A User's Guide to NICER Data & Analysis


It is strongly recommended that, before starting analysis of NICER data, user review the NICER Mission Guide for an overview of the mission, the instrument subsystems, and the data file structure.


NICER Data Reduction using NICERDAS

NICERDAS is a set of NICER-specific tools distributed with HEASoft version 6.24 or later.

NICERDAS includes the following tools:

nicercal - Apply standard calibration to NICER observation
nicerclean - Apply standard NICER screening
nicerl2 - Apply NICER Level2 standard calibration and filtering
nicermergeclean - Merge NICER MPU data and apply standard screening
nicerpi - Apply gain calibration to NICER event file (calculate Energy/PI)
nicertimecal - Apply clock calibration to NICER event file
nicerversion - Report NICER software version
niextract-events - Fast event filtering, similar to extractor
nimaketime - Create standard NICER screening GTI
nimpucal - Apply standard NICER calibration to MPU data
nimpumerge - Merge NICER event data from multiple MPUs (or observations)
niprefilter2 - create augmented NICER-specific filter file
niprefilter - create NICER-specific filter file

NICERDAS uses the HEASARC CALDB to access calibration data.


NICER Data Reduction using NICERDAS

Standard Data Reduction Steps:

  1. nicercal: Apply NICER calibration
  2. nimaketime: Apply standard NICER time screening
  3. nicermergeclean: Merge and clean event data; also used to merge data from multiple observations if desired

nicerl2: A script which runs the filtering, calibration and merging of NICER events. This wrapper script calls nicercal + nimaketime + nicermergeclean.

If users are interested in doing pulse searches or other detailed timing studies, the NICER event files produced by nicerl2 can be corrected to the solar system barycenter using the barycorr task. See the discussion below for more information about using barycorr with NICER data.

After data have been cleaned and calibrated (using nicerl2, for example), science products (time-filtered events, spectra and/or lightcurves) can be extracted using xselect.


Using nicerl2

The nicerl2 task applies standard NICER-recommended calibration processes to an entire NICER observation, as well as standard screening.

This is a high level task that calls multiple sub-tasks as a convenience to the user. It performs the following standard "Level2" processing steps.

  1. nicercal - apply standard NICER calibration
  2. nimaketime - create standard screening good time intervals
  3. nicermergeclean - combine per-MPU data and filter/screen

Please see the help files for these individual tasks for more information about what they do and how they can be used.

Main task parameters:


   indir [filename]
          Input directory name. The directory should be a single NICER
          observation directory, which in turn contains
          xti/{events_uf,events_cl,hk,auxil} subdirectories.

   ufdir = "xti/event_uf" [filename]
          Subdirectory containing per-MPU "uf" event files, which are the
          input to the task. This directory name is relative to the
          observation directory.

   cldir = "xti/event_cl" [filename]
          Subdirectory where the output files as listed above are placed.
          This directory name is relative to the observation directory.

   ufafile = "$CLDIR/ni$OBSID_0mpu7_ufa.evt" [filename]
          Name of the master "ufa" event file to be created. Use the
          variables "$CLDIR" for the output directory name, and use
          "$OBSID" for the observation ID number.

   clfile = "$CLDIR/ni$OBSID_0mpu7_cl.evt" [filename]
          Name of the master "cl" event file to be created. Use the
          variables "$CLDIR" for the output directory name, and use
          "$OBSID" for the observation ID number.

   mkfile = "auxil/ni$OBSID.mkf" [filename]
          Name of the MKF filter file, to be used as input, relative to
          the observation directory. Use the variables "$CLDIR" for the
          output directory name, and use "$OBSID" for the observation ID
          number.

Examples:

  1. Apply NICER calibration and screening to data from observation 1706221428, overwriting previously existing files that are in the event_cl directory:
prompt> nicerl2 indir=1706221428 clobber=YES

The input directory is a NICER observation number 1706221428. The output files are placed in the observation directory, in the cleaned event directory, i.e.:

1706221428/xti/event_cl/

Note that you should be in the root directory which contains the NICER data directory 1706221428.


