INTEGRAL Science Data Center (ISDC) =================================== http://www.isdc.unige.ch/integral Known issues ============ Package: OSA Version: 10.1 Rel. Date: 04-Sep-2014 ------- General ------- 1. Values of parameters of "real" type with more than 7 digits are handled badly by the Graphical User Interface (GUI). This can lead to errors when the analysis scripts are run. In particular, using the JEM-X GUI, if the user wants to enter timeStart and timeStop values through the GUI, the values will be truncated if, after you edited the fields, you close and reopen the hidden parameter window. Therefore, in case you want to enter accurate time constraints, make sure you reenter the timeStart and timeStop values each time you have to reopen this window. ---- IBIS ---- ISGRI ***** 1. Systematic uncertainties of 1% should be added to ISGRI counts, fluxes. 2. In the mosaic built 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. 3. 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. 4. With OSA10, new 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. 5. The position of the low-energy threshold is increasing with time. A safe lower limit for the response is 18 keV 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. 6. 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. 7. 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. 8. ii_pif will crash if the input catalog inCat contains more than 500 sources. 9. 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 ~12 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. PICsIT ****** 1. The spectra extraction with the PIF method is not reliable for the moment (executable "ip_spectra_extraction"). The user should extract the spectra from images (count rates from intensity maps and errors from significance maps) and then convolve them with the RMF/ARF. --- SPI --- 1. SPI is a complex gamma-ray instrument almost always dominated by background contributions. The scientific validation of the SPI data analysis going on at ISDC and at different instrument team sites is as of today far from complete. Users are encouraged to look critically at any result obtained with the ISDC software, and to use external comparisons and simulations when possible. Spurious results can be derived, for example, when using a wrong set of parameters and/or an incorrect background modeling. 2. The SPI instrument is equipped with a Pulse Shape Analysis (PSD) electronic which carries out a parallel processing of the single detector events in order to provide additional information about their pulse shape. The PSD information was intended to help reducing the background. Unfortunately, the in-flight background conditions are such that even the best experts have failed to achieve significant improvements with the PSD. Consequently, all the PSD related processing is currently disabled in the analysis pipeline. PSD events are simply used as standard single events. The basic user choice is then to analyze only single (+PSD) events specifying detector list of 0-18 in the analysis, or to consider double and triple detector interaction with pseudo detectors 19-84. 3. Different instrumental responses are now included in our system, characterizing SPI before, between, and after, the detector 2, 17, 5, and 1 failures. The spi_science_analysis pipeline cannot currently handle a time variable response. The easiest is to analysis the possible cases independently (our software then selects automatically the appropriate response), and to combine the results later on. It is possible however, to analysis different mixtures of different data together using one of the three responses as they are not too different. Great care should be taken in this case anyway to avoid possible biases (see the Tips and Tricks section of our documentation). The spimodfit analysis can instead handle multiple responses which are appropriately used during the data processing. The final response accounts for the multiple responses accordingly. 4. The "spiros" imaging software is quite a complex tool with many different options and parameters. Not all possible cases have been fully scientifically validated. The best tested modes include "imaging" and "spectral extraction". Other modes such as "timing" and "spectral timing" and other background methods are being further tested and validated. The spimodfit software is an on-going project which has now reached a stable configuration, but not all the features have been scientifically tested. 5. At least in one case, a long (staring) pointing which is split up into several science windows in the ISDC system is not handled correctly in the SPI data analysis. It concerns: ScWs 008200220010.001 008200220020.001 008200220030.001. Only the first pointing is properly included in the analysis, while the subsequent ones are ignored. Please report if you find any other such cases. 6. The "spiros" lightcurve production has shown some crashes when running large data-sets and a time binning of one ore more days is selected. The program handles correctly a resolution of one Science Window, however, so the user is encouraged to use this finer time binning and merge the results afterwards in case he/she finds similar problems during the analysis of a long data-set. 7. "spimodfit" handles time variability through the use of splines. The spline order can be 0 to 5, 0 corresponding to a piecewise constant function (with one scaling parameter per interval) and 3 corresponding to cubic splines. In many cases, when using n-order splines (with n equal or greater than 1), the fitting algorithm fails to find the optimal parameters. This is thought to be due to over-parametrized time variability because of the additional parameters of the splines. In addition, crashes of "spimodfit" have been reported when using a large dataset, with about 1000 pointings or more, their origin is still unclear and is under investigation. ----- JEM-X ----- 1. The JEMX lightcurves are deadtime corrected. DEADC in the lightcurve files are set to 1.0 (for XRONOS compatibility). s(but see also Issue no. 7). 2. Due to changes of the on-board configuration, the detection efficiency has changed significantly several times during the mission history. In particular for pointings between revolutions 26 to 45, this means that the measured fluxes of stable sources - in particular at low energy - will strongly depend on the time when the data was taken. These changes are not corrected for in flux units (counts/cm^2/s in the given energy interval) but taken into account in spectral responses. 