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diff --git a/doc/sbp_extract.org b/doc/sbp_extract.org new file mode 100644 index 0000000..efd525e --- /dev/null +++ b/doc/sbp_extract.org @@ -0,0 +1,55 @@ +#+TITLE: Surface Brightness Profile Extraction +#+AUTHOR: Weitian LI <liweitianux@live.com> +#+DATE: 2016-06-03 + + +* NOTE: how =dmextract= calculate the area of extraction region +The =dmextract= can take either *events* file or binned *image* file as the input file. However, the extracted SBP may be *different*, mainly due to the area calculation. + +Above all, let's have a look at how the SBP quantities are calculated ([fn:dmextract]): +#+BEGIN_EXAMPLE +MEAN_SRC_EXP = TOTAL_SRC_EXP / EXP_AREA +SUR_BRI = COUNTS / AREA ~ AREA^{-1} +FLUX = COUNTS / EXPOSURE / MEAN_SRC_EXP ~ EXP_AREA +SUR_FLUX = FLUX / AREA ~ EXP_AREA / AREA +#+END_EXAMPLE +where =AREA= is the area of the source extraction region, while the =EXP_AREA= is the area of the exposure map with respect to the source extraction region. +Note we explicit distinguish these two areas, as we'll see below that it is responsible for the different extraction results. +Also note that we ignored the background contribution here for simplicity. + +For both the two cases, the *excluded point sources* (i.e., =[exclude sky=region(???.reg)]=) are taken into account, and they are also *excluded* when calculating the extraction region (i.e., the =AREA= column/values of the produced HISTOGRAM file by =dmextract=). +Therefore, no extra procedure is required in order to subtract the areas of these excluded point sources from the surface brightness extraction region. +The excluded regions corresponding to the point sources are recorded in the FITS header using the keyword =DSVAL?=, so =dmextract= can properly handle this. + +** Image file as the input +Using the already binned *image* file as the input file is *recommended*, and it is simper than the events file case, e.g., +#+BEGIN_SRC sh +$ dmextract infile="img_c7_e700-7000.fits[bin sky=@sbprofile.reg]" \ + outfile="sbprofile.fits" \ + exp="expmap_c7_e700-7000.fits" \ + opt=generic +#+END_SRC + +Generally, the image file is as such generated: +#+BEGIN_SRC sh +$ dmcopy infile="evt2_c7_clean.fits[energy=700:7000]\ + [sky=region(skyfov.fits[ccd_id=7])]\ + [bin sky=1]" \ + outfile="img_c7_e700-7000.fits" +#+END_SRC +where we use the *ACIS-S* (=ccd_id=7=) observation as an example, and the =skyfov.fits= contains the FoV/region of each CCD chip. +Also note that the =skyfov.fits= should be generated by the CIAO tool =skyfov= with the parameter =aspect= specified, otherwise, the produced =skyfov.fits= dose not precisely describe the CCD chip regions, due to the ignorant of aspect solution. + +As the =skyfov.fits= is incorporated to generate the image, the regions that describe the CCD geometries are also recorded into the FITS header using the same set of keywords =DSVAL?=, similar to the case of excluded point sources. + +Therefore, when =dmextract= calculate the area of the source extraction region, the regions that *beyond the CCD edges* are excluded as well. So the =AREA= is exact the *valid* area of the extraction region with both the point source regions and out-of-CCD regions excluded. + +The exposure map is generated by the CIAO tool =fluximage= and has the identical dimensions as that of the input image. So the =EXP_AREA= is the same as =AREA=. +Thus the final =SUR_FLUX= is independent of the area of the extraction region, which is what we want. +As a result, it is recommended to bin the events file into an image, and then extract SBP from the image. + +** Event file as the input + + +* References +[fn:dmextract] CIAO =dmextract= help: http://cxc.cfa.harvard.edu/ciao/ahelp/dmextract.html |