As of Heasoft-6.22, NuSTARDAS contains an additional filtering mode (mode "3") that can be used for periods of low background when short-lived trapped electron belts may be present. You can see an example of this new filtering mode in the background report for sequence 30302016002. As always, we leave the final decision about which filtering mode to best optimize the data analysis methods to the user.
There are regions of the NuSTAR orbit in which satellite flies through the Earth’s radiations 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, where NuSTAR flies directly through a region of the inner Van Allen belts and particle-induced triggers dominate the NuSTAR telemetry. The on-board computer samples the rate of particle hits in the anti-coincidence shield and turns off telemetry from the NuSTAR detectors while the particle background gets too high (i.e., a “hardware” trigger).
The instrument team is conservative in setting the threshold for turning off the detector telemetry and so that there are periods of enhanced background as NuSTAR enters/exits the SAA and also as it flies through a short-lived radiation belt just to the East of the SAA. For bright sources the effect of the enhanced background can be negligible, while for fainter sources or sources that are sensitive to an enhanced background, additional filtering of an observation may be required.
This filtering is built into the NuSTARDAS nupipeline via the nucalcsaa FTOOL, which has a variety of user settings*. To assist the user in determining the optimal settings to use, the NuSTAR SOC generates a report for every observation that demonstrates the effect of various nucalcsaa settings. These reports can be accessed for all observations via the NuSTAR SOC webpage and from the link above.
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.
The link above points to an automatically generated set of reports for each NuSTAR sequence ID. Each sequence has a PDF report that contains multiple pages illustrating the effect of the various SAA filters. The inputs to nupipeline are given at the top of each lightcurve on the left panels, while a map of the count rate as a function of orbital position (NuSTAR ground trace longitude and latitude) is given in the right panels. The count rates in both the lightcurve and the orbital track plots are integrated over the entire field of view and so include both source and background counts.
For sources that are bright (here bright implies 1 counts per second or higher), the effect of the SAA filtering will likely be negligible. For fainter sources it is up to the user to determine whether or not to use the SAA filtering and which algorithm/mode to use. A description of the modes are given in the NuSTARDAS Software User's Guide*.
The user needs to balance the reduction in the background rates vs the loss in effective exposure. The remaining exposure time after the filtering is given as a percentage of the unfiltered exposure time in the top right above each orbital plot.
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. It is possible that even for faint sources no filtering may be required; the tentacle region specifically is highly dependent on the space weather conditions during the observation.
*See heasarc.gsfc.nasa.gov/docs/nustar/analysis/ for the NUSTARDAS software users guide, the NuSTAR observatory guide, and details about NuSTAR data analysis.