Any general X-ray spectral analysis package requires two type of input:
A generic mission-independent X-ray spectral analysis
package (e.g. XSPEC) requires that both of the above types of information
are provided prior to the spectral analysis process in the form of input files.
The two types of information are separable, with
this memo setting out the OGIP standard format for the PHA file.
However it is important to note that the auxiliary information is invariably
highly dependent upon the criteria used to construct a given PHA dataset
(e.g. upon the observation date, the size and position of the extraction
cell in detector co-ordinates etc. ), along with the value of
any number of housekeeping parameters during the observation.
Thus the PHA data file must contain
sufficient information to allow the auxiliary information to be
generated either inside or outside the spectral analysis package.
We use XSPEC to set the requirements since it is
the most sophisticated package generally available to date, and
in widespread use throughout the community.
- a file containing the data. This is usually the
number of counts observed in each spectral channel/bin of
and commonly known as the PHA (Pulse Height Analyser)
file for historical reasons.
- auxiliary information required during the spectral analysis process.
The precise requirements for this type of input are obviously
highly detector- and observation-specific.
- calibration data (detailing the transformation between an
input X-ray spectrum and the number of counts in
each detector channel, the spectral response of the
detector etc. ).
The OGIP standard format for the calibration files
associated with spectral analysis is given in
OGIP Calibration memo CAL/GEN/92-0021
- the background in the detector during the observation
- any miscellaneous corrections applicable during the
The PHA data file formats specified here were developed with the
following goals in mind:
- The format must be FITS
- The file must contain all information required by a spectral
- The file must contain a complete creation and modification
- The file must contain enough information for the response matrix for
the particular observation to be generated using this file and
any associated the basic calibration files (BCFs -see George 1992).
- For those instruments for which a background prediction
algorithm is used, the file must also contain enough information
to do this.
As will be discussed in Section 3 below, within the OGIP standard format for
PHA files, the information is divided between several standard
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Last modified: Thursday, 21-Oct-2021 12:50:38 EDT