The executable j_cor_gain corrects all science data received at Full, Restricted, Spectral/Timing and Spectral modes for the condition of the individual pixels in the detector, temporal and spatial changes in the detector gain. The telemetry energy bins (PHA channels) of each event are converted to PI energies, using gain determined by
The JEM-X detector gain varies significantly at the beginning of each revolution for a few hours after the instrument has been switched on. The pattern is very similar each time and modeled in the gain correction step (if the parameter COR_gainModel is not 0).
Due to aging of the instruments the JEM-X units (in particular JEM-X1) have become sensitive to not only temperature, but also the total hardware trigger-rate of the detector. For COR_gainModel=4, the trigger rate dependency is taken into account, while for COR_gainModel=3 the algorithm will not include this dependency in the gain smoothing process.
For some cases, this modeling fails and a Warning with an error code of -321126 is reported. The gain correction program continues using linear interpolation values of the gain correction parameters. This is an ad hoc solution to ensure pipelines continue to run, but implies that the energy determinations of the events will be somewhat off. The corresponding events are flagged as having bad gain determination and may be ignored by subsequent steps or lead to distorted results.
In many cases where linear interpolation is used, gain correction of events will be fine. The user should look at the full detector spectrum produced by level BIN_S to see whether the Xe line peaks at PI channel 209. The default gain smoothing model is COR_gainModel=4. If the smoothing goes wrong with model four and you don’t like the look of the linear interpolation results, try using model 3. All of the foregoing considerations about choice of model number can be ignored if there is an IC table with all the gain corrections already determined. The log file will show whether a gain history table (JMXi-GAIN-OCL) has been used to correct the data. (See below).
The j_correction script will automatically fill this parameter with the DOL of an available IC gain history table if one is available for the revolution to which the SCW belongs.
More details can be found in Section 3.2 and Appendix B.3. j_cor_gain parameters included into the main script are given in Table 7.
For the quality of gain corrections of JEM-X, all users should look at the
web-page
http://outer.space.dtu.dk/users/oxborrow/sdast/GAINresults.html