INTRODUCTION

Any general X-ray spectral analysis package requires two type of input:

1. a file containing the data. This is usually the number of counts observed in each spectral channel/bin of the detector, and commonly known as the PHA (Pulse Height Analyser) file for historical reasons.
2. auxiliary information required during the spectral analysis process. This includes:
   • 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 (George et al. 1992a)
   • the background in the detector during the observation
   • any miscellaneous corrections applicable during the observation
   The precise requirements for this type of input are obviously highly detector- and observation-specific.

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.

The PHA data file formats specified here were developed with the following goals in mind:

1. The format must be FITS
2. The file must contain all information required by a spectral analysis package.
3. The file must contain a complete creation and modification history.
4. 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).
5. 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 FITS extensions.