A: References to published work are encouraged, including work citable by a Digital Object Identifier (DOI).

Unpublished work that is based on public data sets or public software can be referenced as work "by the proposal team", without identifying the PI or CoIs by name.

It may be occasionally important to cite exclusive access datasets or non-public software that may reveal (or strongly imply) the investigators on the proposal. It is suggested proposers use language like "obtained in private communication" or "from private consultation" when referring to such potentially revealing work, again without identifying anyone by name.

A: Category C proposals are not carried over into the next cycle. The the time of the proposal deadline a PI of an approved category C observation might not know if the observation will be performed before the end of the current cycle and might want to repropose the same observation for the new cycle in case it will not be observed in the current cycle.

The previously approved category C observation does not need to be mentioned in the new proposal. The target list is public and such a case will get highlighted in the technical report for the reviewers. You are, however, free to acknowledge that the target is in the public list of accepted targets with category C, without saying that it is from your proposing team.

If the observation is performed before the start of the new cycle, it will generally not be duplicated, i.e., no observation of the target based on the same science case will be performed.

For further clarification it is recommended to discuss in the team expertise document (see 2., above) that this is a resubmission of an accepted C target from the current cycle and to explicitly state that the observation in the new cycle is only requested if the C target of the current cycle is not observed.

A:

(1) Observations which are part of the NuSTAR Guest Observer Program become public 6 months after their receipt by the proposal's Principal Investigator (PI) unless the PI waived exclusive use (= data public immediately) or requested and was granted extended exclusive use (= data public after one year).

(2) Observations made under the aegis of another mission's Guest Observer Program which also awarded NuSTAR observing time become public at the same time as the exclusive-use period for that other mission's data expires.

(3) All other observations, i.e., those not performed for any NuSTAR general observer program, are made available in the NuSTAR public archive and have no exclusive-use period. This includes observations performed for calibration, directors discretionary time, and any remaining legacy program targets. The only exception will be observations with significant quality issues which may have their release delayed until the NuSTAR SOC determines the impact on science analysis.

A: The first NuSTAR data release on August 29, 2013 contained observations of celestial calibration targets placed both on-axis and at a range of off-axis angles. The targets were a sample of AGN and X-ray binaries, all of them being point sources. See the next question and answer for how to identify the specific observations which went public.

A: The timing of recorded events in the NuSTAR instrument can drift depending on changing thermal conditions within the spacecraft. Clock offsets from GPS values are measured during each ground station contact and are nominally adjusted if the offsets are larger than +/- 10 msec, but clock offset errors can be as large as 100 msec. Science users who require event timing more precise than this are encouraged to use a clock correction calibration file. After applying the clock offset correction, clock offset errors are nominally less than 100 μsec and relative timing accuracy may be possible to better than 60 μsec.

A: There are two main methods to produce a barycenter corrected NuSTAR event file.

A: Use the "usrgtifile" keyword, but set the "usrgtibarycorr=no" keyword to prevent nuproducts from applying a second barycentric correction to the input GTI files.

A: Yes, NuSTAR has on-board ¹⁵⁵Eu radioactive calibration sources on arms that can deploy into the light-path for FPMA and FPMB. They have only been deployed once on orbit (2012 June 24-25th). The original plan was to deploy them ~1/year to track changes in gain and spectral response. However there is a risk that the deployment arms could become stuck when deployed and so there are currently (August 2013) no plans for future deployments. Changes in the calibration of the instrument are sufficiently tracked with the large number of repeat observations of celestial sources during the mission.

A: An adjustment of the onboard laser metrology system (which tracks the relative motion of the NuSTAR optics bench and the focal plane bench) was performed on March 17th, 2020 to improve the longevity of the laser metrology system. Analysis of any observation taken after March 16th, 2020 will require NuSTARDAS version 1.9.2 (or later), released in HEASoft v6.27.1. The software update is transparent to the user requiring no syntax changes when calling Stage 1 of nupipeline. There is no change to the quality of the science data.

Co-adding the effective areas of the two NuSTAR instruments without first accounting for the cross-normalization may introduce systematic errors in the model parameters. However, this should only be significant for analysis of high signal-to-noise observations when the systematic errors will be large compared to the statistical errors.

For analysis of low signal-to-noise observations (e.g. to test for detectability), when the statistical errors will be large compared to systematic errors, or when the user wishes to combine two observations from different epochs for the same telescope, we recommend the following two methods:

A: This is caused by having multiple files in the "auxil" subdirectory of the format NUSTAR_TLE_ARCHIVE.*.

