Spectral files are produced for each ScW:
scw/0051004X0010.001/isgri_spectrum.fits (X=1..5)
In these files you find the spectra of the desired sources plus the background spectrum. In the header of each data structure, the Name and ID of each source are given, and the first extension of the file scw/0051004X0010.001/isgri_spectrum.fits is an index, summarising the content of all the extensions. With the help of this index you can easily see which extension contains the spectrum of the source you are interested in.
To get an average spectrum with a better signal to noise ratio, one can sum up spectra of a source from different science windows. This can be done with the spe_pick tool. In the following example we create a joint spectrum of 4U 1700-377 from all the available science windows (included in the og_ibis.fits file).
cd $REP_BASE_PROD/obs/isgri_gc spe_pick group="og_ibis.fits" source="4U 1700-377" rootname=4U1700
As a result two files with spectra of 4U 1700-377 will be created: 4U1700_sum_pha.fits contains the final average spectrum of 4U 1700-377, while 4U1700_single_pha2.fits stores all the original spectra of 4U 1700-377 that were averaged up. The corresponding Ancillary Response File (ARF) - which contains the energy dependence of the effective area of the instrument - is also produced and written to the 4U1700_sum_arf.fits and 4U1700_single_arf2.fits files. Note that the correct ARF is automatically chosen by the program from the list displayed in Table 3 and in case your observation spans more than one epoch, several ARFs are averaged up to produce the best ARF for your particular dataset.