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Дата изменения: Fri Mar 19 00:20:04 2004
Дата индексирования: Sat Dec 22 10:28:45 2007
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Поисковые слова: radio source

XMM-Newton Science Analysis System


omsource (omsource-1.9.5) [xmmsas_20040318_1831-6.0.0]

Meta Index / Home Page

Photometry

Photometry on sources is done in a similar fashion to that done by the SAS tasks omdetect and ommag.

POINT-SOURCE PHOTOMETRY. The counts for a source are extracted within the aperture radius and the background (see the next paragraph) subtracted. If necessary, the background-subtracted counts are extrapolated to the coincidence-loss area corresponding to a circle of 12 unbinned pixels, using the point-spread function (this correction is tabulated as psf1 corr). A correction is then made for coincidence-losses (this correction is tabulated as coinc corr). For UV data, a further correction is made to extrapolate the counts to a radius of 35 pixels using the point-spread function (this correction is tabulated as psf2 corr). The count-rate is then corrected for the CCD dead-fraction and an instrumental magnitude derived from it. The user can change the aperture radius to exceed the coincidence-loss radius, in which case the photometry will be computed as for an extended source.

EXTENDED-SOURCE PHOTOMETRY. For each pixel within the aperture a coincidence-loss correction factor is computed by assuming this pixel is centred on a point-source and then computing the correction-factor as for a point-source. The pixel value is then corrected using the correction value. The background is computed from the pixels within the background annulus, which can be elliptical. The apertures can be elliptical, and the user can fix the ellipse parameters (the semi-minor axis, ellipticity and position-angle of the major-axis). Please note that the accuracy of this method has not yet been evaluated.

Source type A source can be of two types, point-like or extended. The former refers to an unresolved source with a 2-dimensional pixel distribution similar to that of the point-spread function, whereas the latter refers to any source that has an extent greater than that of the point-spread function.

In the Source-type group box there are two radio-buttons: textbfPoint source - if this is enabled the photometry in the normal way for a point source. Extended source - if this is enabled the photometry is performed as for an extended source. The user can change the axial-ratio by clicking on the a/b spin-box and position-angle by clicking on the Pos angle spin-box) and the inner and outer background radii for the background determintaion (by clicking on the Inner bck and Outer bck spin-boxes, respectively). Note that the background region will have the same ellipticity and position-angle as for the photometry aperture.

Background determination The Background determination group box allows the user to select one of three ways to subtract the background, by clicking the approriate button. Annulus If this is checked then an annulus region with the inner radius set by the Inner bck spin-box and outer radius set by the Outer bck spin-box is used to extract the counts to compute the background mode.

User regions If this is checked then the circular background regions will be used to determine a mean background per pixel which will be used for the background mode.

User value If this is checked then the value displayed in the background label will be used.