j_ima_iros

j_ima_iros processes data (shadowgrams) from one science window at a time. Combining the sky images or the source candidate lists must be done by subsequent software packages. j_ima_iros can analyze a maximum of 15 shadowgrams (f.i. 15 separate energy bands) from one science window. For an initial survey of the observed sources it is best to divide the data into a limited number of energy bands, e.g. four bands.

j_ima_iros generates its sky images by a simple backprojection algorithm. The necessary information for this step is to see if the path from the detector pixel to the sky pixel is free or blocked by e.g. collimator or mask. Since the geometry of the JEM-X detector with its collimator is complex the raytracing from shadowgram pixels to sky image pixels is quite time consuming.

Therefore the Aperture Response Function is calculated off line by a separate set of subroutines. For every detector pixel (= shadowgram pixel) a list is generated containing all sky-image pixels potentially illuminating this detector pixel. Even when stored in compressed format these "backprojection lists" are of substantial size: about 375 Mbyte (for each JEM-X unit) for sky images with 511x511 pixels of 1.5 arcmin. The advantage is that the backprojection process (which must be repeated many times during the IROS-process) is quite fast - and, very important, the duration of the backprojection step is independent of the steadily increasing complexity of the raytracing model required by our improved understanding of the intricacies of JEM-X.

The backprojection lists are stored as separate IC files JMXi-BPL.-GRP under the general IC tree.

The search for sources is conducted in three predefined energy bands irrespective of the user selection of energy bands. Source results will be reported for all energy bands. Sources found in two or three of the search bands will define a 'basic source set' for this observation.

Optionally, the user may specify a 'user catalog' (parameter: usrCat) to enhance and control the performance of j_ima_iros. A subset of the sources in the catalog will automatically be selected corresponding to the current field of view.

For strong source candidates found during the first source detection phase of j_ima_iros a catalog match will be searched. If found, the catalog coordinates will be used during the IROS process.

The user may want to force j_ima_iros to analyze a source which is not detected with sufficient strength to be included in the basic source set. This can be done by assigning specific values to the 'FLAG' column of the user catalog.

A value of one will force j_ima_iros to derive flux values for the source. The flux will be derived from a fit including the basic source set plus the desired source. If several sources are marked in this way, a separate fit is performed for each source together with the basic source set.

A value of three for the FLAG will force j_ima_iros to include this source in the basic source set. It can be useful for very strong sources near the edge of the FOV that are not detected because they are too close to the edge or the exposure time is too short. But beware! Forcing irrelevant sources into the IROS process will normally deteriorate the fit.

The pixel size in the sky image is 1.5arcmin and the dimensions are 511 511 pixels.

The distribution of pixel values in the reconstructed and cleaned image (RECONSTRUCTED) is not Gaussian (see Fig. 24) which implies that the detection significance limit should be set somewhat higher e.g. to 12, than a ``number of sigma's''

**Figure 24:** The distribution of pixel values in a RECTIFIED image with only one or two weak sources around the center of the FOV (thick histogram). A Gaussian fit is shown with the dashed curve and it is evident that the wings of the distribution are wider that the fit.

The presence of the collimator introduces a vignetting of off-axis sources. Also at off-axis angles greater that 2.4 degrees a source is not visible through the mask from the entire detector surface. These two effects together form the vignetting. The vignetting or throughput factor as a function of angle in two direction is shown in Fig. 25. The quadratic cell shape of the collimator introduces the deviation from circular symmetry.

With OSA5.1 images it was realized that the flux determined from the images was a decreasing function of the off-axis angle. This effect is corrected in OSA6 by applying a correction map placed in the BPL IC files to the images before they are written to disk. Therefore the offline tool 'mosaic_spec' can be used to extract source spectra from j_ima_iros images as well as from mosaic images.

**Figure 25:** Simplified version of the vignetting array.