XSPEC User's Guide

32
We are now ready to fit the data. Fitting is initiated by the command fit. As the fit proceeds,
the screen displays the status of the fit for each iteration until either the fit converges to the
2
minimum , or the user is asked whether the fit is to go through another set of iterations to find
the minimum. The default number of iterations is ten.
XSPEC12>fit
Chi-Squared Lvl Fit param # 1 2 3
204.136 -3 7.9869E-02 1.564 4.4539E-03
84.5658 -4 0.3331 2.234 1.0977E-02
30.2511 -5 0.4422 2.174 1.1965E-02
30.1202 -6 0.4648 2.196 1.2264E-02
30.1189 -7 0.4624 2.195 1.2244E-02
---------------------------------------------------------------------------
Variances and Principal axes :
1 2 3
4.14E-08 | 0.00 -0.01 1.00
8.70E-02 | -0.91 -0.41 -0.01
2.32E-03 | -0.41 0.91 0.01
---------------------------------------------------------------------------
---------------------------------------------------------------------------
Model: phabs[1]( powerlaw[2] )
Model Fit Model Component Parameter Unit Value
par par comp
1 1 1 phabs nH 10^22 0.4624 +/- 0.2698
2 2 2 powerlaw PhoIndex 2.195 +/- 0.1288
3 3 2 powerlaw norm 1.2244E-02 +/- 0.2415E-02
---------------------------------------------------------------------------
---------------------------------------------------------------------------
Chi-Squared = 30.11890 using 31 PHA bins.
Reduced chi-squared = 1.075675 for 28 degrees of freedom
Null hypothesis probability = 0.358
2
The fit is good: reduced is 1.075 for 31 – 3 = 28 degrees of freedom. The null hypothesis
2
probability is the probability of getting a value of as large or larger than observed if the model
is correct. If this probability is small then the model is not a good fit. The matrix of principal axes
printed out at the end of a fit provides an indication of whether parameters are correlated (at least
local to the best fit). In this example the powerlaw norm is not correlated with any other parameter
while the column and powerlaw index are slightly correlated. To see the fit and the residuals, we
use the command
XSPEC12>plot data resid
The result is shown in Figure C.