3. The JEM-X detector gain varies significantly for a few hours after the instrument has been switched on. This mostly affects the beginning of each revolution but can also happen if the instrument was temporarily shut down for e.g. solar flares. The pattern is very similar each time and modeled in the gain correction step even in complicated cases. Nevertheless, it could in principle fail, in which case linear-interpolation gain correction values would be used, which could lead to distorted spectra. Users are advised to check this possibility in case of highly unusual source spectra e.g. by consulting http://outer.space.dtu.dk/users/oxborrow/sdast/GAINresults.html 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. The source coordinates found by j_ima_iros may deviate a little from the true positions and this can occasionally cause inaccurate flux reconstructions. If a good source position is available, it is better to force these coordinates by use of a user catalogue. An example is given in the cookbook (but see also point 8 below). 6. Lightcurves from weak sources may be contaminated with counts from stronger sources in the FOV. This happens because the source extraction does not take into account the presence of the other sources. 7. 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 8. In OSA 7.0 and later, the source position reported in columns RA_OBJ and DEC_OBJ of JMXi-SRCL-RES will always be the one found by j_ima_iros. Columns RA_CAT and DEC_CAT reflect the catalog position if a user catalog has been defined. The SPE and LCR levels will read the RA_OBJ and DEC_OBJ columns and do the extraction using those. In order to force the use of the catalog positions - which is recommended - the JMXi-SRCL-RES table must be manipulated e.g. by an ftool, to update columns RA_OBJ and DEC_OBJ. 9. Light curve extraction is unchanged in OSA 10 compared to previous versions in order to allow the easy generation of short-bin light curves. However, long-term stability is not assured in this case; the user interested in long-term light curves or who doesn't need time bins shorter than the length of a science window is advised to generate light curves from the imaging step, as explained in the cook book. 10. 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. 11. A count-limiting mechanism, the grey filter, is actived when sources corresponding to more than 0.75 Crab on-axis are in the field of view. The grey filter is adjusting itself automatically, according to the rate of events accepted as X-rays and the filling level of the onboard telemetry buffer. Ideally, a grey filter should randomly reject events. However, the mechanism implemented is only pseudo-random. Therefore some care should be taken in interpreting power spectra of arrival times of events from very bright sources with a very significant grey filter, as QPO artifacts may show up. Normally, the automatic grey filter is varying over a science window. This fortunately has the effect of "averaging" out power spectra artifacts, as they are specific to a particular grey filter setting. Therefore, 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 User Manual for further explanations. 12. Since 30 March 2012, there is a new instance of the Instrument Model Group (IMOD files version 22) produced by the JEM-X Team. The usage of these new IMOD files is highly recommended, and will be automatic upon update of your copy of the Instrument Characteristics files. Note however that the data are reliable only above 5 keV. 13. For the time being it is not trustable to extract spectra of strong sources with "mosaic_spec" from images obtained with the PIF option. 14. The flux of a given source can be obtained either with the "standard" extraction or with mosaic_spec. In cases when the fluxes obtained with the two methods differ, it is advised to consider the one obtained from the standard extraction (SPE level). 15. When analysing very-Near Real Time data (i.e. within a few hours from the data transmission to ground, and well before the entire revolution is completed) JEM-X gain calibration might fail. This happens when there are not enough data collected as yet to describe the gain evolution for the current revolution. In this case 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. --- OMC --- 1. The automatic extraction of fluxes and magnitudes produce reliable results only for point-like sources. 2. For extended sources or high-energy source counterparts with large uncertainties in their position, the OMC planning assigns multiple adjacent sub-windows to cover the whole area. In that case, multiple boxes are found with different ranks but with the same OMC ID. From OSA 6.0 onwards these adjacent sub-windows can be correctly analyzed by using IMA_wcsFlag=yes (default in OSA 7.0). In this case, o_src_get_fluxes creates a virtual 11x11 pixel sub-window inside the whole area centred at the source position. After that, OSA works on this new sub-window and ignores the previous sub-windows mosaic. This is an internal software trick, these virtual sub-windows do not exist as standard sub-windows (o_ima_build, for example, will not create these virtual sub-windows as 11x11 pixel images). Note that with IMA_wcsFlag=no, these adjacent sub-windows will not be analyzed correctly as the software treats each box individually. 3. If the source coordinates are inaccurate by more than 2 OMC pixels (~35"), the software analysis will not be able to re-centre the target and the derived fluxes and magnitudes obtained with default analysis parameters will not be correct. 4. If another star is within a few pixels of the source of interest, it can introduce systematic errors in the derived fluxes and magnitudes. The strength of this effect can be different for different pointings, since the relative position in the sub-windows will slightly change for different rotation angles. 5. Since OSA 4.0, the detection of saturated sources has been improved significantly. However, some of the bright sources slightly saturating one or few pixels might not be detected as saturated sources. As a consequence, their derived magnitudes are not correctly computed. The observer should check whether the source might be saturating the CCD for a given integration time, and reanalyze the data rejecting the shots with the longest integration times. 6. Due to thermoelastic deformations, the alignment of the OMC optical axis with the S/C attitude reference (after correcting the OMC misalignment) may diverge by up to 30" (~2 pix). From OSA 5.0 onwards, the derived coordinates are corrected at the time of computing the WCS support by using the photometric reference stars, giving an accuracy better than 2" in most cases.