Preparing for Further Analysis

If spectral analysis is the goal for the processing, this is the end of ESAS, and you can use your favorite spectral analysis program with the P-fovt.pi, P-bkg.pi, P.rmf, and P.arf files. (For the pn, you can use the P-fovtootsub.pi file with the P-bkg.pi file.)

If imaging analysis is the goal of the processing, you will now need to do a spectral analysis to determine 1.) the strength and spectral shape of the residual SP emission, and 2.) the strength of the SWCX emission. Using the results from the spectral fitting described below, one then proceeds with the ESAS tasks proton, swcx, and combimage, which are described in §7.

If only spectral analysis is to be done, RMFs and ARFs, and background spectra are desired (and if so the “withimages=no” should be set for the tasks above), mosback and pnback are as far through the processing as one needs to go for a single exposure and region selection. The processing must be repeated for all instruments individually, as well as for any selections for different regions on the sky and for different exposures of the same instrument. Before rerunning mosspectra and pnspectra for a different region on a given instrument (MOS1, MOS2, or pn), mosback or pnback should also be run and the important output files should be either renamed or moved so they are not overwritten. This includes the source and background spectra as well as the RMF and ARF files. For example, the process of completing one spectral set is as follows:

mosspectra eventfile=mos1S001-allevc.fits
withsrcrem=yes maskdet=mos1S001-bkregtdet.fits
withregion=yes regionfile=regmos1.txt
keepinterfiles=yes pattern=12
elow=350 ehigh=1100 ccds="T T T F F F T"
mosback inspecfile=mos1S001-fovt.pi
withimages=yes elow=350 ehigh=1100
ccds="T T T F F F T" withqdp=yes
mv mos1S001-fovt.pi mos1S001-fovt-reg1.pi
mv mos1S001-bkg.pi mos1S001-bkg-reg1.pi
mv mos1S001.rmf mos1S001-reg1.rmf
mv mos1S001.arf mos1S001-reg1.arf
mv mos1S001-fovimspdet.fits

mosspectra eventfile=mos2S002-allevc.fits
withsrcrem=yes maskdet=mos2S002-bkregtdet.fits
withregion=yes regionfile=regmos1.txt
keepinterfiles=yes pattern=12
elow=350 ehigh=1100 ccds="T T T F F F T"
mosback inspecfile=mos2S002-fovt.pi
withimages=yes elow=350 ehigh=1100
ccds="T T T F F F T" withqdp=yes
mv mos2S002-fovt.pi mos2S002-fovt-reg1.pi
mv mos2S002-bkg.pi mos2S002-bkg-reg1.pi
mv mos2S002.rmf mos2S002-reg1.rmf
mv mos2S002.arf mos2S002-reg1.arf
mv mos2S002-fovimspdet.fits

pnspectra eventfile=pnS003-allevc.fits
withsrcrem=yes maskdet=pnS003-bkregtdet.fits
withregion=yes regionfile=regpn.txt
keepinterfiles=yes pattern=0
elow=350 ehigh=1100 quads="T T T T"
pnback inspecfile=pnS003-fovt.pi
withimages=yes elow=350 ehigh=1100
quads="T T T T" withqdp=yes
mv pnS003-fovt.pi pnS003-fovt-reg1.pi
mv pnS003-fovt-oot.pi pnS003-fovt-oot-reg1.pi
mv pnS003-bkg.pi pnS003-bkg-reg1.pi
mv pnS003.rmf pnS003-reg1.rmf
mv pnS003.arf pnS003-reg1.arf
mv pnS003-fovimspdet.fits
mv pnS003-fovt-os.pi pnS003-fovt-os-reg1.pi

One author likes to do the above "mv-ing". The other author of this cookbook prefers to create directories for each instrument region combination. This procedure is demonstrated in the appendix.

Note that the task pnback produces an OOT subtracted spectrum with the file name prefix-fovt-os.pi which should be used for spectral analysis. The spectra, however, are therefore count rates rather than counts which makes the use of cstat statistics problematic.

Figure 16: Fitted MOS1 (black), MOS2 (red), pn (green), and RASS (blue, see Section 6.6 below) spectra from the entire field of view of the Abell 1795 observation. The upper panel shows the fit with the model background subtracted (the poorness of the fit is due to fitting one emission temperature to a multithermal spectrum). The lower panel shows the same fit but the data do not have the particle background subtracted, thus the deviation at higher energies. The diagonal lines in both plots show the contribution of the fitted residual SP background.

In most cases for chi-squared statistics the data are going to be too sparse, particularly at higher energies, to be usefully fit without binning. Binning can be done conveniently by using the Ftool grppha. In the binning process with grppha the background spectrum, RMF, and ARF files can also be associated with the source spectrum simplifying data entry into the spectral fitting program. The following commands will group the data with a minimum of 50 counts per channel and place the output spectrum in a file with the -grp qualifier added to the file name.

chkey BACKFILE mos1S001-bkg.pi
chkey RESPFILE mos1S001.rmf
chkey ANCRFILE mos1S001.arf
group min 50

chkey BACKFILE pnS005-bkg.pi
chkey RESPFILE pnS005.rmf
chkey ANCRFILE pnS005.arf
group min 50

Of course, you will probably want to add more qualifiers to some of these file names, particularly if you are going to be fitting more than one observation of the same source. However, this is just a minimal example.

Figure 17: Various components of the MOS1 spectral fit shown in Figure 16. The black points and curve show the background subtracted and fitted data. The red points show the observed spectrum without the particle background subtracted. The light blue curve shows the fitted cluster spectrum, the dark blue curve shows the fitted cosmic background spectrum. The green curve shows the fitted Al K$\alpha$ and Si K$\alpha$ instrumental lines. The violet (it may appear red depending on your printer) curve shows the fitted SP component.

Figure 16 shows the fitted MOS1, MOS2, and pn spectra from the entire field of view of the Abell 1795 observation. The difference between the two panels shows the effect of subtracting the model QPB. At lower energies the source clearly dominates over the background but at $E\sim4$ keV the falling flux and falling instrumental effective area coupled with the relatively flat QPB allows the latter to dominate. Note, however, that Abell 1795 is a relatively bright object and thus the QPB will be relatively more significant at lower energies for many other observations. The particle background even in this observation will also be much more important when the outer annuli of the cluster are considered in § 6.

Figure 17 shows the contributions of the various spectral components to the observed spectrum of Abell 1795 (demonstrated for the MOS1 detector). For a source as bright and extended as Abell 1795, the object spectrum dominates all other components except at the highest energies. However, it is clear that observations of fainter sources such as the cosmic background will be very significantly affected by the particle background and possible residual SP contamination over the entire energy band.