A: The NuSTAR flight software performs housekeeping operations every second, which results in short periods when the instrument cannot acquire photons (deadtime). In the early observations (prior to August 2012), the flight software performed these housekeeping operations at the same phase of every second, producing small "dips" every second. For bright sources, this can be detected in the power spectrum as peaks at 1 Hz (and multiples of 1 Hz). Additional peaks might appear at 0.25 and 0.50 Hz, due to other housekeeping operations. After this problem was detected, the phase of the second when the housekeeping operations are performed was randomized, which eliminated the spikes from the power spectrum, since the small gaps were no longer coherent. Observations from August 2012 on should not be affected by this issue.

A: Any ds9 region file can be used to extract source spectra and lightcurves. However, ARF corrections and light-curve PSF corrections can only be applied when the appropriate region types are used. For point source extractions the user should use a 'Circle' ds9 region with the standard ARF generation flags. For extended sources arbitrary source extraction regions can be used.

A: For point source extractions, the software currently supports ds9 circles, annuli, and ellipses. However, ghost rays are currently only supported for circular regions. Any geometry can be used to extract counts from extended sources. While the ARF and lightcurve corrections are applied in this case, ghost ray corrections are not applied to extended sources.

A: Additional combinations may be implemented in future releases of the NuSTARDAS package.

A: The motion of the NuSTAR optics relative to the detectors is measured by a metrology system that consists of two lasers positioned on the optics bench pointing back along the 10m mast to position sensing detectors (PDS's) on the focal plane bench. The system is designed to register the motion of the focal point of the optics and for most observations the travel is of order a few mm on the detectors which translates to < 1 arcmin in the sky frame. The processing of metrology data within the NuSTARDAS module 'numetrology' corrects for distortions introduced by the response of the PSD0s. In sporadic cases the laser spots fall outside this calibrated range and the corresponding time intervals are filtered out during the data screening.

The value of the exposure time (not livetime corrected) lost because of the laser spots outside the calibrated range is reported in a specific keyword of the cleaned Level 2 event files named 'NUPSDOUT'. For example see the observation of Her X-1 on 2012-11-19 (obsID 30001030002).

To recover the lost exposure time, at the expense of some positional uncertainty, a specific 'nupipeline' input parameter named 'psdcal' allows use of the raw coordinates of the laser spots without applying the distortion correction. This procedure is safe for the analysis of bright sources, however users must check the sky images to verify the accuracy of the reconstruction of celestial positions.

For details see the NuSTAR Data Analysis Software guide, sections 3.2 and 6.3.

The NuSTARDAS applies livetime corrections to each lightcurve bin via the "nulccorr" FTOOL, which is automatically called by nupipeline/nuproducts when extracting lightcurves if the parameter "correctlc" is set to "yes" (the default). The lightcurve bin width is set via the hidden "binsize" parameter when calling nupipeline/nuproducts (the default binsize value is 10 seconds). It is assumed that the user has selected an appropriate lightcurve bin size for the source.

The NuSTAR focal plane modules have a rate-dependent livetime (see the Userss Guide). To correct a lightcurve for this, the "nulccorr" FTOOL reads the instrument livetime (recorded at a 1 Hz cadence in the instrument housekeeping FITS files), integrates the livetime over each lightcurve bin, and then applies the livetime correction to that bin.

For example: If a lightcurve is binned to 100-seconds, then the livetime is integrated over each 100-second bin and used to correct the lightcurve rate. If a source has variability on 10-second timescales the user should use 10-second (or smaller) bins, not 100-second bins.

A: The channel to energy conversion is linear: E = Channel Number * 0.04 keV + 1.6 keV (lower energy range of bin) Channel 0 corresponds to 1.60-1.64 keV, channel 1 corresponds to 1.64-1.68 keV, etc. You can also check the ebounds extension of RMF files (with the 'fv' command) to confirm the relationship between PI value and energy.

A: There are regions of the NuSTAR orbit in which satellite flies through the Earth's radiation belts. The charged particles interact in the detector and the anti-coincidence CsI shield, resulting in an enhanced background in the detectors. The most famous example is the South Atlantic Anomaly (SAA), where NuSTAR flies directly through a region of the inner Van Allen belts and particle-induced triggers dominate the NuSTAR telemetry. The impact of the SAA on NuSTAR background event rates is primarily between ~300 and ~330 degrees longitude and < +1.5 degrees latitude, while the "tentacle" region is around ~270 degrees longitude and above about -2 degrees latitude. The strength and size of the SAA and the strength of the background in the tentacle region both depend on the space weather conditions at the time of the observation.

