niprelflaregti estimates a GTI that can be used in NICER screening, based upon precipitating electron events.
Precipitating electrons are detected when a reconnection or electron scattering event occurs in the earth's magnetosphere, which causes electrons to flow to NICER's orbital location. Unlike trapped charges, precipitating electrons are not trapped, and will likely end their lives when they reach the earth's atmosphere just below NICER's orbit.
Precipitating electrons that NICER detects are largely low energy (<20 keV) and enter through NICERs optical pathway like X-rays do. They are difficult to predict because they do not produce large numbers of high energy charge depositions (>20 keV), which are recorded as overshoot events. Precipitation events tend to occur in the polar horn regions, are highly variable, and can produce very high count rates, hence the name "flares."
Instead of modeling precipitation events as background, the NICER team currently recommends to screen them out.
To do this, the worflow consists of two parts.
It is unlikely the user will need to run this task. Instead they can run the standard tools nicerl2/niprefilter2/nimaketime, which creates the recommended columns and applies the recommended screening.
The task will search for PREL flares by searching for time intervals (or GTIs) where the PREL index exceeds prelthresh. This is the starting point for PREL flares.
Then, the flare GTIs are expanded using the enlarge and dilateerodeo operators of the ftadjustgti task. These tend to expand the GTIs by a set amount (default 10 seconds), as sell as to coalesce any GTIs that are neighboring.
Finally, the task checks the filter file. While certain column values in the filter file are high compared to a nominal threshold the flare interval is expanded. By default, the flare interval is expanded until either the overshoot rate goes below 1.5 ct/s/FPM, or the PREL index value goes below 1.5, or the COR value exceeds 2.5 GeV/c. Note that these are the default values, they can be changed as specified below.
The user can also choose to keep the flare intervals or the non-flare intervals using the "keep" parmeter.
Estimate a non-flare GTI from the specified filter file.
niprelflaregti n1234567890.mkf nonflare.gti keep=NONFLARE clobber=YES