Background subtraction issues

The usual method for correcting a spectrum from an imaging detector for background is to extract a spectrum from another part of the image and subtract that from the source spectrum. This assumes that the source region has the same background spectrum as the background region. However, instrument performance often varies across the face of the detector either due to mirror vignetting or changes in detector efficiency. There are three possible strategies for dealing with this.

  • Instead of using a different region of the same image take background from the same region of the detector for a different image, for instance a deep field observation. This has the advantage that it eliminates the effects of any changes in instrument performance over the field. It has the disadvantage that it assumes that the X-ray background is the same in the source and background images and it assumes that the non-X-ray background does not vary over the time between the source and background observations.

  • A second alternative is to use background from the source image but correct the background spectrum for the position-dependent effects. This depends on knowledge of the spectrum of the background, both X-ray and non-X-ray, and requires considerable care because the X-ray background may have to be corrected by a different amount than the non-X-ray background. It is also difficult to know how to take this operation into account in the error analysis.

  • The final possibility is to use background from the source image and to fit simultaneously the source and background spectra. The background spectrum is fit using a model for the X-ray and non-X-ray backgrounds and the source spectrum is fit using a the background model plus whatever model is required for the source. The parameters for the background model are linked between the source and background spectra. This method has the advantage that it is statistically well-defined but it is more cumbersome. Again it is necessary to be careful about variations of the X-ray and non-X-ray backgrounds across the image.

The ROSAT satellite has some vignetting of the X-ray background and also a radial dependence of the non-X-ray background. The ASCA satellite has strong vignetting of the X-ray background and little spatial dependence of the non-X-ray background (except at the edges of the GIS). ASCA also has X-rays scattered into the image from outside the field-of-view through reflections off only one of the pair of foils or off the back of a foil. The extra X-rays mitigate the vignetting effect of the telescope and make the X-ray background flatter across the detector.


Keith Arnaud (kaa@genji.gsfc.nasa.gov)

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Last modified: Wednesday, 04-Jun-2003 11:35:11 EDT