The on-board computer samples the rate of particle hits in the anti-coincidence shield and turns off telemetry from the NuSTAR detectors when the particle background is too high (i.e., a "hardware" trigger).

The instrument team is conservative in setting the threshold for turning off the detector telemetry and so there are periods of enhanced background as NuSTAR enters/exits the SAA. In addition there may be an enhanced background when NuSTAR flies through a short-lived radiation belt just to the East of the SAA (the "tentacle"). For bright sources the effect of the enhanced background can be negligible, while for fainter sources or sources that are sensitive to varying background levels, additional filtering of an observation may be required.

A: The impact of the nucalcsaa filters depends on the space weather conditions during the observation. The NuSTAR SOC provides background filtering reports to aid the observer in making a decision about which filtering mode to use. The default pipeline option is not to use any SAA filtering.

There are some post-processing filters that remove spurious triggers (a.k.a "noise events") from the NuSTAR cleaned event files. The rate of these noise events is ~2e-3 counts per second in a typical source extraction region with a diameter of 1 arcminute, though this rate varies based on which focal plane module is used and on the location on each focal plane. In the case of sources that produce moderate rates (when the incident rate on the focal plane exceeds 100 counts per second), "good" source counts can be vetoed by the noise filter. This can result in a mismatch between the measured flux between FPMA and FPMB (since they have subtly different filters) and an underestimate of the source flux.

We recommend that for bright sources a user should produce a lightcurve with 1-second bins using nuproducts with the default energy range (3-79 keV). The resulting lightcurve should be a good estimate of the incident source rate. If any of the 1-second bins exceed 100 counts per second, we recommend re-processing the data using a modified value for the "statusexpr" keyword in nupipeline:

statusexpr="(STATUS==b0000xxx00xxxx000)&&(SHIELD==0)"

in addition to any other keywords. This status expression changes the behavior of the "nufilter" FTOOL that produces the cleaned output event files (e.g. files of the form "nu10311002008A01_cl.evt"). nupipeline must be re-run from scratch for the cleaned event files to be updated. To repeat: using this modified statusexpr keyword or any variations of other status-related keywords will not affect lightcurves produced by nuproducts if the cleaned event files have not been regenerated using nupipeline.

Updated Jan 2023: Note that the previous versions of this FAQ omitted the "SHIELD==0" filter in the suggested statusexpr. In this case, events coincident with a particle hit in the CsI "shield" that surrounds the detectors are not screened out of the cleaned event files.

For bright sources, NuSTAR often turns off the on-board event cosmic ray rejection, opting to instead flag the events and send them down to the ground. This is to avoid rejecting events that have a chance coincidence between a source photon and an (unrelated) particle hitting the CsI shield. For observations when the shield rejection is turned off, this can result in a higher (~factor of 2) non-X-ray background, which dominates the NuSTAR spectrum for energies above ~20 keV.

The SHIELD==0 option above applies a software filter that removes all events with a cosmic ray hit (e.g., SHIELD=1) from the cleaned event files, and removes this additional background term. So we recommend using the above statusexpr as a baseline for most observations.

For timing observations or observations of bright, hard source where the source flux is well above the background level up to 80 keV, we recommend using the version of statusexp without the SHIELD==0 addition listed below. Turning the shield veto on in software can result in removing source counts that have a chance coincidence with a particle hit in the CsI shield, and may influence the resulting power spectrum by preferentially removing the (more numerous) low-energy photons. We therefore suggest using the original filter expression in this case:

statusexpr="STATUS==b0000xxx00xxxx000"

In general, no. The NuSTAR pile-up window is so short that the instrument has only found evidence for pile-up in sources that produce incident count rates in excess of 10,000 counts per second (and then only at the few percent level). If you are working on such a bright source (i.e. the Sun and/or the flaring state of Sco X-1), please contact the NuSTAR SOC for details.

