5.10 High Energy Astrophysics Applications

The natural form for storing data for high energy astrophysics is as a table of events. Consequently, binary tables are used extensively for high energy astrophysics data. One example of a design for such tables is that for data from the High Resolution Imager on ROSAT.

• The primary header, in addition to the required keywords, contains data set identification and history. NAXIS is set to zero; there is no primary data array.
• The first extension is a ``Good Time Intervals'' table, identifying those time intervals during which the data received from the satellite can be regarded as valid measurements. Each row of this table consists of two double-precision floating point fields, listing the start time and end time of a time interval where the data are valid.
• The next extension is the ``Events'' table, including most of the data from the native time-ordered event file.
• The actual columns in the ROSAT tables contain an information structure governed by the way in which the ROSAT readings are organized.

The kind of information obtained from any high-energy experiment could be organized in a similar table. The header could, in addition to the card images describing the structure of the table, use keyword=value statements to convey such information as the conversion between instrument and sky coordinates, zero points for temporal and spatial scales, conversion between table values and physical values, and the instrument configuration. The table would contain such information as the coordinates (in the detector reference frame) of the individual events, the time of an event, the energy of an event, and the strength or intensity of an event. Example 7 in Appendix A shows a different FITS header for high energy astrophysics data, from the Advanced Satellite for Cosmology and Astrophysics (ASCA).