Up: Known Limitations
Previous: Known Limitations
- 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.
(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 !
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.
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).
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.
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.
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.
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).
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.
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.
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.
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.
ii_pif will crash if the input catalog inCat contains more than
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-
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.
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.
Up: Known Limitations
Previous: Known Limitations