A strong limitation of the spiros imaging and spectral mode lies in the underlying assumption that the source flux is constant over the whole observation period. If this assumption is wrong and a relatively bright source does vary significantly, the results obtained in these modes will be biased.
One possible indication that a source flux change with time is when the analysis residuals remain large, even after all cares have been taken in the analysis (see next section). In these cases, we recommend to switch to the spiros timing modes (imaging-timing to obtain images, timing-spectra to obtain spectra, and timing to get light curves) and to let some of the suspected sources vary during the observation. To tell spiros to let a certain source vary, open your source catalogue with fv and edit the ``SEL_FLAG'' column and set it to 2 (normally, this column is set to 1 for the sources to be used and 0 for the sources to be ignored). It is probably best to first let vary only one source (the brightest of the suspects) and to check the residuals again. If the residuals are still too large, try with a second source and so on. But do not forget that, as usual, the best solution will be the simplest model capable of reproducing the observed counts with statistically reasonable residuals. Note that it is a particular bad idea to simply set many (or all) sources to be variable! In such a case, spiros will be unable to derive a reliable solution and the results may be arbitrary.
A detailed discussion with exmaples is provided in two separate documents: ``SPI Compact Source Analysis'' and ``SPI Complex Case Analysis'', both available from the Documentation section of the ISDC homepage [11].