Photon Event Tables (see Table 2.1) show up in two flavors: as a table where all the photons of an observation are positionally ordered into smaller sky areas (maps) for fast access (file type: PEM, that is Photon Events Map-sorted) or as tables without this structure (file type: PET, Photon Event Table). The structure is transparent to the user.
Table 2.1: Complete set of labels for the PSPC Photon Event Table (PET)
Because this table contains in addition to the event data a lot of basic information as well as the manipulation history in the header section, it should never be treated by MIDAS commands that affect the data or header information (e.g. due to the specific structure a small map sorted photon event table (PEM) cannot be time sorted without destroying its structure). The number of defined rows corresponds to the number of listed photons, while the number of defined columns depend on the number of parameters associated with each photon. Each parameter can be referenced via the corresponding column label. In the case of the ROSAT PSPC a photon event is completely described by seven parameters, as shown in Table 2.1. In general, a PET may hold only a subset of these labels.
As long as no time sorting process has been applied, a PET is not ordered with respect to the photon arrival times.
Table 2.2:
Example of a file (type BIN) with binned spectra
of a source and three kinds of background