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Building Ancillary Response Files (ARFs)

Although the RMFs are obtained from the CALDB as ready-made files, ARFs need to be created for every extracted spectrum. To do this, the task called xrtmkarf is used. Generally, an exposure map should be used, corresponding to the same time interval as the spectrum (see the exposure map thread). If the spectrum PC.pi corresponds to the whole of observation 00374210000, then the commands to use would be:

>xrtmkarf expofile=sw00374210000xpcw3po_ex.img
> Name of the input PHA FITS file [ ] PC.pi
> PSF correction active?(yes/no) [ ] yes
> Name of the output ARF FITS file [ ] PC_exp.arf
> Source X coordinate [ ] -1
> Source Y coordinate [ ] -1

                Running 'xrtmkarf_0.5.6'
                 Input Parameters List:
Name of the input RMF file                    :'CALDB'
Name of the input mirror effective area file  :'CALDB'
Name of the input filter transmission file    :'CALDB'
Name of the input arf file                    :'CALDB'
Name of the input exposure map file           :'sw00374210000xpcw3po_ex.img'
Name of the input vignetting file             :'CALDB'
Name of the input spectrum file               :'PC.pi'
Name of the input PSF file                    :'CALDB'
Name of the output ARF file                   :'PC_exp.arf'
Source SKYX                                   :'-1.000000'
Source SKYY                                   :'-1.000000'

vig_flag (in)  : 1
  min exposure : 0.000000 in 0 0
  max exposure : 15649.213867 in 519 488
 CENTER of the ROI (SKY)  503.303464 558.035812
 CENTER of the SRC (SKY)  503.303467 558.035828
xrtmkarf_0.5.6: Info: WMAP region boundaries (DETX1:DETX2,DETY1:DETY2):[ 484 : 524 , 539 : 579 ]
xrtmkarf_0.5.6: Info: Source position (DETX,DETY): [503.303467 , 558.035828]
  min ROI exposure : 8683.248047 in 488 552
  max ROI exposure : 15537.960938 in 515 542
xrtmkarf_0.5.6: Info: Processing '/usr/local/swift//caldb34/data/swift/xrt/cpf/rmf/swxpc0to12s6_20010101v011.rmf' CALDB file.
 OFFAXIS ANGLE: 2.286438
vig_flag   : 1
>>> using a vignetted exposure map
xrtmkarf_0.5.6: Info: Processing '/usr/local/swift//caldb34/data/swift/xrt/cpf/arf/swxpc0to12s6_20010101v011.arf' CALDB file.
xrtmkarf_0.5.6: Info: Processing '/usr/local/swift//caldb34/data/swift/xrt/cpf/psf/swxpsf20010101v004.fits' CALDB file.
 PERC. OF FLUENCE WITHIN THE ROI:  88.241035 % (AT 1.002500 keV )
xrtmkarf_0.5.6: Info: 'PC_exp.arf' file successfully written.
xrtmkarf_0.5.6: Exit with success.

The method is the same no matter what period of time (one snapshot, multiple Obs IDs...) is used for the spectrum; the only part which differs is the exposure map used.

When an exposure map is included, the PSF correction must be active (i.e. yes as the input to the second xrtmkarf prompt); this ensures the ARF is corrected for hot columns, bad pixels and any loss of counts caused by using an annular extraction region (if the source is piled-up). If -1 is given at the prompts for the source coordinates, the position of the source is extracted from the header of the spectrum (and will, thus, correctly be the centre of the extraction region used).

Note that the relevant RMF to use is given in the output of xrtmkarf. In the above example, this is:

xrtmkarf_0.5.6: Info: Processing '/usr/local/swift//caldb34/data/swift/xrt/cpf/rmf/swxpc0to12s6_20010101v011.rmf' CALDB file.

Alternatively, the FTool quzcif can be used to determine the RMF required for a given observation. When prompted for the Calibration Dataset Codename, the response matrix will provide the information about the RMF. Here is an example of the inputs required and resulting output:

Name of Mission[SWIFT]
Name of Instrument[XRT]
Name of Detector (- if not required)[-]
Name of Filter (- if not required)[-]
Calibration Dataset Codename[matrix]
Requested Date in yyyy-mm-dd format[2009-12-02]
Requested Time in hh:mm:ss format[08:45:16]

Boolean selection expression for Boundary params(- if not required)[datamode.eq.windowed.and.grade.eq.G0:2.and.XRTVSUB.eq.6]

/usr/local/swift//caldb34/data/swift/xrt/cpf/rmf/swxwt0to2s6_20070901v011.rmf   1

xrtmkarf can also be used to create ARFs for extended sources, by including extended=yes on the command line.

Sometimes (for example, when correcting light-curves, or determining the count rate of a spectrum in XSPEC), knowledge of the actual correction applied is useful. This value is given by the PERC. OF FLUENCE WITHIN THE ROI line in the xrtmkarf output. So, for the example above, the correction factor would be 100/88.218671 = 1.13.

Note that if you are using spectra from the online XRT product generator, the relevant correction factor will be given in the file called *