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,
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
> 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.