Creating ACIS RMFs with mkacisrmf

Introduction

The tool mkacisrmf was a new addition in CIAO 3.2 and represents an entirely new method for creating ACIS imaging response matrices. This tool contains all the functionality of the previous tool mkrmf. Unlike its predecessor, however, mkacisrmf separates the RMF calculation process into two components: an "ideal" component which describes the CCD spectral response prior to the effects of CTI, and a spatially varying component which incorporates the changes in the response produced by CTI.

The version 5 "phase 2" response file (acisD2000-01-29p2_respN0005.fits) was released in CALDB 3.2.0 (21 November 2005). The How CIAO 3.3 and CALDB 3.2.0 Affect Your Analysis section of the CIAO 3.3 release notes explain how they will affect your analysis.

Technical Details

This new method was motivated primarily by a desire to provide a more rapid means of developing ACIS response calibration products. Accompanying the new tool is a new CCD analysis reference data (ARD) file which describes both the ideal response and the spatial variation produced by CTI. In contrast to traditional CCD FEFs, the CTI-induced spatial variations can be generated directly from numerical simulations of the CCD response obviating the need for laborious fitting at each position on the CCD. This human-intensive fitting was the primary bottleneck in generating ACIS FEFs. Once the "scatter matrix", describing the spatial variations, has been generated automatically, small scale adjustments are included to account for differences between the simulated response and the actual CCD response as measured using data from the onboard calibration source.

The algorithm contained in mkacisrmf is based upon the the algorithm in Alexey Vikhlinin's tool calcrmf2 (previously available as Contributed Software) and described in his memo "Updates to the RMF model in the ACIS FI CCDs" (PDF). The CIAO tool is essentially a direct translation of this prototype code and includes a number of enhancements including an improved interpolation scheme to calculate the response at intermediate energies between the available calibration points. All functionality of the previous tool mkrmf is available including the ability to produce weighted response matrices for arbitrary spatial regions.

Analysing & Re-analysing Data

All new analyses on the calibrated chips should be done with mkacisrmf instead of mkrmf. It is necessary to upgrade to at least CALDB 3.2.0 to have access to the most recent calibration files.

The mkacisrmf tool can be used to create RMFs for all -120 ACIS observations taken in any mode except for GRADED mode. For GRADED mode, it is possible to create responses for the back-illuminated chips (ACIS-S1 and S3) only. For more information, refer to the ACIS GRADED Mode Data section of the mkacisrmf thread.

Also, the response files must be used with data that has the time-dependent gain adjustment and CTI correction (if available) applied. (There is no CTI correction for the back-illuminated ACIS chips, S1 and S3.) For instructions on how to run the new tool, see the mkacisrmf analysis thread.

Re-analysing data with mkacisrmf:

The results from mkacisrmf and mkrmf are very similar. Most users will not lose any accuracy by finishing the analysis with RMFs created by mkrmf. The exceptions are the following cases:

• The new RMFs are 10-15% more accurate at energies lower than 1 keV. If your analysis is sensitive at this level, create a new RMF with mkacisrmf.

• Calibration for the ACIS-S1 (ccd_id=5) back-illuminated chip has been greatly improved. Anyone doing imaging analysis on this chip should use mkacisrmf.

Further information on running this tool is given in the mkacisrmf analysis thread.

Using Consistent Calibration

specextract, a new script for creating ACIS spectra for extended sources, is available as of CIAO 3.3. This script uses the most appropriate response tool to create the RMF (mkrmf or mkacisrmf). specextract should be used for the analysis of extended sources only and is designed to replace acisspec; refer to the specextract thread for instructions.

The CIAO script psextract still uses mkrmf to create the RMF files (Figure 1). Users who wish to take advantage of the improvements in mkacisrmf may run it standalone to generate new RMFs after using the script to create the spectrum and ARF files (Figure 2); instructions are available in the Using mkacisrmf with the psextract scripts section of the mkacisrmf thread. The same is true for the acisspec script; if you do not wish to repeat the analysis with specextract, mkacisrmf may be run separately to create a new RMF.

In all cases, it is important that the calibration applied to the event file is consistent with the RMF tool chosen. A systematic energy shift will occur when the event data and the RMF have inconsistent gain files. The following gain should be used when running acis_process_events:

The newest version of the gain file (here the N0005 version) is picked up automatically when acis_process_events is run. Note that the version 4 gain file (acisD2000-01-29gain_ctiN0004.fits) is good enough for use with this tool as well; see the How CIAO 3.3 and CALDB 3.2.0 Affect Your Analysis section of the release notes for information on the changes between version 4 and 5.

If you are using mkrmf and therefore need to apply the version 2 gain file, set the gainfile parameter to point to it:

unix% pset acis_process_events \
      gainfile=$CALDB/data/chandra/acis/bcf/gain/acisD2000-01-29gain_ctiN0002.fits

Figures

Figure 1. Using mkrmf in an analysis thread.

Figure 2. Using mkacisrmf in an analysis thread.