Running the Tools Separately

If you want more control of the data reduction, you may want to run the nicercal, nimaketime, and nicermergeclean separately, rather than running the nicerl2 task.

Step 1: run nicercal

nicercal applies standard NICER-recommended calibration processes to an entire NICER observation (in a standard NICER observation ID directory structure, and thus works on an entire observation directory. The default screening is to remove over-shoot and under-shoot events (since they cannot be calibrated) but to keep X-ray events and forced triggers

An example of using nicercal to apply NICER calibration to data to observation 1706221428.

prompt> nicercal indir=1706221428 outdir='$INDIR/xti/event_cl' clobber=YES

The input directory given above (indir) is a NICER observation directory which contains data from observation id 1706221428.

The output files are placed in the observation directory under 1706221428/xti/event_cl/ni1706221428_0mpu*_ufa.evt. (Note that single quotes are used to prevent $INDIR from being expanded by the Unix shell.) The default calibrations files are taken from CALDB (Note that the NICER CALDB can be installed locally or the HEASARC CALDB can be accessed remotely by the user.)

Step 2: run nimaketime

NICER has a task named "nimaketime" which applies recommended screening criteria.

prompt> nimaketime infile="$obs/auxil/ni${obsroot}.mkf" outfile="$obs/auxil/standard.gti"

The output of nimaketime is a Good-Time Interval (GTI) file which users can use downstream (here named standard.gti); nimaketime does not apply the GTIs to the events data.

Note that $obs is the observation directory name and $obsroot is the observation number. Type fhelp nimaketime to see what kind of screening is applied.

nimaketime can apply the following screening criteria. The exact filtering string is also given in quotes after each item.

  • nicersaafilt=YES
    Excludes times within SAA as defined by NICER_SAA. This is a NICER-specific definition of the SAA stored in the NICER CALDB.
    "(NICER_SAA==0)"

  • saafilt=NO
    When YES, excludes times within the generic SAA defined by prefilter. (NOTE that this is set to "no" because normally you do not need to apply both the generic and NICER-specific screening. Use either nicersaafilt=YES or saafilt=YES, but not both.)
    "(SAA==0)"

  • trackfilt=YES
    Excludes times when NICER is not in target tracking mode.
    "(ATT_MODE==1 && ATT_SUBMODE_AZ==2 && ATT_SUBMODE_EL==2)"

  • ang_dist=DIST
    Excludes times when the angular distance between the target and NICER's boresight is greater than DIST (by default, 0.015 degrees). "(ANG_DIST < 0.015)"
  • elv=MINELV. Excludes times when the distance to the earth limb is less than MINELV (by default, 30 degrees).
    "(ELV > MINELV)"

  • br_earth=MIN_BR_EARTH
    The same as ELV for the bright earth. Default value of MIN_BR_EARTH = 40 degrees.
    "(BR_EARTH > MIN_BR_EARTH)"

  • cor_range=A-B
    Specify a range of magnetic cut-off rigidity (COR) to accept (in GeV/c). This can be used for filtering of high background regions. You can use the notation cor_range="A-" to specify a minimum but no maximum, cor_range="-B" to specify a maximum but no minimum, and the default cor_range="-" accepts all CORs.
    "(COR_SAX > A && COR_SAX < B)"

  • min_fpm=MIN_FPM
    Specify a minimum number of enabled detectors. (The default MIN_FPM = 38.)
    "(NUM_FPM_ON > MIN_FPM)"

Step 3: run nicermergeclean

Up until this point, data are kept on a per-MPU basis. The data must be merged and cleaned using the task nicermergeclean.

First, create a list of the individual event lists from all 7 MPUs:

prompt> ls $obs/xti/event_cl/ni${obsroot}_0mpu[0-6]_ufa.evt > ufalist.lis

Then, run nicermergeclean using the list of individual MPU event files as input:

prompt> nicermergeclean infiles=@ufalist.lis \
ufafile=$obs/xti/event_cl/ni${obsroot}_0mpu7_ufa.evt \
clfile=1706221428/xti/event_cl/ni1706221428_0mpu7_cl.evt \
gtifile=standard.gti
  • The first ls command lists the calibrated "ufa" files. The second nicermergeclean command merges and cleans the inputs. The GTI (standard.g.) is the result of the previous nimake.me command (see previous slide).
  • Here $obs is the observation directory name and $obsroot is the observation number.
  • The GTI file standard.gti is the output from nimaketime.

