Marx 5.0.0

Creating CIAO-based ARFs and RMFs for MARX Simulations

While marx strives to accurately model of the Chandra Observatory, there are some differences that need to be taken into account when processing marx generated data with CIAO. As described on the caveats page, marx does not incorporate the non-uniformity maps for the ACIS detector, nor does it employ the spatially varying CTI-corrected pha redistribution matrices (RMFs). As such there will be systematic differences between the marx ACIS effective area and that of the mkarf CIAO tool when run in its default mode. Similarly, the mapping from energy to pha by marx will be different from that predicted by mkacisrmf.

Creating an ARF to match a marx simulation

As mentioned above, marx does not implement the ACIS QE uniformity maps. The following mkarf parameters may be used to produce an ARF that is consistent with the marx effective area:

   mkarf detsubsys="ACIS-7;uniform;bpmask=0" \
         maskfile=NONE pbkfile=NONE dafile=NONE

An aspect histogram is also required by mkarf. The asphist CIAO tool may be used to create an aspect histogram from an aspect solution file. The marxasp program may be used to create the aspect solution file for a dithered marx observation. See the various examples for its use.

Creating an RMF to match a marx simulation

Marx maps energies to phas using the FEF gaussian parametrization utilized by the mkrmf CIAO tool. The newer mkacisrmf tool uses a more complicated convolution model that does not appear to permit a fast, memory-efficient random number generation algorithm that marx would require. In contrast, a gaussian-distributed random number generator is all that is required to produce pha values that are consistent with mkrmf generated responses.

The Chandra CALDB includes several FEF files. The one that marx-5.0 employs is acisD2000-01-29fef_phaN0005.fits, and is located in the $CALDB/data/chandra/acis/cpf/fefs/ directory. This file must be specified as the infile mkrmf parameter.

For a point source simulation, look at the marx2fits generated event file and find the average chip coordinates. The CIAO dmstat tool may be used for this purpose. The CCD_ID and the mean chip coordinates are important for the creation of the filter that will be passed to mkrmf to select the appropriate FEF tile. For simplicity suppose that the mean point source detector location is at (308,494) on ACIS-7. Then run mkrmf using:

  fef="$CALDB/data/chandra/acis/cpf/fefs/acisD2000-01-29fef_phaN0005.fits"
  mkrmf infile="$fef[ccd_id=7,chipx_hi>=308,chipx_lo<=308,chipy_hi>=494,chipy_lo<=494]"
      axis1="energy=0.25:12.0:0.003" axis2="pi=1:1024:1"
      outfile=marx.rmf

See the CIAO threads for other ways of running mkrmf. The important thing is to specify the correct FEF and tile.

This page was last updated Jan 25, 2012 by John E. Davis.
Technical questions should be addressed to marx-help at space mit edu.