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Дата изменения: Thu Jun 29 00:51:54 2006
Дата индексирования: Sat Dec 22 10:49:44 2007
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Поисковые слова: дисперсия скоростей

XMM-Newton Science Analysis System


omdetect (omdetect-5.6) [xmmsas_20060628_1801-7.0.0]

Meta Index / Home Page / Photometry

Point-like Sources

When the source-detection algorithms has run omdetect will compute raw count-rates for each detected source. For point-sources (ie those sources having widths consistent with those of the point-spread function) the count-rates are obtained by aperture photometry. The background is subtracted using an annulus region lying just outside the circular aperture region.

These ``raw'' count-rates have to be corrected for coincidence-loss, dead-fraction and the time-dependent sensitivity degradation before instrumental magnitudes can be computed (using the task ommag). The default aperture is 12 unbinned pixels, but for some sources a smaller aperture may be used. For these cases an aperture correction is also applied using the point-spread function. For the filters UVW1, UVM2 and UVW2 a further point-spread function correction is made to extrapolate the count-rates to an unbinned aperture radius of 35 pixels.

The following summarizes the procedure used to correct the source extracted count-rates for coincidence-losses-

• Extrapolate (if necessary) the source count-rate to the 12 unbinned pixel aperture, using the point-spread function for the filter.
• Extrapolate (if necessary) the background count-rate to the 12 unbinned pixel aperture, by using the scaling factor (144 / aperture radius ** 2).
• Apply a coincidence-loss correction to the total (source+background) count-rate.
• Apply a coincidence-loss correction to the background count-rate.
• Subtract the background coincidence-loss corrected count-rate from the coincidence-loss corrected total count-rate- this is the coincidence-corrected source-count rate.

  Please note that these coincidence-loss corrections do not contain the factor 1/(1-deadfraction).

The following further corrections will then be applied by ommag-

• If a UV filter a further point-spread function correction is applied to extrapolate the source count-rate from 12 to 35 unbinned pixels.
• The ``dead-time correction ''- 1/(1-deadtime)
• The correction for the time-dependent sensitivity.