A: A rip in the multi layer insulation (MLI) at the exit aperture of the optic module aligned with detector focal plane module FPMA has resulted in an increased photon flux through the optic that has manifested itself as a low energy excess in FPMA spectra of bright sources. The effect is largest for 3 keV, with an exponential decrease between 5-8 keV. The rip occurred in early 2016, progressively enlarged through 2017 and then appeared to stabilize in 2018. To compensate for the increased flux, the amount of MLI covering the opening, F_MLI (in fraction of opening), in the on-axis ARF of module A has been adjusted as a time dependent component released in three new ARFs valid in different ranges:

DATE                            FILE                   MLI FRACTION
Launch to  01/01/2017           nuA20100101v007.arf     F_MLI=0.96
01/01/2017 to 01/01/2018        nuA20170101v007.arf     F_MLI=0.91
01/01/2018 onwards              nuA20180101v007.arf     F_MLI=0.86

These are available from CALDB v2000429 and subsequent releases. The changes in the on-axis ARF affects all FPMA data from for the entire mission. No changes have been made to FPMB. The impact on data before 2017 is small and the correction removes a 1% excess in FPMA flux between 3-4 keV. The correction for data after 2018 removes a 4% excess flux at 3 keV and then declines at higher energies, becoming insignificant at 6 keV.

A: For historic reasons NuSTAR PHA- and associated RMF files generally have different DETNAM keywords. This means XSPEC will give a warning like:

"Warning: RMF DETNAM keyword (DET1) is not consistent with that in spectrum (NONE)"

This is an alert for a situation where the user might be unintentionally mixing an RMF and a spectrum that do not "go together". This warning can usually be ignored for NuSTAR.

A: The RA and Dec values in the NUMASTER table are based on the nominal observatory pointing J2000 coordinates calculated from the star tracker on the NuSTAR optical bench. The first ObservationID of an observation set usually contains only the time period when the observatory is slewing across the sky and when the star tracker is occulted by the Earth. No nominal observing mode (science quality) data are contained within files associated with these observationID's and so the RA and Dec values are null.

Note that ObservationID's beginning with 0 are for non-pointing periods, usually associated with spacecraft calibration tests, and so (in all but the first calibration observations) will not contain stable observatory pointing periods.

A: The exposure time in the numaster table is the normal mode (observing mode code = 01) livetime-corrected exposure time within an observationID. The data have been screened by the data analysis pipeline (nustardas using default parameters) to exclude periods when:

Obs mode code   Reason

    02          The target is occulted by the Earth.
    03          The observatory is slewing.
    04          The observatory is passing through the South Atlantic Anomaly 
                (SAA) and the instrument is turned off.
    05          The on-board radioactive calibration sources are in the 
                field of view.
    06          No valid attitude solution from the optical bench star tracker 
                is available.

A: The roll angle tabulated in the NUMASTER table is the average position angle of the detector field of view on the sky. This is defined as the angle East of North of the direction of the +DET1Y focal plane axis. At PA = 0 degrees the +DET1X axis points East and the optical axis is approximately 1 arcminute NE of the center of the focal plane field of view.

A: Every NuSTAR observation is assigned a unique 11 digit number of the form CPPttxxxvvv that is a combination of an 8 digit target identification number and a 3 digit visit number, where:

C is the "source" category defined as:

      0 Non-pointing data (e.g. IOC) or safe hold
      1 Calibration observations, e.g. Crab nebula
      2 Solar system objects (e.g. the Sun)
      3 Galactic compact sources (stars, CVs, X-ray binaries, isolated 
        neutron stars)
      4 Non-ToO Supernovae, Supernova remnants, and Galactic diffuse emission
      5 Normal galaxies
      6 Active galaxies and quasars
      7 Galaxy clusters and extragalactic diffuse objects
      8 Proposed ToO's and Directors Discretionary Time
      9 Non-proposal ToO's 

PP is two digits used to identify the program type as:

      00 is assigned to the first 2 year primary mission (2012 to 2014)
      01, 02 etc increments for each additional year of operation 
         (synchronized with possible GO time)

tt is the program type within that PP. The value is reset at each PP. The tt values are the following:

       01 single observation of an object.
       02 multiple observation (monitoring) of the same object.
       10-59 reserved to tiling/mosaic/raster scan programs. These programs 
             include several different targets (pointings) not at the same 
	     sky position but are very close.       
      60-99 reserved to survey programs. These programs comprise observations 
            of several objects identified to carry out a specific science 
	    investigation.

    xxx is the target number unique for a given C and PP.

    vvv is the observation visit number for a given target. Multiple 
        observations may be planned by the observer or due to operational 
	scheduling requirements. The starting value is 001.

The time period covered by an Obs(ervation)ID will begin at the time the command to slew to the target was executed and will end at the time the next slew command was executed.

A: A standard NuSTAR observation of a celestial source requires two slews and so results in two ObservationID's (obsIDs).

