General Description of a FITS Rate File

The Rate Files are implemented using the FITS binary table extension format. This requires that the Rate File be preceded by a FITS primary array, but in most cases this will simply be a null array (with NAXIS = 0). A null primary array is not compulsory, however, and the primary array may be used to store any related data (e.g., a map of the source). The header of the Rate Data File extension must contain a minimum set of keywords which describe the data and define various times associated with the observation. This is then followed by the Rate Table which may be given in one of four formats:

• Event: A table of time measurements for a discrete set of events, e.g. the arrival times of X-ray photons.
• Equally Spaced: A table of binned intensity measurements at equally spaced time intervals.
• Unequally Spaced: A table of binned intensity measurements at unequally spaced time intervals.
• Packet: Histogram array of timing or PHA information stored for a single reference time. The interval between each row maybe equispaced or unequally spaced.

For unequally spaced and event data, a TIME column that specifies the time of each event or bin measurement is required. The times may be specified either as an absolute time or as a time offset relative to a reference time specified in the header. For equally spaced data, the time associated with each measurement is calculated from a start time and the delta time interval which are given as keywords in the Rate File header. Rate Data Tables which contain binned data require a column containing the intensity measurements. In the case of multichannel instruments (for which more than one energy band is sampled), this column may contain an array of simultaneous intensity measurements in different energy bands. The Rate Data table may have additional columns containing various corrections either to the time or to the intensity columns. Alternatively, the correction information may be included in a separate FITS extension appended to the Rate file.