Known limitations

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 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 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 modified 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 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.