Solar Wind Charge Exchange Background

Table 7: Common SWCX Emission Lines.
Line Energy (keV) In swcx Call
C VI 0.37 not yet implemented
C VI 0.46 not yet implemented
Ne IX 0.92 NEIX
Ne IX 1.02 not yet implemented
Mg XI 1.35 MGXI

Solar wind charge exchange (SWCX, see ยง6.4) emission, more so than the soft proton background, probably affects all observations. Also like the SPF background, the level of the background is not known a priori and must be modeled from the data. For sources like the soft X-ray background which cover the entire instrumental FOV, this can be very problematic as there is no method to distinguish a SXRB X-ray from a SWCX X-ray. Again, like the SPF background, an estimate of the level of SWCX background can be derived from spectral fitting and then subtracted from images. In cases of diffuse emission (e.g., most clusters of galaxies) which does not cover the entire FOV a more robust estimate can be made by explicitly fitting SWCX lines in the on- and off-emission spectra.

Figure 27: Images of the model SWCX background in detector coordinates (upper panel, the output of swcx) and in sky coordinates (lower panel, output of rotdet2sky). The images are very similar morphologically to the exposure maps as they are produced by simply scaling exposure maps calculated for the SWCX energies. The fine structure in the sky image is due to the transforming where occasionally two original pixels get cast into a single transformed pixel, and sometimes none do. However, for any reasonable pixel binning or smoothing this effect is washed out.

The solar wind charge exchange (SWCX) component is treated in a similar manner as the soft proton background. Scale factors are determined during the spectral fitting process for a user-determined number of emission lines that are then used to create model count images. Unlike proton, where the normalization is for the full FOV, here the normalizations are per square arcminute. The swcx routine is called:

swcx imagefile=mos2S002-fovimdet-350-1100.fits
specfile=mos2S002-fovt.pi elow=350 ehigh=1100
rmffile=mos2S002.rmf arffile=mos2S002.arf
lines="OVII" gnorms='1.83305e-7'

where imagefile is the image for the appropriate detector in detector coordinates. specfile=mos2S002-fovt.pi provides a file for the extraction of the EXPOSURE keyword, and must be the spectrum used in the spectral fits where the magnitude of the SWCX contamination was determined. objrmf=mos2S002.rmf and objarf=mos2S002.arf are the response and redistribution matrix files. The values of elow, and ehigh are as defined above and must be the same values as used in mosspectra, pnspectra, mosback, and pnback. linelist='OVII OVIII' sets the energies for the SWCX lines which are being modeled and subtracted. gnormlist='4.38E-07 3.63E-07' are the fitted Xspec normalizations for the SWCX lines where the line normalizations are fitted per arcmin$^{-2}$. Figure 27 (upper panel) shows the MOS1 SWCX background image for the Abell 1795 observation for the $0.4-1.25$ keV band in detector coordinates. As for the particle background and SP maps, rotdet2sky is run to convert the image from detector to sky coordinates (Figure 27, lower panel).