Particle Background

If selected by setting withimages=yes and entering non-zero values for elow and ehigh, mosback and pnback also produce particle background images in detector coordinates for the specified energy band. Figure 25 (upper panel) shows the MOS1 particle background image for the Abell-1795 observation for the $0.4-1.25$ keV band in detector coordinates. The distribution is relatively flat although there is one noticeable feature: the CCDs tend to have different apparent brightnesses. This is primarily due to the differences in total FWC exposure for each CCD and therefore the total number of counts for a given CCD (the counts are normalized by exposure). MOS1 CCD 6 is brightest because of its loss earlier in the mission. MOS1 CCD 4 and MOS1 CCD 5 are also slightly brighter than the rest due to having lost exposure due to time spent in the anomalous state.

In general, images in detector coordinates are not particularly useful; one needs them in sky coordinates. Transformation of the detector coordinate images into sky coordinate images is done simply with rotdet2sky. Here is an example where we transform a QPB background image:

rotdet2sky intemplate=mos1S001-fovimsky-350-1100.fits
withdetxy=false withskyxy=false

The output images, x-bkgimsky, are in sky coordinates with the same projection and pixel size as the source images x-fovimsky-350-1100.fits produced by mosspectra andpnspectra.

The parameters for rotdet2sky are the name of the image whose coordinate system is to be matched, intemplate, the name of the input image, inimage, the name of the output image outimage. The withdetxy=false and withskyxy=false are probably not necessary. Figure 25 (lower panel) shows the MOS1 image after casting into sky coordinates which requires a reflection, an offset, and a rotation. As will be seen below, the proton and swcx routines produce output images in detector coordinates; rotdet2sky can be used to rotate these images as well.

Figure 25: Images of the model particle background in detector coordinates (upper panel, the output of mos_back) and in sky coordinates (lower panel, output of rotdet2sky). The differences in the general colors of the events in the different CCDs are due to differences in the FWC data exposure times. For a CCD with less FWC exposure there are fewer sample events, and therefore each event must have a greater weighting.