Multiple Point Source Subtraction

The situation where there are a large number of point sources within the central ring, such as a 'deep survey' field, has many similarities to the case of the extended source. Choosing a nearby background region for each source is often impossible because of source confusion and anyway is time consuming. One method of subtracting these sources is to use an annulus at a large radius as described above. However, this method may cause over-subtraction and since many of the sources are likely to be faint, this can be a serious problem.

An alternative method is to use background from within the central ring. Although much of the area may be contaminated by sources, there is usually a reasonable amount of blank sky available. The method for producing such a background file is very similar to the annular background described in the 'Extended source' section.

• Produce a square spectral_image with sides just larger than the circle required$^{\star}$
• Select the regions to be excluded using iregion.
• Export the regions into an ARDFILE using iregion export.
• Use ardqual to remove the sources.

$^{\star}$ Never xrtsort directly to a circular or annular spectral_image. The pixels at the edge of the circle will be only half filled and there is no way of subsequently correcting for this in the software.

An example of this method is shown in Figure 3.

A restriction of this method is that a single vignetting correction has to be applied to the whole background file. This means that the range of off-axis angles that can be used with this method is limited. A circular background region with off-axis angles 0-12 arcminutes or an annular box with angles 10-17 are acceptable.

When removing sources from the background region, careful consideration must be given to the point spread function of the instrument. Very high and very low energies are the most affected by photon scattering and so if sources are inadequately removed the spectral characteristics of the background will be strongly biased. A description of the PSPC PSF is given in Hasinger et al. 1992. Faint sources (< 100 counts) can be adequately excluded with a circle of 1-2 arcminutes radius, centered on the source. Brighter sources should be treated individually. Isys can be used to obtain a radial profile to ascertain the distance from the source at which the source counts fall below the level of the background. Photons should be excluded from a circle of this radius centred on the source. Its often a good idea to smooth the image, before extracting the profile.