NICER Calibration RecommendationsReturn to: Analysis Threads | Analysis Main Page
OverviewOver 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: 2024-02-13 IntroductionThe 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 CALDBPlease 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. NICER Calibration releases are documented at the NICER Calibration Documents page. 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. Background. NICER background models continue to evolve and improve. As of NICERDAS 12 (HEASoft 6.33), background model auxiliary data files are now included within NICER's CALDB. Current Best RecommendationThe NICER team recommends the NICER CALDB release "xti20221001" to be used for all analysis. This release containes the following new components:
For more detailed calibration release information please see the NICER Calibration Documents page. ARF and RMF files are generated for your specific observation. Users should now generate all responses, tailored to their observation with the nicerarf and nicerrmf tasks. For more information, please see the NICER Responses analysis thread. However, instead of generating responses manually, these products can be generated using the nicerl3-spect task. As of the xti20240206 release, the nicerrmf task is able to adjust the low energy trigger efficiency curve to match threshold settings during orbit day for data taken after the optical light leak that developed in May 2023. Background As of NICERDAS 10 (released October 2022), NICER background models are now included within NICER's official software. The NICER team does not yet have a definitive recommendation about which model to use in all circumstances. Here are some guidelines:
Domain of ApplicabilityThe 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 1.5% in the 0.4 - 10 keV range. The current recommended systematic error vector is shown in Figure 1. This file is in CALDB and can be applied to any spectrum using the niphasyserr task, which is automatically done by nicerl3-spect. Below 0.4 keV there are several uncertainties that start to compete (gain scale, RMF parameters, intrinsic Crab spectrum including Carbon edge profile). Above 10 keV, expect systematic errors to be larger due to uncertainties in the optic throughput and detector efficiency.
Effects of optical loading and threshold change. After the light leak of May 2023, NICER changes its low energy threshold setting for observations taken during orbit day. This affects data in the 0.25 - 0.45 keV range. HEASoft 6.33 and CALDB xti20240206 are able to take into account for response estimation (nicerrmf) and SCORPEON background estimates. However, users can expect larger errors in the 0.25-0.38 keV range. 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 improvements released in 2021 ("optmv12" gain), we 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. Also a strong noise peak may intrude into the energy band of interest (0.25 - 0.45 keV range). For that reason, the user should select a smaller undershoot range (0-50 preferred) if possible. Going outside of the 0-500 range for undershoots is not recommended. Several odd behaviors become apparent above this range which are difficult or impossible to calibrate. 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 RecommendationsObsolete Release: xti20221001The NICER team declares the NICER CALDB release "xti20221001" to be obsolete. Users may continue to use this release for existing projects but shoud migrate to the best recommended calibration release as soon as practicable. This release contained the following new components:
Obsolete Release: xti20210707The NICER team declares the NICER CALDB release "xti20210707" to be obsolete. Users may continue to use this release for existing projects but shoud migrate to the best recommended calibration release as soon as practicable. This release contained the following new components:
Obsolete Release: xti20200722The NICER team declares the NICER CALDB release "xti20200722" to be obsolete. This release contained the following new components:
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 ReleasesWe document here older recommendations which are now obsolete, but which may have been used for previous analysis.
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.
Please see the NICER CALDB documentation page for more descriptions of changes. Modifications
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- HEASARC Director: Dr. Andy Ptak
- NASA Official: Dr. Andy Ptak
- Web Curator: J.D. Myers
- Last Modified: 13-Feb-2024