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The background subtraction of point sources tends to be relatively easy
because of the low intrinsic background of the PSPC. Use Isys
to determine a suitable background area and then sort with xrtsort.
This should be either an annulus around the source, with an inner radius of
at least 3 arcminutes, or if source confusion makes this impossible, a
circular box as near as possible to the source. The background box should
be as large as possible without including any sources or intersecting the
window support structure. If a spectrum or time series is being
subtracted, the background datafile must have exactly the same range of
spectral bins and time bins as the source file.