The first ObsID starts with the slew from the previous target to the new target. This slew is performed in STELLAR ACS mode and can take up to an hour for the observatory to reach the new target. However this mode is unsuitable for science observations as it will include a roll maneuver of about 1 deg/day to maintain the solar array oriented to the Sun. So a second, usually short, slew maneuver is performed in INERTIAL ACS mode which freezes the observatory attitude, pointing the observatory at the celestial target for extended periods. The length of an observation is limited by how long the orientation of the solar panels in INERTIAL mode can remain within operational limits, usually about 1 week.

To maximize efficiency the STELLAR slew is timed to arrive at the new target when it is occulted by the Earth. The following INERTIAL attitude slew maneuver is timed to complete within the same occultation period. An additional period of observatory settling is also allowed to complete before the celestial target exits Earth occultation and the science observation can begin. As an example, here are the entries in the as flown timeline around the observation of Mkn 421 on 2013 January 2nd:

    observationID Name           Start               End                  ACS

    60061256002   NGC5728        2013-01-02 04:20:05 2013-01-02 18:10:00  I
    60002023001   Mkn421	 2013-01-02 18:34:29 2013-01-02 18:40:00  S
    60002023002   Mkn421	 2013-01-02 18:40:02 2013-01-02 23:00:00  I
    60021009001   COSMOS_MOS009	 2013-01-02 23:16:40 2013-01-02 23:25:00  S

    (ACS I = INERTIAL mode, S = STELLAR mode)

The slew in STELLAR mode from NGC 5728 to Mkn 421 began at 18:10:00 and completed at 18:34:29. The slew in INERTIAL mode on Mkn 421 began at 18:40:00 and completed at 18:40:02. The observatory in held in INERTIAL mode from 18:40:00 to 23:00:00. The position of Mkn 421 was occulted by the Earth from 18:22:24 until 19:00:44 and so the observatory will have completed all slews and attitude settling by the time the target emerged from behind the Earth. The observation ended when the observatory slew to the next target (COSMOS_MOS009) at 23:00:00. Observations of the same target at later dates will continue the visit numbering scheme. For example the next observation of Mkn 421 was on 2013 January 10th and contained obsID's 60002023003 & 60002023004.

Note that some observations may have an additional obsID if it was deemed necessary to refine the observatory pointing. The small maneuver is usually performed within the first 10 orbits of an observation.

Some targets may only have a single obsID associated with the observation if they are part of a survey program. These programs are usually planned as a mosaic of positions where the distance between the tiles in the mosaic are less than 10 degrees. No STELLAR mode slew is required to set the orientation of the solar array when observing these tiles contiguously.

A: The filename format for the NuSTAR science files uses the following convention: "nuObservationID[M][xx]_[ll].ss" where:

'nu' is the prefix indicating the mission name (NuSTAR);

'ObservationID' is an 11-digit number identifying the observation (ObsID);

'M' is a one-character string that identifies the Focal Plane Module (A or B);

'xx' is a code to identify the observing mode as defined below: 

    01 (SCIENCE): normal observing scientific mode;
    02 (OCCULTATION): Earth in the field of view;
    03 (SLEW): data taken during a spacecraft slew;
    04 (SAA): South Atlantic Anomaly passages;
    05 (CALIBRATION): on-board calibration radioactive source in the field of 
view; 
     06 (SCIENCE_SC): attitude reconstruction from the spacecraft bus star 
trackers;

'll' indicates for event files the processing level ('uf' for Level 1 and 
Level 1a files, 'cl' for Level 2 files). For other data files it describes 
their content (e.g. 'met' for raw metrology data, 'mast' for mast aspect 
solution file, 'ex' for exposure maps);

'ss' is the file extension and indicates the data type (e.g. 'evt' for event 
files, 'img' for sky images, 'hk' for housekeeping files, 'lc' for 
light-curves, 'pha' for energy spectra).

A preliminary catalog designation for the NuSTAR satellite of 99105 was provided before launch and this is what is recorded in the archived Two Line Element (TLE) files in the NuSTAR archive. After launch NuSTAR was assigned the USSPACECOM catalog number 38358 but the TLE files are not updated with this number. The catalog number is not used in any calculations during the analysis of NuSTAR data.

A: A typical acknowledgment in a work containing NuSTAR analysis is the following: "This research has made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. Data analysis was performed using the NuSTAR Data Analysis Software (NuSTARDAS), jointly developed by the ASI Science Data Center (SSDC, Italy) and the California Institute of Technology (USA)."

In addition, if using the HEASARC service made a significant contribution to a research project, please make the following acknowledgement in any resulting publication: "This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC."