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(OBSOLETE) NICER Precomputed Responses

Overview

This page describes a previously recommended method for generating NICER responses (ARF and RMF files), which is now obsolete. It is included for archival purposes only. These files will not be updated with improved calibration information.

Read this thread if you want to: Recall the obsolete method of using precomputed ARFs and RMFS.

Last update: 2022-10-26

Introduction

This page describes a previously recommended method for generating NICER responses (ARF and RMF files), which is now obsolete. It is included for archival purposes only. It is intended that this page be available so that analysts may understand the previous process, but not recommended for current use.

To read a longer discussion of NICER's ARFs and RMFs, as well as the current recommended methods, please see the NICER Responses analysis thread.

In the techniques described below, the user will retrieve precomputed ARF and RMF files, and then combine them for their specific observation.

How are NICER ARFs and RMFs Delivered?

NICER's primary delivery mechanism for calibration data is HEASARC's Calibration Database (CALDB) system.

If you need help to install NICER software or calibration please see the thread titled Setting Up a NICER Analysis Environment to download or update your NICER CALDB.

The CALDB area only contains files designed for the currently recommended ARF and RMF techniques. In the obsolete techniques described below, the analyst downloads files outside of CALDB and then uses them.

Retrieving an ARF and RMF from CALDB for Spectral Analysis (Obsolete)

For spectral analysis with XSPEC or any other analysis program, you will need both an ARF and RMF for your observation. As noted above you can retrieve the on-axis full-array files from CALDB, which will apply to most situations. Read the next sections if have spectra for individual modules or a subset of detectors.

To retrieve the on-axis full-array ARF from your installed CALDB, issue the following command (obsolete): quzcif NICER xti 99 - SPECRESP now now - and a response similar to the following should appear: /path/to/caldb/data/nicer/xti/cpf/arf/nixtiaveonaxis20170601v003.arf 1 Note that the actual path to your CALDB area may be different, and the version number (vNNN) may be different. The "nixtiaveonaxis" indicates that this is the "average" (actually sum) on-axis ARF. The "1" indicates that the ARF is stored in the first data extension. To retrieve the full-array RMF from your installed CALDB, issue the following command (obsolete): quzcif NICER xti 99 - MATRIX now now - and a response similar to the following should appear: /path/to/caldb/data/nicer/xti/cpf/rmf/nixtiref20170601v001.rmf 2 Note that the actual path to your CALDB area may be different, and the version number (vNNN) may be different. The "nixtiref" indicates that this is the "reference" (array-average) response matrix. The "2" indicates that the matrix is stored in the 2nd table extension.

You may either use the CALDB files in-place or copy them to your local directory for use in XSPEC. CALDB files will always have a unique name. When newer calibration files become available, the older ones are not deleted.

Calculating ARF and RMF for Different Subset of Modules (Obsolete)

As noted above, the ARF and RMF available within CALDB are limited to the combination of all 52 detectors (full-array). If you have data where NICER was configured with only a subset of detectors enabled, then the above full-array products will not be useful. You may also want to examine a subset of detectors for various reasons. (Example: excluding a noisy detector)

The NICER team provides a way to obtain a custom ARF or RMF for a specific set of detectors.

Prerequisites:

  • Know which subset of detectors you are interested in
  • Ability to download .tar.gz files from the internet and extract them.
  • Run scripts from the command line.

To use this feature, you will need to download the obsolete auxiliary data which contains per-module ARF and RMF files. The information is stored outside of CALDB, thus necessitating the additional download.

The obsolete downloads available are:

Download the desired file using your browser. After downloading, you should have a .tar.gz file. We will use the example of nicer-arf-rmf-xti20200722.tar.gz. You will want to unarchive this file. tar xvzpf nicer-arf-rmf-xti20200722.tar.gz and the result will be a folder named nicer-arf-rmf-xti20200722/ in your current directory. Change into this directory with cd nicer-arf-rmf-xti20200722/ and you should find folders named arf/ and rmf/. (for the earlier releases, only per-module ARFs are provided, and no per-module RMFs).

To calculate a new ARF, this is the obsolete process. Change to the arf/ subdirectory. cd arf/ You will find a directory of 52 per-module ARFs of the form nixtionaxis20170601_detid??_v004.arf where detid?? is the detector ID number 00 .. 67.

There is also a "list file" named nixtionaxis20170601_arfweights.lis, which lists each per-module ARF and a weight of 1.0 or 0.0. Based on your knowledge of which detectors are enabled and/or selected, edit the list file and change any deselected detectors to a weight value of 0.0. Example. If detector ID 01 is disabled, the top of the list file should read: nixtionaxis20170601_detid00_v004.arf 1.0 nixtionaxis20170601_detid01_v004.arf 0.0 nixtionaxis20170601_detid02_v004.arf 1.0 ... Note the 0.0 for line detid01. If you wish, you may also remove an individual line completely, which is equivalent of setting the weight to 0.0.

Finally, run the command (obsolete) ./nicer-add-arfs and the resulting sum should appear as a file named nixtionaxis20170601_combined_v004.arf. This is your new summed ARF with only the desired detectors.

Note: if you get an error "command not found" or "permission denied" you may need to set the execute permissions of the script. This command can do the trick: chmod ugo+x ./nicer-add-arfs

To calculate a new RMF, the process is largely the same but also obsolete. Change to the rmf/ directory, edit the RMF "list file" to enable only the desired detectors by setting their weights to 1.0 and disabled detector weights to 0.0, and then run (obsolete) ./nicer-avg-rmfs The result will be named nixti20170601_combined_v002.rmf.

Note that these small scripts are not standard HEASoft tools, nor are they officially part of the NICERDAS suite of tools. They are quick tools to help you generate new response files. Each tool contains a header which describes how to use them, and gives some options to modify the operation of the task, such as where it places its output. If you are familiar with shell scripts, you can easily modify the scripts as desired.

When complete, you should have a summed ARF and (optionally) an averaged RMF that you can use within XSPEC as described above.

Will the Precomputed Responses Be Maintained Forever?

What happens if you want to use the "old" method of using precomputed responses instead of the new response calculators? Will the precomputed responses be maintained forever? The short answer is, no.

NICER is committed to maintaining an online presence for the precomputed responses released in July 2020 for at least six months after the next big calibration release. This means that the NICER team will maintain the precomputed responses on-line until at least February, 2022. After this date, the precomputed responses may become unavailable.

However, the NICER team will not update the precomputed responses. This means that although they are available for download, they will not represent the state of the art, and most importantly cannot be used for off-axis or high optical loading conditions.

These Instructions Are Obsolete

The NICER team continues to revise and improve response matrix products.

To read the currently recommended methods, please see the NICER Responses analysis thread.

Modifications

  • 2020-10-16 - fixed typos
  • 2020-07-22 - initial draft
  • 2021-04-16 - add navigation bar
  • 2021-07-16 - move precomputed ARF/RMF discussion to its own thread