j_ima_src_locator

j_ima_src_locator is a tool to locate point sources in an image. It is included in the OSA 7 distribution but is it is not part of the pipeline.

The primary goal of this tool is to analyze images from JEM-X made by j_ima_iros or j_ima_mosaic but it can also be used as a generic tool for a FITS image file. The main condition is that the PSF measured in pixels does not vary significantly over the image.

Currently j_ima_src_locator operates on a single image, but the functionality to get the images from a DAL observation group (OG), a DAL science window group (SWG), or a DAL index table (IDX) is being prepared.

Significance map

The starting point for the search for sources is the significance map. If the image is accompanied by a significance map this can be given as a parameter with its DOL.

Detection significance of sources

It is assumed that the significance given in the significance map is evaluated pixel by pixel e.g. based on the number of counts that contribute to each image pixel. The significance of an excess in a pixel in the image is evaluated as the number of excess counts divided by the square-root of the variance (number of sigmas). In order to deal with the fact that the signal from a source is distributed over several adjacent pixels - the number is given by the PSF - a better source significance evaluation is done by combining several pixels around the source.

The distribution of significance values in the map is analyzed and a suggested value for the parameter 'detsigMin' is given in the log file (search for the string 'Suggest').

Variance map

If the search for a significance map has failed the alternative is to use a variance map. For an individual image it can be given by parameter as a DOL. The significance map is constructed by dividing the input image with the 'local RMS' or the square root of the variance map.

Bootstrap variance

In the case of a missing variance map then the 'local RMS' map (square root of the variance) is built by finding the RMS value by a sliding box excluding the pixels that are the highest contributors to the RMS. This method will reduce the effect of the extraordinarily high values of the RMS if there is a source inside the sliding (square) box.

Source fitting

Once a source candidate has been spotted a gaussian fit with both position, sigma and amplitude as free parameters is done around the initial position in the intensity image and both the derived sigma and the peaksize are reported. The best source position is given as the fitted peak position.

Currently there is no way to e.g. freeze the width of the PSF.

Output data structure

The output data structure is named: JMX1-SLOC-RES and is similar to JMXi-SRCL-RES which makes it possible to run e.g. q_identify_srcs and cat2ds9. There are some extra columns with additional information such as SIGMA (result of fitting), PEAKSIZE ('volume' of peak as fitted),

Table 25: j_ima_src_locator parameters

Name	Type	Description
inDOL	string	DOL of an image or of a DAL group default ``jmos.fits[1]''
varDOL	string	DOL of corresponding variance map default ``''
sigDOL	string	DOL of corresponding significance map If 'inDOL' is a group the two parameters above are ignored. If 'inDOL' is an image then 'sigDOL' is used as significance map if not empty; then 'varDOL' is used as variance map if not empty default ``''
slocRes	string	File for SLOC-RES (override) Currently not active. Prepares for jemx_science_analysis script default ``./jmx1_sloc_res.fits(JMX1-SLOC-RES)''
outFile	string	Name of output FITS file with source results Name of output file, that will be placed where DAL_CURRENT_DIR points default ``jmx1_sloc_res''
catDOL	string	DOL of user defined X-ray catalog FITS table with columns RA_OBJ, DEC_OBJ, FLAG for the positions where a source fit is requested irrespective of the existence of a source default ``''
sigmaPSF	real	Defines the (gaussian) PSF width in pixels. Start point for the fitting default ``1.5''
sigmaFlat	real	(avoid use) If set to a positive number then the input image .. will be flatfielded .. with a gaussian of this sigma before further analysis default ``0.0''
erode		Determines how much of the edge of the image where the noise often is overwhelming should be excluded from source finding. A high value implies a wide edge to be removed and vice versa. default ``5.0''
maxNumSrcs	integer	Maximum number of sources to search for A limit to the number of sources. prevents the output file to expand to an unreasonable size. default ``14''
maxNumCatSrcs	integer	Maximum number of catalog sources to include used to define the amount of memory to allocate to the internal data arrays default ``14''
boxwidth	integer	Edge dimension of sliding box The edge size of the box used for finding the local RMS. Should be about five times larger than the FWHM of the PSF default ``11''
detsigMin	real	Detection significance lower limit" With the current scheme put to 14. If you get too many spurious sources then choose a higher value default ``-0.3''
imaTypes	string	Image types to search for sources .. Only applicable if the input is a group. A good choice is "RECONSTRUCTED" default ``RECONSTRUCTED''
saveI	boolean	Flag for saving input image default ``n''
saveF	boolean	Flag for saving flat-fielded image default ``n''
saveM	boolean	Flag for saving folded image default ``n''
saveK	boolean	Flag for saving array of amplitudes default ``n''
saveC	boolean	Flag for saving array of chi squares default ``n''
saveS	boolean	Flag for saving array of significance default ``n''
saveR	boolean	Flag for saving array of local RMS values. The 'save?' parameters above should only be set to 'yes' is you are working on a single image default ``n''
chatter	integer	Level of chattiness default ``3''