The nicermergeclean task is a high-level task that merges data from multiple NICER MPU slices, and applies standard screening. It is equivalent to running the tasks nimpumerge (for merging MPU data) and nicerclean (for applying screening).

An input to the task is a set of several MPU event files, which have been calibrated but not finely screened. These are typically named niNNNNNNNNNN_0mpuN_ufa.evt, where the "ufa" indicates unscreened but calibrated data. The output of the task is a single merged and screened event file, which is typically named niNNNNNNNNNN_0mpu7_cl.evt, where the "cl" indicates a cleaned event file. Here "mpu7" means all MPUs, numbered 0-6, have been merged into one file designated as "mpu7."

Another input to the task is a GTI (good time interval) file which contains the desired screening good time intervals. Note that nicermergeclean does not by itself create a GTI file, but rather uses one created externally by the user. The NICER team recommends to use nimaketime first to create a GTI file using standard screening criteria before running nicermergeclean.

This task runs the following lower-level NICER tasks to complete its work.

  1. nimpumerge - to merge per-MPU data into a single file.
  2. nicerclean - to apply screening criteria.

In addition to applying good time screening, the nicerclean task also applies per-event screening based on event type and pulse height range. See the documentation for nicerclean for more information. By default, events with pulse invariant (PI) pulse heights above 20 (i.e. 200 eV and above). This can be changed using the pirange parameter. For example, use pirange=35:1000 to select events with nominal pulse heights in the range 0.35 - 10.00 keV. When trumpetfilt=YES, nicermergeclean performs standard filtering to exclude background events using the event PI_RATIO column. This is known as "trumpet filtering" since the PI vs PI_RATIO cloud looks like a trumpet. See the help for nicerclean for more information:

prompt> fhelp nicerclean

Specific Examples: merge per-MPU data from multiple observations

Create a list of the UFA files from obsid 1234567890, using these commands on the Unix command line:

prompt> ls 1234567890/xti/event_cl/ni1234567890_0mpu[0-6]_ufa.evt > ufalist.lis

This creates a file, ufalist.lis, which contains a list of all the UFA files from all the MPUs from OBSID 1234567890 Then append the UFA files from OBSIDS 1111111111:

prompt> ls 1111111111/xti/event_cl/ni1111111111_0mpu[0-6]_ufa.evt >> ufalist.lis

Then use the ufalist.lis as input to nicermergeclean to merge the data from the 2 OBSIDS:

prompt> nicermergeclean infiles=@ufalist.lis \
ufafile=merged_ufa.evt \
clfile=merged_cl.evt

If you want to apply time screening, then you will need either:

  • Merge the filter files (i.e. the .mkf files) from 1234567890 & 1111111111, using the general HEASoft tool "ftmerge", then run nimaketime on result, OR
  • Make individual observation GTI files using nimaketime on 1234567890 & 1111111111, then merge the resulting GTI files using the general HEASoft tool "ftmgtime" (combining the data using "OR" logic to accept times that are either of the GTIs). For example for the two NICER observations above, suppose you've run nicermaketime on both, outputting the GTI files as <obsN>/auxil/standard.gti (where <obsN> is either 1234567890 or 1111111111. Then to create the merged gti file, you should run the following commands on the command line. These commands create a list of GTIs from the first observation, appends the list of the GTIs from the second observation, then merges the GTIs using OR logic with ftmgtime, overwriting ("clobbering") previous output:

    prompt> ls 1234567890/auxil/standard.gti > combined.gti
    prompt> ls 1111111111/auxil/standard.gti >> combined.gti
    prompt> ftmgtime @combined.gti merged.gti OR clobber=YES

Merging MPU-Merged Data (MPU7 files) from Different Observations

If you have multiple OBSIDs for a given target, it's possible to merge the mpu7_ufa files from each OBSID to create a combined event file for all the observations of the target.

