Nicer

NICER data extraction

NICER is a soft X-ray observatory on board the ISS. Biggest difference to many other modern X-ray satellites are its silicon drift detectors and X-ray concentrators (instead of focusing mirrors), which together do not permit imaging capabilities. While this simplifies the data extraction, it is also required to simulate a background for each observation.
Previously downloaded NICER observations can be found under /satdata/X-ray/nicer/.

Preparation:

To apply the the latest calibration and filtering, first run 'xtiprepare'. The script will produce cleaned event files inside the subdirectory 'reprocessed' inside 'prepdir'.

#!/bin/csh

set datadir = /data/X-ray/nicer/data/2200570101
set prepdir = '/userdata/data/$user/where/ever/you/want'

xtiprepare --datadir=$datadir  --prepdir=$prepdir  

If you notice an increased background in the resulting spectra, you can apply additional filter criteria during the preparation. See 'xtiprepare --help' for more information.

Spectrum and Light-curve Extraction:

To produce light-curves, spectra and apply barycenter correction 'xtiextract' can be used as follows.

#!/bin/csh

set prepdir = '/userdata/data/$user/where/ever/you/want'
set dirname = 'extraction_123'

xtiextract --name=$dirname --prepdir=$prepdir

The produced spectrum and light-curve can then be found in the subdirecory 'dirname' in $prepdir. Currently no observation-specific response files are generated, but instead soft links to the current ARF & RMF files included in the CALDB are generated.

Background generation:

As we cannot produce a background spectrum from an off-source region on the detector, a background spectrum has to be modeled. Currently there are two different tools available for that purpose. One based on external parameters during the observation (i.e., space weather, solar angle and cutoff-rigidity of earths B-field) amd one based on the internal distribution of measured events. Both are still in active development and might not be able to produce realistic spectra for all observations. In that case a simple reference observation of one of the RXTE background fields might be used instead.
Both tools are further described on the instruments webpage:
https://heasarc.gsfc.nasa.gov/docs/nicer/tools/nicer_bkg_est_tools.html

Issues to keep in mind:

Feel free to expand this list whenever you came across any new issues.

Optical loading:

When optical light is reflected onto the detectors, this can significantly increase the low-energy background below 3keV. This is most significant for solar angles below ~70 degrees. However, due to its position on the ISS this can also happen at any solar angle when light is reflected by other part of the ISS.

Polar Horns:

Unlike other observatories in LEO the ISS is on an orbit with a quite high inclination of 51.6 degrees. This means the ISS passes through the so called polar horn regions, where the outer Van Allen belt reach LEO. These regions can feature an increased population of energetic electrons, which can induce a strong background signal in NICER, particular at higher energies.
See this "science nugget" for more information:
https://heasarc.gsfc.nasa.gov/docs/nicer/science_nuggets/20180809.html

Overly aggressive filtering:

It can happen, that e.g. optical loading leads to a filter parameter exceeding the standard filter criteria for most of certain observations. If that happens you should check the .mkf-file and see whether any of the parameters exceeds the default filter criteria specified in the fhelp of nimaketime. This can be done e.g. using fplot:

 fplot *.mkf TIME "FPM_UNDERONLY_COUNT SUN_ANGLE FPM_OVERONLY_COUNT SUNSHINE "

Such an observation might still be of interest, however. In that case one might want to increase the allowed range e.g. for "underonly_range" in "xtiprepare".

Large event files:

The downloaded eventfiles are generally compressed and are extracted to memory before the data processing. This fails for large eventfiles of several GB. In this case one has to unzip the file event_cl/niXXXXXXXXXX_0mpu7_ufa.evt.gz before running xtiprepare.

Active FPMs:

NICER has 56 focal-plane modules (FPMs) of which 52 are usually active. For different reasons, e.g. annealing, sometimes some FPMs are switched off. And sometimes you have excluded certain FPMs because of their increased background (especially FPM 14 & 34). In both cases you have to be careful to scale the ARF or generate a new one. So one should check the event files how many detectors where active during the observation.
More info here: https://heasarc.gsfc.nasa.gov/docs/nicer/analysis_threads/arf-rmf/

Damage to the optical bench

On May 22, 2023, between 13:00 and 14:00 UTC, NICER appears to have developed a visible-light leak in the X-ray Timing Instrument (XTI) optical bench. This leads to and increase in the undershoot rate, background at soft energies and can lead to detectors switching off, telemetry saturation, and spurious timing features. This only affects observations performed during orbital days. Night time observations are unaffected.