NICER Calibration Recommendations

Overview

Over time, the NICER team improves its detector performance model. The NICER team recommends to use the best calibration available for your your scientific analysis.

Read this thread if you want to: know the recommended NICER calibration practices.

Last update: 2021-07-20

Introduction

The NICER team gradually improves its detector performance model. This includes both the energy scale (gain), spectral response (ARF and RMF), and timing calibration.

This document describes the NICER team's recommended practices for calibration of NICER data. Please check this page routinely for the most up to date recommendations.

NICER's Primary Calibration Files are in CALDB

Please be aware that you should be using the HEASARC's Calibration Database (CALDB) system.

CALDB provides not only a way to archive and distribute calibration data, but it also provides a way to "tag" each calibration data set with a unique release identifier. This tagging capability provides a way for analysts to describe in the papers the source of their calibration data.

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.

There are several key components of the NICER calibration data.

Energy scale (Gain). A correct NICER energy scale is key to getting accurate spectral modeling. Please see the thread titled "Gain Calibration" for more information about gain calibration and how to update the calibration of your data.

Reponse matrices (ARF and RMF). A correct NICER response is also key to getting accurate spectral modeling. You will need both the Ancillary Response Files (ARFs; effective area) as well as the Response Matrix Files (RMFs; redistribution). Please see the thread titled "NICER Response Files (ARF/RMF)" for more information about how to use NICER Response Files with your data.

Timing. NICER timestamps are extremely accurate. The standard NICER calibration process should correct for known temporal offsets. Please see the thread titled "Definition of Time in NICER Data Files for more information on how NICER timestamps are defined.

Current Best Recommendation

The NICER team recommends the NICER CALDB release "xti20210707" to be used for all analysis. This release containes the following new components:

  • CALDB Release: xti20210707
  • NICER Team Recommended as of: 2021-07-20
  • Gain: code-named "optmv12" - changes for high undershoots (200-500 undershoot count/s)
  • RMF: now generated by the 'nicerrmf' task (includes effects of optical loading)
  • ARF: now generated by the 'nicerarf' task (includes effects of off-axis vignetting and enabled/disabled detectors)
  • Spectral fitting energy range: 0.25-10 keV
  • Spectral fitting systematic error: 1% (i.e. use "syst 0.01" in XSPEC)
  • Timing: unchanged since post-launch

ARF and RMF files are generated for your specific observation. Unlike the previous release, which utilized precomputed responses that the user would combine manually, the software release HEASoft 6.29 (NICERDAS 8) now supports computing a response for your specific observation. For more information, please see the NICER Responses analysis thread.

Domain of Applicability

The current fits to the Crab are in the energy range 0.24 - 14.0 keV. We expect that most spectral fits can be performed in that energy range with good success. Please note that with this release, performance for on-axis sources has not changed significantly. The major improvement is for off-axis targets, and for sources with high optical loading. The following discussion describes what conditions and assumptions should be considered when analyzing NICER data.

Systematic errors. Relative systematic errors can be expected to be less than 1% in the 0.4 - 10 keV range. Below 0.4 keV there are several uncertainties that start to compete (gain scale, RMF parameters, intrinsic Crab spectrum including Carbon edge profile) so errors may be as large as 5-10% in that range. Above 10 keV, expect systematic errors to be 10%. For bright sources we recommend to set 1% systematic errors using the XSPEC "systematic 0.01" command. Although this does not quite capture the energy-dependent systematics, it will be a good rule of thumb for most observations.


Figure 1. NICER recommended systematic errors. In the 0.4-10 keV range, the spectral systematic errors is within 1% rms. Outside of this band, the recommended systematic errors are ~10%. The NICER team is working to provide a standard tool in the future to add energy-dependent systematic errors to a spectrum.

Off-Axis Performance. We expect that for sources within 100 arcseconds of on-axis (ANG_DIST < 0.0278), performance should be comparable to on-axis results. We are aware that the current off-axis model has special deficiencies for energies above 8 keV and off-axis angles of >150 arcsec. Response errors may be 100% or more. Users of the response calculators should be especially careful of off-axis targets that have high energy tails.

