THE XMM-NEWTON ABC GUIDE, STREAMLINED
Fit an EPIC Spectrum with Xspec (Locally Installed)
We will do some simple spectral fitting, using as an example the source and background that was extracted from the Lockman Hole observation (Obs ID 0123700101) and its RMF and ARF files. All tasks will be called from the command line.
Nearly all spectra will need to be binned for statistical purposes. The procedure grppha from FTOOL provides an excellent mechanism to do just that. The following commands not only group the source spectrum for Xspec but also associate the appropriate background and response files for the source. This is done simply by calling the task and editting parameters as is appropriate:
>ftools >grppha Please enter PHA filename mos1_pi.fits ! input spectrum file name Please enter output filename mos1_grp.fits ! output grouped spectrum GRPPHA chkey BACKFILE back_pi.fits ! include the background spectrum GRPPHA chkey RESPFILE mos1_rmf.fits ! include the RMF GRPPHA chkey ANCRFILE mos1_arf.fits ! include the ARF GRPPHA group min 25 ! group the data by 25 counts/bin GRPPHA exitNext, use Xspec to fit the spectrum, editting the parameters as needed.
>xspec XSPEC> data mos1_grp.fits ! input data XSPEC> ignore 0.0-0.2,6.6-** ! ignore unusable energy ranges, in keV ! set a range appropriate for the data XSPEC> model wabs(pow+pow) ! set spectral model to two absorbed power laws 1:wabs:nH> 0.01 ! set model absorption column density to 1.e20 2:powerlaw:PhoIndex> 2.0 ! set the first model power law index to 2.0 3:powerlaw:norm> ! default model normalization 4:powerlaw:PhoIndex> 1.0 ! set the second model power law index to 1.0 5:powerlaw:norm> ! default model normalization wabs:nH> 0.01 ! set model absorption column density to 1.e20 XSPEC> renorm ! renormalize the model spectrum XSPEC> fit ! fit the model to the data XSPEC> setplot device /xw ! set the plot device XSPEC> setplot energy ! plot energy along the X axis XSPEC> plot ldata ratio ! plot two panels with the log of the data and ! the data/model ratio values along the Y axes XSPEC> exit ! exit Xspec Do you really want to exit? (y) yThe spectrum is shown in Figure 1.
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