Spectral Analysis of the Background

To find the spectra of the cosmic background in a PSPC image, the following procedure may be adopted. It is probably best to just use data from within the central ring for this purpose to ensure uniform point source rejection to a well defined threshold.

1. Use xrthk to restrict the master veto rate to 170.

2. Use xrtsort to produce a square spectral_image which includes the inner ring. The spatial binsize should be about 15 arcseconds; any number of spectral channels may be used, but 32 or 64 is possibly best. A 256 by 256 by 32 spectral channel file should produce good results.

3. Use the quality setting programs to set pixels outside a radius of upto 17 arcminutes BAD and blank out the sources and window support structure from the spectral image (this procedure is described in the section on background subtracting multiple point sources).

4. The spectral_image can now be compressed into a spectrum using project.

5. Remove the particles (and detector noise) from the spectrum. This is accomplished by using xrtsub to produce a spectrum of the particle background by :

        a) xrtsub spectrum spec_image part_file=parts

           This just uses xrtsub to generate the particle file, throw
           away the subtracted file that it produces. This assumes that the
           spectral file is called spectrum.sdf and the spectral image,
           spec_image.sdf

        b) subtract spectrum parts

           Use to subtract the particles from the background spectrum

5. Exposure correct the spectrum with:

        > xrtcorr offax1=n.n

xrtcorr will use an inappropriate default for the off-axis angle of this spectrum. It must be supplied with the area weighted mean off-axis angle of the photons, this is $R / \sqrt{2}$, where R is the radius of the circular region specified with the quality setting programs. OFFAX1 is in arcminutes.

6. xrtresp will attach a detector response structure using instrument information calculated by xrtcorr. The default matrix should be used with observations after mid 1991.