j_cor_gain

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

• Temporal gain variations across the detector are determined by the energy channel positions of the fluorescence lines of Cd-109 and Fe-55 sources placed above certain points on each detector. The peak positions are determined every four minutes (256 sec) to give current time variation of the detector gain, and compared to the Cd/Fe positions at a reference time when the Xe fluorescence lines of the gas over the entire detector are known. The values of the found Cd/Fe calibration line positions are kept in the gain history table and are used for gain conversion from PHA to PI channels in two possible ways. For gainModel=0, a linear interpolation between gain corrections is used for each event or spectrum at a given time. For gainModel=2 a time-decay model is used to smooth out statistical noise and discharge glitches at the calibration points. It is recommended that gainModel=2 is used for all processing. gainModel=1 is obsolete and should not be used (it is only included for backward compatibility).

  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 have become sensitive to not only temperature, but also the total hardware trigger-rate of the detector. For COR_gainModel=4 (the default value for JEM-X1) the trigger rate dependency is taken into account, while for COR_gainModel=3 (which is the default value for JEM-X2) 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. If the smoothing goes wrong with the default model (4 or 3, depending on the JEM-X unit) and you don’t like the look of the linear interpolation results, try using a lower value for this parameter. 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).

• spatial gain variations across the entire detector area. These are initially determined by pre-flight calibration, but have been updated using in-flight calibration based on on the Xe 30 Kev fluorescence lines which can be observed over the whole detector.

• Unusual gain fluctuations hidden from the calibration sources (e.g. hidden gain suppression on the plate) and rare telemetry/procedure changes that can't be dealt with automatically, are handled by having the instrument team deliver instrument characteristics gain history tables, JMXi-GAIN-OCL, to ISDC. These are used automatically by the OSA software instead of the usual gain history tables, if one exists for a given revolution. All the revolutions after 948 have an IC table because of aging and occasionally eccentric behaviour of the instruments. Some revolutions prior to 948 also have IC tables due to odd behaviour. Delivery of these files is constantly ongoing and users should always check that they have the latest IC/IMOD files before beginning an analysis. For a complete overview of the gain calibration of every revolution see
  http://outer.space.dtu.dk/users/oxborrow/sdast/GAINresults.html

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 and Appendix . j_cor_gain parameters included into the main script are given in Table .

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