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Miscellaneous on software behaviour

  1. On December 2015, the JEM-X Team produced the latest instance of the Instrument Model Group (IMOD files version 25). With these IMOD files, the data are fully reliable in the range 3-20 keV.

  2. Detector gain variations. The JEM-X detector gain varies significantly for a few hours after the instrument has been switched on. These effects are normally taken care of for the CONS(consolidated) data by IC Gain History Tables which are now produced on a revolution by revolution basis by the JEM-X support team. CONS generally has an energy determination within +-2% for spectra integrated over an entire science window and can be used for all energy-sensitive applications.  Noisy revolutions with poorer energy determination do occur occasionally and users should check the JEM-X calibration page to get an overview of the goodness-of-energy -determination for the revolutions they are looking at:
    http://outer.space.dtu.dk/users/oxborrow/sdast/GAINresults.html
    For help fitting gain data in complicated revolutions contact Dr. Carol Anne Oxborrow at oxborrow@space.dtu.dk.

  3. Near Real Time data. When analysing Near Real Time data (i.e. within a few hours from the data transmission to ground, and well before the entire revolution is completed) the JEM-X gain calibration is not available. The NRT data from JEM-X are produced prior to the generation of these IC-tables and can therefore only be used for non-spectral applications: e.g. lightcurves covering all energies, source location, mosaicking with a single energy bin etc. For NRT data we suggest to run the analysis with parameter COR_gainModel=2 to force the use of a simplified fitting model. The default model (COR_gainModel=-1) can be used again at revolution completion.

  4. If the gain correction step fails then take a look at the gain history table. Gain correction failure is often signaled by all corrected events having a non-zero STATUS value due to bad gain determination (64). If the gain history for your revolution shows multiple switch on/offs, this may be confusing j_cor_gain. Then remove all gain history values up to the switch on/off just before your SCW being analyzed. For help fitting data in these complicated revolutions contact Dr. Carol Anne Oxborrow at oxborrow@space.dtu.dk .

  5. Grey filter artifacts. A count rate limiting mechanism, the grey filter, is activated when the total count rate exceeds the telemetry capacity. (With the current telemetry allocation this corresponds to about 100 counts/s or about 1.5 Crab). At the beginning og each science window the grey filter is set to zero (no action). If needed, the grey filter will adjust itself automatically, according to the filling level of the onboard telemetry buffers. Ideally, the grey filter should reject events in a completely random way. However, the mechanism implemented is only pseudo-random. Therefore, some care should be taken in interpreting power spectra of arrival times of events from bright sources affected by significant grey filtering, as QPO-like artefacts may show up. As the grey filter varies over a science window and the artefacts are specific to each particular grey filter setting there may be some "averaging" out of the power spectra artefacts. Anyway, if noticing transient features in the power spectra of very strong sources it should be checked if this is limited to a period of a specific grey filter setting. Please check the JEM-X Observers Manual for further explanations.

  6. The JEM-X lightcurves are deadtime corrected. DEADC in the lightcurve files are set to 1.0 (for XRONOS compatibility).

  7. Data from early revolutions. Due to changes of the on-board configuration at a number of occasions, the detection efficiency has changed significantly several times during the mission history. In particular, for all pointings between revolutions 26 and 45, this means that the measured fluxes of sources - in particular fluxes at low energy - will strongly depend on the specific data taking time.

  8. If you mix FULL and REST data then be sure to give chanMin/Max that match REST channel limits, for example:
            chanMin:   64  128  160 192
            chanMax:  127  159  191 223
    

  9. It has been noticed that in mosaics of JEM-X images a plus-like depression in the background around certain sources can occur. This can happen for sources that are too weak to be noticed in the search for sources in the individual science windows. The cleaning process excludes (known) source areas. It operates horizontally and vertically since the systematics are strongest in these directions. However, adding many images can amplify the effect of an unnoticed source since the distribution of position angles is quite narrow, in particular for the sources near the galactic center, which is also where the probability to find a source in the depression caused by a neighboring source is highest. If a source is situated in such an indentation its peaksize is reduced accordingly, whereas there is no change for the source causing the feature. This is solely an image feature so j_ima_iros flux determinations are unaltered.

  10. In case of mosaicking using the Aitoff-Hammer projection in galactic coordinates, if the position of the center of the mosaic image is given by the user with equatorial coordinates (RA, Dec) leading to a galactic latitude , there is a potential risk that the obtained mosaic is wrongly orientated compared to its coordinates and/or apparent sources are misplaced.

  11. if the parameter IMA_burstImagesOut=yes and a burst is detected, it will not be possible to produce mosaics, because this choice leads in principle to an uneven number of images per science window, which is not supported by the mosaicking.


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Next: Known Issues and Limitations Up: Data Analysis Previous: Running the analysis in   Contents
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