extension indicator are the output of the final maximum likelihood algorithm. For stronger sources a search for time variability is performed and spectra are derived and fitted with standard emission models. Finally, large optical catalogues are scanned for possible counterparts of the detected X-ray objects. Summaries of the results of the Standard Processing are produced and distributed --together with the standard data sets-- to the relevant ROSAT observers by the ROSAT Archive Service.