13. Fitting an EPIC Spectrum in XSPEC

In this chapter, 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) in §6 (or §7 if you used the GUI), and its RMF and ARF files. All tasks will be called from the command line.

Prior to fitting, users are encouraged to read this Guide's discussion on statistics in §14.1. More information can be found in Humphreys et al. 2009, ApJ, 693, 822.

For this example, we are interested in just a quick fit to see what we are dealing with, so we will rebin the data. The procedure grppha from FTOOL provides an excellent mechanism to do 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_grp30.fits  ! output grouped spectrum
      GRPPHA[] chkey BACKFILE bkg_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 30                         ! group the data by 30 counts/bin
      GRPPHA[] exit

Next, use Xspec to fit the spectrum, editting the parameters as needed. We'll use two lightly absorbed power laws as a first try.

      XSPEC> data mos1_grp.fits      ! input data
      XSPEC> ignore 0.0-0.2,6.6-**   ! set a range appropriate for the data, in keV
      XSPEC> model pow               ! set spectral model to a power law
      1:powerlaw:PhoIndex> 2.0       ! set the power law photon index to 2.0
      2:powerlaw:norm>               ! default model normalization
      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) y

The spectrum is shown in Figure 13.1.

Figure 13.1: An EPIC MOS spectrum fitted with Xspec.