  1. Create a list called ufalist.lis of all the mpu7_ufa files. This assumes all the NICER OBSID directories are under the current working directory, and that the standard NICER directory structure is used:
    prompt> ls */xti/event_cl/ni*_0mpu7_ufa.evt > ufalist.lis
  2. Merge the files with nimpumerge:
    prompt> nimpumerge infiles=@ufalist.lis outfile=merged_ufa.evt mpulist=7
  3. You can then use nicerclean to screen this data:
    prompt> nicerclean infile=merged_ufa.evt outfile=merged_cl.evt
  4. If you want to apply time screening, then you will need either:
    • Merge filter files (.mkf files) from the individual observa.ons using ftmerge, then run nimaketime on result, OR
    • Make GTI files using nimaketime on each observation, then merge the resulting GTIs with ftmgtime (combining the files using "OR" logic).

Selecting data with XSELECT

You can then use the HEASoft xselect package to generate data products from the screened, merged events data. For example:

prompt> > xselect
XSEL> set datadir 1234567890/xti/event_cl
XSEL> read event ni1234567890_0mpu7_cl.evt
XSEL> set binsize 1
XSEL> extract curve
XSEL> plot curve
XSEL> set phaname PI > extract spectrum > plot spectrum
XSEL> save curve
XSEL> extract spectrum
XSEL> save spectrum
XSEL> exit

Barycentering Photon Arrival Times

You can apply barycentering to NICER data using the HEASoft barycorr command.

i prompt> barycorr infile=orig.evt outfile=bary.evt \
orbitfiles="$obs/auxil/ni${obsroot}.orb" \
ra=<RA of target> dec=<Dec of target> \
refframe=ICRS
  • RA and Dec should be the target coordinates in J2000 in degrees
  • Here $obs is the observation directory name and $obsroot is the observation number
  • Use the .orb orbit file for input, and not the filter file (.mkf). The .orb file has the highest precision. In rare cases, the standard filtered orbit solution may not be functioning. In that case the SPS_ORBIT extension can be used ni${obsroot}.orb[SPS_ORBIT]
  • HIGHLY RECOMMENDED: use refframe=ICRS, which selects a modern ephemeris. If you don't do this, there can be millisecond-errors! Deepto Chakrabarty translated new updated JPL ephemerides DE421 and DE430, which are now included in the HEASoft 6.22 distribution. Select them with the ephem=JPLEPH.430 to select DE430.

Background Correction

As noted above, NICER events can include various background components in addition to the good science events. These background components are usually minimized through standard event selection and cleaning, but residual background events may remain in even "clean" science data. This residual background should be modeled and subtracted from NICER lightcurves &/or spectra for X-ray source analysis.

Non-cosmic background contamination can be most readily seen at low energies, E < 0.3 keV and at high energies, E > 6.0 keV.

  • The low energy background component is dominated by detector noise which produces a "noise peak" near near E ≈ 0.2 keV. The height of this noise peak may be particularly large in cases of low values of Sun Angle or Bright Earth Angle, when optical impinges on the XRCs and FPMs. Optical light leakage can also produce enhanced low energy background. The noise peak varies from FPM to FPM, and is particularly large in FPM #34. If unusual variations are seen at low energies, users should examine the values of SUN_ANGLE and BRIGHT_EARTH from the makefilter (.mkf) file
  • The high energy background component is usually caused by charged particles which are not removed by the standard screening. This background component often manifests itself as a significant count rate in the channels above 12 keV.

The NICER CALDB

The NICER caldb can be accessed using standard HEASoft caltools. We can also use the HEASARC pycaldb package (experimental) to examine and explore the NICER CALDB.

Using quzcif with the NICER CALDB

A standard quzcif call would be

	prompt> quzcif NICER XTI - - MPU_GAIN now now -
	Found: ftp://heasarc.gsfc.nasa.gov/caldb/data/nicer/xti/bcf/gain/nixtiflightpi20170601v001.fits   1

Last Updated: March 27, 2019