Point Source Subtraction

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.