Converting OM data to OGIP II format (use in xspec)


The purpose of om2pha is to produce the necessary files for OM photometric data to be analysed with xspec, e.g., together with the simultaneous X-ray data.

Expected Outcome

  • single file in OGIP TYPE:II format that contains a count rate for each filter

SAS Tasks to be Used


Useful Links


Last Reviewed: 25 May 2023, for SAS v21.0

Last Updated: 22 January 2016




The output of the OM chains contains brightness values for each source in units of raw count rates, magnitudes and fluxes. This information can be found in the source list file for each exposure. In addition, there is a combined source list containing the brightness values for all exposures within an observation. Consistency checks of observed OM brightnesses with models that fit the X-ray data are thus possible with the combined source list alone.

To facilitate a graphical comparison of OM data with X-ray data in the form of a Spectral Energy Distribution (SED) or to fit a model to the combined X-ray and OM data, the SAS task om2pha produces a file in the OGIP II format that can be read into spectral fitting programs such as XSPEC.

If OM are to be analysed in combination with X-ray data, it may be easiest to do a complete run with xmmextractor.


Change to the directory in which the output of omichain are stored (see the thread OM image mode data processing chain). Copy the canned OM Response Files into this directory - these are not needed to run om2pha itself but are needed, subsequently, to run XSPEC with the file created by om2pha (see also the Products below).

Define the two environment variables SAS_ODF and SAS_CCF to point to the ODF directory and ccf file, respectively (full path).

The om2pha tool operates on the basis of the combined source list, *OMCOMBOBSMLI0000.FIT, which contains the coordinates and source properties for each source for each filter that was used during the observation. The exact coordinates of the source of interest as listed in the source list needs to be found out. The interactive tool omsource can be used to find out the coordinates, see Interactive photometry thread.


An example call is,

   om2pha srclist=P0721010601OMCOMBOBSMLI0000.FIT ra=262.08222 dec=-14.2659 output=pds456.ds

where here the coordinates of source number 32 (given in this call) slightly differ from the coordinates in Simbad.


The only output is the file pds456.ds, the name indicated in the example above, as output file. It is in fits format and contains two extensions, the second one, 'SPECTRUM' contains six columns: SPEC_NUM, CHANNEL, RATE, STAT_ERR, RESPFILE, AND ROWID (name of filter). Each row can be understood as a spectrum of one value. The values in the RESPFILE column correspond to the names of the respective canned response file that needs to be in the same directory and can be obtained from the list of canned OM Response Files. If a central repository is used, the RESPFILE column needs to be edited to contain the full path.

With the response files correctly located, the output file can be read into, e.g., XSPEC. The application is described in the xspec thread.

Also OM grism spectra can be converted to a format that spectral fitting packages can digest. This is described in the Section Use of the Canned OM Response Files.



To identify the desired source in the source list file, the coordinates have to be provided on the command line. They may differ from Simbad coordinates, so one has to carefully check that the correct source is selected.