The epreject task also offers a method to flag so-called "soft flare events" in EPIC pn timing mode data: The timing mode was designed for observing bright variable sources with a very high time resolution. Up to now it has been possible to use the spectra down to 500 eV in this mode. Below this energy the data are affected by so-called soft flares which are caused by stack overflows generated by high energy particles. These soft flares have a sharp rise (from 0 counts/s to several thousand counts/s in 0.10-0.15 s) then decaying back to 0 counts/s slightly slower in about 0.3-0.4 s). In some cases (extremely bright flares) they can lead to a FIFO overflow thus causing a small gap in the data of about 0.08 s. These gaps are handled by the SAS.
epreject provides a method to mitigate the effect that these flares have on the data by flagging such soft flare events so that they can later be screened from the data.
With this method it becomes possible to use the spectral information in the data down to the lowest energies detectable in the pn timing mode i.e. 200 eV.
This is particularly interesting for timing studies of isolated neutron stars, X-ray binaries and other variable objects, such as magnetic CVs, with very soft spectra.
In order to perform the soft flare event selection, the withsoftflarescreening parameter in epreject must be enabled. The programme then generates a light curve in 0.1 s bins in the specified PHA range (parameter softflareenergyrange, default: 40-50 ADU) using the output event lists from the epframes task. The light curve is smoothed with a boxcar function whose width can be modified via the parameter softflaresmoothparams. This smoothed light curve is then used to perform the soft flare event selection: each event is checked to see whether it has a PHA value in the user given PHA range and the light curve is above the user given threshold (parameter softflarethreshold1; this value has to be optimised for each observation as the contribution of the source photons in the given PHA range varies a lot from observation to observation).
Again, epreject has no destructive effect on the event file: A flag column labeled Flare is added to the rawevents file. A "1" in this column denotes that an event has been flagged as a soft flare event. This column can be used to screen the data.
The user should check the epreject task description for information on caveats to keep in mind when interpreting the pn timing mode data down to 200 eV.