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XMM-Newton Science Analysis System


ommergelists (ommergelists-1.2) [xmmsas_20211130_0941-20.0.0]


Description

This program has originated due to the new OM functionality of source-detection on mosaiced sky-images. It matches the sources from the first input observation source-list file (ie those detected from the individual exposure images) with those from the second input observation source-list file (ie those that have been detected on the mosaiced sky-image), and propogates these matches into an output source-list with the following caveats:
  1. Any unmatched sources from the second-list having a raw count-rate (including the background) $\leq$ maxrawrate value are added to the output observation source-list file.

  2. Often fields are imaged through more than one filter. An object exists in a source-list if it is detected through a minimum of one filter. Non-detections of the known source through other filters are not flagged. However, if the second source-list is constructed using, for example, a deeper mosaicked image then it can potentially contain new detections of a known source which has already been found in the shallow images of individual exposures, through different filters. Provided new detections in list 2 of known sources already found in other wavebands in list 1 have intrinsic + background count-rates $\leq$ maxrawrate then they are propogated into the new combined source-list.

  3. Deeper exposures provide a more significant detection and better constrained source properties. If there is a source match between the two lists through a specific filter, the properties of the most significant detection are adopted within the output list. An exception to this rule occurs if a source with intrinsic + background count-rate $>$ maxrawrate. In these bright-source cases the properties derived or combined from individual images are adopted within the output list. This exception prevents systematic errors related to combining different CCD dead-times. frame-rates and coincidence-losses from propogating into the bright-source sample.

The maxrawrate value is an optional input parameter, with a default value of 5 counts/sec; below this threshold a coincidence-loss correction to photon rates is not required within photometric conversions. The reason why there is a maximum raw-rate cut-off is due to the way OM photometry is conducted. A coincidence loss correction is applied to the count-rates and this correction depends on the frame-time and dead-time fraction. These CCD properties vary from exposure to exposure and so coincidence-losses cannot be corrected in mosaicked images with the current SAS algorithms. By propogating only the faint sources from the second list into the output table we avoid systematic bright-source photometry error, while retaining the full potential size of the source sample within the output.

Since this cut-off is determined by the input parameter maxrawrate, the user has complete control over what sources are retained. It should be noted that for the photometry of the sources detected on the mosaiced sky-images, no coincidence-loss correction will have been applied and the dead-fraction correction will have been incorporated into the exposure images.

Ommergelists will compare the celestial coordinates of the matched sources from both source-lists and compute mean RA and Dec offsets and add them to the coordinates of the second source-list to correct for any small differences in their origins (it will give a warning about any significant offsets indicating bad astrometry).

The program can also produce graphs comparing various properties of the common sources in the two lists, which can be a useful check for possible problems. It can also produce a ds9 region file that allows the new sources detected on the mosaiced sky images to be easily identified.

XMM-Newton SOC -- 2021-11-30