Absolute flux scale. The absolute NICER flux level is not pegged to another observatory or flux reference point. However in the process of fitting the Crab data, there is no doubt that NuSTAR's "absolute" flux calibration (Madsen et al. 2017) has guided the calibrator's hand. Note that in the work here, the Crab spectra have been corrected for instrumental deadtime using the filter file's mean deadtime measurements. For the Crab, the included deadtime correction factor is about 8%.

Flux dependence. We are not aware of any flux dependence on the response matrix. Above ~2 Crab pile-up becomes significant and will distort the spectrum in a complicated and non-linear fashion. At much fainter fluxes, background will be the dominant systematic flux error rather than the response components.

Optical loading dependence. All of the results here are based on filtering to get low optical loading (near-dark conditions). We selected array undershoots less than 50 ct/s for all Crab data (underonly_range=0-50). However, because of the new optmv12 gain and response calculators in this release, we can now recommend calibrated results in the range of 0-500 undershoot ct/s. Optical loading will broaden the energy response as well as modify the trigger efficiency parameters, and these effects are taken into account in the RMF calculator.

Point-like or off-axis nature of target. The NICER response calculators in this release are capable of calculating the response to point-like as well as diffuse sources. The possible diffuse surface brightness profiles supported are: gaussian, uniform flat sky, and user-specified radial surface brightness profile.

Older Recommendations

Obsolete Release: xti20200722

The NICER team declares the NICER CALDB release "xti20200722" to be obsolete. This release contained the following new components:

  • CALDB Release: xti20200722
  • NICER Team Recommended: 2020-07-22 - 2021-07-20 (now obsolete)
  • Gain: code-named "optmv11" - ~5eV changes in the 0-2.5 keV range
  • RMF: code-named "6s" - changes 0-0.5 keV and >9 keV (as much as 50% in some circumstances)
  • ARF: code-named "CONSIM135p" - updated areas to match RMF (as much as ~5% changes)
  • Spectral fitting energy range: 0.25-10 keV
  • Spectral fitting systematic error: 1% (i.e. use "syst 0.01" in XSPEC)
  • Timing: unchanged since post-launch
The newly released ARF and RMF files must be used together. The photon energy grid has changed from previous releases so you cannot mix and match old ARFs/RMFs. Please see the NICER CALDB documentation page for more descriptions of changes.

Also please note that the ARF and RMF available in CALDB area array-averaged 52-detector responses. If you have a different number of detectors you can see the following page: "NICER Response Files (ARF/RMF)" which describes how to create response files specialized for your observation.

Even Older Releases

We document here older recommendations which are now obsolete, but which may have been used for previous analysis.

  • CALDB Release: xti20200202 (OBSOLETE)
  • NICER Team Recommended: 2020-02-02 - 2020-07-22 (no longer recommended)
  • Gain: code-named "optmv7he" (OBSOLETE)
  • RMF: code-named "v1.02" (OBSOLETE)
  • ARF: code-named "CONSIM135o"
  • Timing: unchanged since post-launch

Please see the NICER CALDB documentation page for more descriptions of changes.

Also please note that the ARF and RMF available in CALDB area array-averaged 52-detector responses. If you have a different number of detectors you can see the following page: "NICER Response Files (ARF/RMF)" which describes how to create response files specialized for your observation.

  • CALDB Release: pre-2020
  • NICER Team Recommended: post-launch through 2020-02-02 (no longer recommended)
  • Gain: code-named "optmv7" (OBSOLETE)
  • RMF: code-named "v1.02" (OBSOLETE)
  • ARF: code-named "v1.02" (OBSOLETE)
  • Timing: unchanged since post-launch

Please see the NICER CALDB documentation page for more descriptions of changes.

Modifications

  • 2020-07-20 - initial draft
  • 2020-07-27 - add recommended energy range and systematic error
  • 2021-04-16 - add navigation bar
  • 2021-07-16 - add CALDB release xti20210707