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Creating HEXTE Responses
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Definitions & requirements

Following OGIP convention, HEXTE response matrices are notionally divided in two parts: the Ancillary Response File (ARF), which accounts for the detector windows and collimator response; and the Redistribution Matrix Function (RMF), which accounts for the redistribution of photon energy amongst detector channels by the detecting medium. The product of the two is known as the response (RSP) and is used by XSPEC for spectral analysis.

In the case of the HEXTE, the response depends primarily on:

  1. Detector ID: HEXTE spectra can be produced from cluster A files (FS50 science and FS52 standard modes), or cluster B files (FS56 science, FS58 standard).

    Spectra produced by SEEXTRCT/SAEXTRCT do not have a DETNAM keyword. It is strongly recommended that users add this keyword to the FITS headers of their spectra before using them with calibration files. For ease of use with the CALDB (see later) and XSPEC, Cluster A spectra should have a DETNAM keyword set to "PWA". For cluster B, an additional complication is due to the loss of spectral capability from detector PWB2 in 1996 March 6 11:27:12 UT. Thereafter, all the counts from this detector end up in PHA channels 1 and 2. Before this date, Cluster B spectra should have DETNAM set to "PWB". After this date, the use of detectors 0, 1, 3 only is denoted by DETNAM="PWB013". This affects both the RMF and the ARF.

    You can easily set the DETNAM keyword using the Ftools; one method is to use fparkey:

     
    fparkey add=yes "PWA" FS50_xxx.pha DETNAM 
    fparkey add=yes "PWB013" FS52_xxx.pha DETNAM 
    
    (The script hxtrsp described below will also set these keywords for you).

  2. Collimator response: This is a correction to the effective area due to the (usually small) off-axis angle of the source in the HEXTE detectors' fields of view. For the HEXTE, this correction is believed to be independent of energy, and so is applied as a constant factor (<1.0) to the ARF.

  3. Channel binning: Although the instrument's response does not depend on how the data channels are binned, XSPEC obviously requires that the channel-binning of the PHA file and its RMF should be identical. Standard Modes data in particular have been rebinned on-board from the original 256 down to 64 channels, so the RMF must be rebinned identically.


The shortcut method

If you are in a hurry, and/or you do not have the HEXTE files installed in a CALDB on your system, you can retrieve the following HEXTE cluster A and B rmfs and arfs from:
ftp://legacy.gsfc.nasa.gov/xte/calib_data/hexte_files/DEFAULT.

These files are adequate for analysis with XSPEC, assuming the following conditions:

  1. You are sure that the data were obtained with the source close to the RXTE pointing axis (fplot the OFFSET values in the XTE filter file to verify).

  2. You are sure that the data were obtained after 1996 March 6 11:27:12 UT when Cluster B detector 2 lost its spectral capabilities.
  3. Your spectra have the full 256-channel sampling. (e.g. most Event data). If not (e.g. Standard Modes data has been rebinned to 64 bins), then these matrices must be rebinned to match the PHA file thus:
    fparkey add=yes "PWA" hexteA.pha DETNAM
    rddescr phafil=FS50_xxx_src.pha chanfil=FS50_xxx.bins
    rbnrmf binfile=FS50_xxx.bins infile=hexte_97mar20c_pwa.rmf outfile=FS50_xxx_src.rmf
    fparkey add=yes value="FS50_xxx_src.rmf" fitsfile=FS50_xxx_src.pha keyword=RESPFILE 
    
    However, if you have the HEXTE calibration files installed in the CALDB, it is much easier to use the hxtrsp script to perform these steps (with an attitude file of "NONE" if condition 1 above is satisfied).

Using hxtrsp to make a response (recommended)

Note: Before running hxtrsp, you must have a CALDB installed on your system.

Attention GO's with an offset pointing!!!
In the special case of a requested offset pointing, you will need to modify the keywords in the header BEFORE running hxtrsp. The keywords RA_OBJ and DEC_OBJ contain the coordinates of the pointing requested; use fkeyprint or fdump to inspect their values in the PHA file. For an offset pointing, these should be changed to the SOURCE coordinates using fmodhead as in the following example:

  1. Create an ASCII file, containing the RA and Dec of the source coordinates, for example:
     RA_OBJ 266.137909 
     DEC_OBJ -28.741100 
     
    We'll call this file position.

  2. Run fmodhead to replace the current values of RA_OBJ and DEC_OBJ in the header of your .pha file:
    fmodhead MYFILE.pha position
    
Changing these keywords to reflect the source coordinates ensures that when you generate a response matrix, the collimator correction will be calculated correctly based on the pointing information in the attitude file. You may use the same file to make the same correction to your PCA data.

Running hxtrsp:

  1. hxtrsp is a Perl script installed under the Ftools environment. To invoke the script, just type "hxtrsp". Optional command-line inputs are:
    hxtrsp -h                                   (provides help information)
    hxtrsp -i pha_file -a attitude_file         (specifies the input files)
    
  2. Enter the name of the PHA file. If you have not yet set the DETNAM keyword, hxtrsp will prompt you for its value and write your input into the PHA file before proceeding further. This will ensure that the corresponding calibration data can be found in the CALDB.

  3. Enter the name of the file containing the RXTE spacecraft attitude information. To compute the collimator response correctly, hxtrsp needs this information. It can be found either in your filter file or in the quaternion file (the file beginning with "FH0e_"). These files are the same as those used by the pcarsp script for PCA data.

    However, if you don't have the attitude information and are confident that the source was observed close to the RXTE pointing axis, you may enter "NONE" and hxtrsp will still work correctly.

  4. hxtrsp will then generate an RSP file with the same root name as the PHA file, and extension ".rsp". This file name will also be written into the PHA file for use with XSPEC.

    For those interested, hxtrsp generates the response with these steps: (i) retrieving the appropriate RMF from the CALDB, (ii) rebinning the RMF (if necessary) to match the PHA file, (iii) retrieving the on-axis ARF and the collimator response from the CALDB, (iv) using these to form an ARF corrected for the off-axis angle, and (v) combining the RMF and ARF to produce an observation-specific response (RSP).


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