Verifying the Output

While the output from the chains is ready for analysis, OM does have some peculiarities, as discussed in §9.1. While these usually have only an aesthetic effect, they can also affect source brightness measurements, since they increase the background. In light of this, users are strongly encouraged to verify the consistency of the data prior to analysis. There are a few ways to do this. Users can examine the combined source list with fv, which will let them see if interesting sources have been detected in all the filters where they are visible. Users can also overlay the image source list on to the sky image with implot by downloading the relevant files to the user's local machine. This can also be done with ds9 or gaia by using slconv to change source lists into region files and downloading the relevant files to your local machine. The task slconv allows users to set the regions radii in arcseconds to a constant value or scale them to header keywords, such as RATE. By default, ds9 region files have suffixes of .reg; gaia region files have suffixes of .gaia. In the example below, we make a region file from the source list for the mosaicked, V-band sky image.

To make a ds9 region file from a source list with the GUI:

1)

In the Available Tools panel, under ``XMM-SAS'', call slconv.

2)

Next to ``Name of source list dataset:table'', enter P0123700101OMS000RSISWSV.FIT:SRCLIST; next to ``Expression that determines the radius of the source circle'', enter 5; next to ``Output mode'', enter ds9; next to ``First part of the name of the output text file'', enter Vband_mosaic.

3)

In the task pop-up window, click ``Run''.

To make a ds9 region file from a source list in the Command Window, type:

slconv srclisttab=P0123700101OMS000RSISWSV.FIT radiusexpression=5
$$ outfileprefix=Vband_mosaic outputstyle=ds9

where

srclisttab - source list file name
radiusexpression - constant or expression (possibly involving keywords) used
$$ to determine the radii of the plotted circles
outputstyle - output format; either ds9 or gaia
outfileprefix - prefix of output file name

We have downloaded the image and region file for examination on a local machine. The mosaicked, V-band sky image, P0123700101OMS000RSIMAGV.FIT, with the region file from slconv overlayed, is shown in Fig. 9.1. There clearly are spurious detections; these can be removed by hand, or by rerunning omichain with a different background-level threshold or source significance.

Figure 9.1: A mosaicked, V-band sky image and corresponding region file viewed with ds9. Spurious detections are clearly present and can be removed either by hand or by rerunning omichain with the appropriate flags.