Description

The mkacisrmf tool generates an OGIP-style RMF file appropriate for analysis of Chandra ACIS imaging data. The response matrix file (RMF) encapsulates the mapping between the physical properties of incoming photons (such as their energy) and their detected properties (such as detector pulse heights or PHA) for a given detector. This mapping is stored in the form of a 2-D matrix. Spectral fitting packages like Sherpa, XSPEC, and ISIS use this redistribution function (the matrix), plus an accompanying effective area function (the ARF), to predict the counts per channel produced by a given spectral model for the source.

As of CALDB 3.1.0, released on 23 June 2005, mkacisrmf can be used to create RMFs for all observations taken at the -120 C focal plane temperature. Calibration for other focal plane temperatures will be released in the future. For the latest information on what calibration is available, see the Chandra Calibration Database website and the Creating ACIS RMFs why topic.

mkacisrmf calculates the RMF on user-defined grids in energy (units of keV) and channel (units of PHA or PI) using tabulated ACIS response calibration information. The user may choose to create an RMF for a single position on a given ACIS CCD, or, alternatively, an RMF which represents the weighted sum of the response function over a given spatial extraction region. In general, the response of the detector is a function of position on the CCD and such weighted RMFs (WRMFs) may be more appropriate for analysis of extended sources. To construct weighted RMFs, mkacisrmf uses weighting information contained in a FITS image of the field in CCD detector coordinates. Such an image is called a weights map (WMAP) and can be created in CIAO directly from the events data using dmcopy or during the spectral extraction process using dmextract.

Users have the option of creating RMFs appropriate for either PHA or PI extracted ACIS spectra. For the case of PI RMFs, the information in the input gain file is used to scale from PHA channels to PI channels dynamically during the RMF computation. If a WMAP file created in CIAO is specified, mkacisrmf will utilize keyword information in the header to locate the appropriate gain file in the CALDB automatically. If an RMF for a single position is desired, the gain file must be specified explicitly.

Examples of the various options for creating RMFs can be found in the CIAO science threads.

The tool mkacisrmf is an entirely new method for creating ACIS response matrices. Unlike its predecessor mkrmf, this tool separates the RMF calculation process into two components: an "ideal" component which describes the CCD spectral response prior to the effects of charge transfer inefficiency (CTI) degradation, and a spatially varying component which incorporates the changes in the response produced by CTI. Accompanying the new tool is a new CCD analysis reference data (ARD) input file which describes both the ideal CCD response and the spatial variation produced by CTI. This new approach provides a number of improvements in speed and accuracy and will become the default tool for ACIS RMF generation once a full set of ACIS response calibration products is available.

Output format

The output RMF is a standard FITS file and contains two extensions. The MATRIX extension consists of the actual energy to detector channel mapping and is an MxN matrix where M and N correspond to the sizes of the grids selected by the user. The EBOUNDS extension is a set of vectors giving the upper and lower energy bounds for each detector channel and is determined from information in the gain file. The EBOUNDS information is used by packages such as Sherpa, XSPEC, and ISIS for plotting purposes. It is not used directly for fitting. The mkacisrmf output format retains the HEASARC OGIP 92-002 standard for 2-D RMFs, and is fully compatible with Sherpa, XSPEC, and ISIS.

CALLING SEQUENCES

There are basically two different calling sequences for mkacisrmf depending on whether the user wishes to create a weighted RMF or an RMF appropriate for a single position on one of the ACIS CCDs.

Creating an RMF to match an extracted spectrum

In this scenario, the user has extracted a spectrum from a given region using dmextract and wishes to compute a matching RMF. For example, the sequence:

  dmextract \
  "data_evt2.fits[(x,y)=circle(4232, 3289,30)][bin pi=1:1024:1]" \
  test_src_pi.fits  wmap="det=8"

  mkacisrmf infile=CALDB                \
            outfile=test_wrmf.fits      \
            energy=0.3:10.0:0.005       \
            channel=1:1024:1            \
            chantype=PI                 \
            wmap=test_src_pi.fits       \
            gain=CALDB
   

will extract a spectrum from within the indicated region and create a weighted RMF appropriate for that region. Here, the wmap option in the call to dmextract creates a weight map (see the documentation for dmextract) which mkacisrmf will use to create a counts weighted RMF over the extraction region. The "infile" parameter is set to "CALDB" and mkacisrmf uses the information in the header of the input wmap file to determine the appropriate Phase 2 response calibration file to use. The energy grid for the RMF is given in keV and in this example ranges from 0.3 to 10.0 keV in 5 eV increments. The channel grid in this case uses PI channels.

Similarly, the gain parameter should be set to "CALDB" as well in order to ensure that an appropriately matched gain file is used. Every Phase 2 response calibration file has corresponding matched gain file in the CALDB.

Setting the "infile" parameter to a value of CALDB, in conjunction with an input wmap file, is the best way to ensure that the most appropriate Phase 2 response calibration file is used for a given dataset. As with any CALDB file, updated files are added to the CALDB to correct problems found with existing files; however, by using the CALDB value for the "infile" parameter, mkacisrmf will automatically select the appropriate file. This method is the recommended way of constructing RMFs with mkacisrmf.

Creating an RMF at a specific location

For this case, it is assumed the user wishes to create an RMF file for a specific location on one of the ACIS CCDs, independent of a given dataset or extraction region. In such a situation, the user must explicitly supply the value of the "infile" parameter, since the tool does not have access to the information in a header to determine which file should be used. For example, the sequence:

  mkacisrmf \
  infile="$CALDB/data/chandra/acis/cpf/p2_resp/acisD2000-01-29p2_respN0002.fits" \
            outfile=test_c7_645_10_rmf.fits      \
            wmap=none                            \
            energy=0.3:10.0:0.005                \
            channel=1:1024:1                     \
            chantype=PI                          \
            ccd_id=7                             \
            chipx=645                            \
            chipy=10                             \
  gain="$CALDB/data/chandra/acis/bcf/gain/acisD2000-01-29p2_gain_ctiN0003.fits"
  

creates an RMF at a position of (645, 10) in CHIP coordinates on CCD 7. Again, the user must ensure that the appropriate matching gainfile is used. The energy and channels grids are the same as in the previous example. Note that when specifying the input response and gain files explicitly, the path to these files must be included.