Finding Sources in the Mosaic Image

j_ima_src_locator is a tool to locate point sources in an image. Since OSA-7 it is included in the OSA distribution but is not part of the pipeline. This tool can be used to search for the gaussian excesses in the mosaic image (but not to identify the detected sources at the moment).

As an example, let us find all the significant excesses in the 7-11 keV mosaic image of the Galactic Center region. We assume that in the mosaic image jmx2_mosa_ima.fits the intensity map is in the extension 2 and the significance is in the extension 4 (which is the case if the default value of IMA_skyImagesOut parameter is used). The command

j_ima_src_locator inDOL=jmx2_mosa_ima.fits\[2] \
   sigDOL=jmx2_mosa_ima.fits\[4] outFile=jmx2_sloc_res

produces the list of the detected sources jmx2_sloc_res.fits. In this file all the significant sources in the mosaic image are listed. However, they are not identified with the known sources. To do the source identification, you can use the q_identify_srcs script:

q_identify_srcs srcl_cat_dol="$ISDC_REF_CAT"\
   srcl_res_dol=jmx2_sloc_res.fits instrument=5

(use instrument=4 for JEMX-1 and instrument=5 for JEMX-2). As a result, the list of sources in the file jmx2_sloc_res.fits looks like it is shown in Fig. 17.

Figure 17: The content of the jmx2_sloc_res.fits file.

When inspecting the result the user might want to get rid of (too many) spurious sources or include more sources that seem to be present. To achieve the latter the parameter 'maxNumSrcs' must have an adequate value and the 'detsigMin' parameter might be lowered.

Analogously 'detsigMin' can be increased to avoid false sources and there is some help in the log file to find a reasonable value. Giving a negative 'detsigMin' will cause j_ima_src_locator to apply a limit of the first found histogram value exceeding one plus the absolute value of the given 'detsigMin'.

The JEM-X coded mask has an almost non-cyclic pattern, which ensures imaging artifacts (ghost sources) are particularly rare. Nevertheless in some peculiar cases, a bright source inside the FOV might cause the presence of ghosts in the mosaic. These artifacts appear at 8.32 degrees from the bright source. To verify that a 'New Source' in the sloc_res file is not instead a ghost, a simple test consists in checking whether there are bright sources at 8.3 deg from it in the FOV.