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ISGRI

  1. The validity of OSA11.0 is currently limited to the recent data, since 2015-12-26 at 02:03:13, i.e., from the beginning of the revolution 1626. Analysis of earlier data will fail due to the lack of calibration files. The validity will be updated to past data as soon as possible. Regular updates for the more recent data will be provided, as the instrument continues to evolve.

  2. (OSA11) energy boundaries for the response matrix are taken from the table in the file
    ic/ibis/rsp/isgr_ebds_mod_0001.fits. They are chosen as close as possible from the boundaries defined int the input parameters. No boudaries from matrices in previous versions of OSA should be used !

  3. The PHA2 files produced by spe_pick contain a reference to the response matrix used for the particular science window. These are referenced in the file system used for the analysis. Therefore, the PHA2 files cannot load a matrix when moved on another file system, unless the user takes care of manually copying the single response matrices and update the entry in the PHA2 file with other tools.

  4. In general, systematic uncertainties of about 1% should be added to ISGRI counts, fluxes. However, this needs to be checked accurately by scientists performing their analysis (see also below).

  5. Since 2016, ISGRI occasionally experiences particularly rapid and unpredictable changes in the detector response, at the scale of up to 5%, which are not corrected in the energy reconstruction and response computation.

  6. In the mosaic build with the option spread=1 the source flux is slightly reduced ( 10%) compared to the weighted average of the fluxes measured in the Science Window.

  7. The maximum number of sources handled by ii_spectra_extract is 200 but it is strongly recommended to only fit spectra of the sources that are effectively active (visible, detectable) during the Science Window. In some cases, especially in the later part of the mission (as the number of usable pixels has decreased), the maximum number of sources which can be meaningfully reconstructed with ii_spectra_extract can be as low as 50.

  8. With OSA10.2, calibration files have been produced including a correction for the variation of gain across the entire mission, as observed in previous OSA versions. However, on single revolution time scale, a drift in counts is still observed. For the latest part of the mission, spectra extracted at the beginning and end of a same revolution can therefore show an artificial difference in counts. The secular drift observed in all bands over the mission life-time is known and due to the evolution of gain: this effect is accounted for by the set of ARFs available in the IC tree. With OSA11, this effect has been corrected, but other instabilities set in at instrumental level with unpredictable paste (see above).

  9. The position of low-energy threshold is increasing with time (see Sect. 12.4.1), due to the detector gain variation, which causes the energy scale to be more compressed when expressed as function of the electric signal in the detector. A safe lower limit for the response is 18keV until revolution 848.
    Between revolutions 848-1090, we recommend to ignore data below 20 keV.
    From revolutions 1090 on, we recommend the user to ignore data below 22 keV.
    Since revolution 1600, we recommend to exclude data below 30 keV from the spectral analysis.
    For imaging, lower energy ranges can be used, down to the low threshold of the instrument, which evolves from about 15 to 25keV along the mission. However, the low threshold and active status of pixels is changed at the beginning of each revolution, according to the instrument status. Using energy ranges at the limit of detector sensitivity might introduce a strong decoding noise in the image, due to the rapid variation of a pixel response with energy. To obtain clean images and optimize detection of weak sources, we recommend to use a low threshold similar to the one recommended for spectral analysis, which, however, slightly reduces the sensitivity to soft sources. We note that a signal from the source is present al lower energies (see Fig. 32), but with unstable response. Therefore, if one does not need an accurate determination of a source flux, but wants to optimize sensitivity, more inclusive energy cuts can be made. The same considerations apply to light curves.

  10. Merging products of OSA 10.2 and OSA 11 is not recommended. Spectra should be fit separately, while images could be mosaicked only in overlapping energy ranges, but with the caveat that count rates of a stable source might be different along the mission (see Sect. 12.4.1). For instance, light curves could possibly show a discontinuity when passing from OSA 10.2 to OSA 11. The analysis strategy for data sets overlapping with the transition should be decided on a case-by-case basis, following scientific needs.

  11. A problem on-board IBIS causes event times to be shifted by 2 seconds under some circumstances (this is rare). The software tries to correct the data. The keyword TIMECORR found in the event files (*-*-ALL or *-*PRP extensions), indicates whether the correction was done. If you are doing an accurate timing analysis and your data contains TIMECORR 0 please take great care: If TIMECORR=1 or 2, the applied correction should be OK. If TIMECORR=3 you should better not use these data. If TIMECORR=4 contact ISDC.

  12. The lightcurve extraction (ii_lc_extract) is performed by building shadowgrams for each time and energy bin. It potentially takes a large amount of CPU time and there is a minimum usable time bin. The time bin must be such that the total number of maps in the file isgr-corr-shad does not exceed 2 GB worth of disk space. The product of the number of time bins in a science window, and the number of energy bands must be less than about 9942.

  13. ii_pif will crash if the input catalog inCat contains more than 500 sources.

  14. At large off-axis angles the IBIS response is not well known and strongly energy dependent. Therefore, the user should be careful when analyzing observations performed at large off-axis angles, above 10 degrees, since systematic flux variations might be introduced. The systematic flux variations are energy dependent, and therefore the user should be careful both with photometric and spectral analysis of sources at large off- axis angles.

  15. Due to lack of a consistent calibration information and the lower end of the energy scale, absolute energy scale of ISGRI is poorly defined below 60 keV. We recommend systematic uncertainty on the absolute energy scale of 1 keV at 30 keV.

  16. Specifying energy range with (e.g. 20.1 - 40 keV) crashes the ISGRI pipeline. It is recommended to choose round energy boundaries, e.g. 20 instead of 20.1 keV.


next up previous contents
Next: PICsIT Up: Known Limitations Previous: Known Limitations

2018-10-24