Is my Source constant?

 

If a source does not vary periodically, or if the data do not allow to search for a periodicity, it might be relevant to run a statistical test to check if the source is variable. Again, it is assumed that the source photons have already been extracted and are contained in the table sourx.tbl. To run a Kolmogorov-Smirnov test   against constancy, a parameter file has to be created first via CREATE/PARFIL KS ks.par (see Table 7.6).

  
Table 7.6: Example of a parameter file for K-S test (UNBINNED)

If the parameter file is called ks.par, the K-S test is then run with the command

MAKE/KSTEST ks

The probability for variability is then displayed on the terminal, as well as some additional information. It is advisable to plot the result with the command

PLOT/KSTEST sourx_ks

This command will plot the observed and expected integral distribution of photons (normalized to 1.0), and the absolute value of their difference (normalized to 0.3). The threshold values for 90%, 95% and 99% confidence level for variability are also plotted.

In many cases, however, a direct test on the photon arrival times is not the best choice. For instance, a background subtraction or a series of corrections to the data (e.g. for vignetting or dead time effects) have to be applied before running the analysis. It is then possible to run a variability test in the case of binned data. If the binned light curve (in counts or count rate, corrected or not) is contained in the table binned_licu, and the relevant column to analyize vs. :TIME is called :MYRATE, a parameter file like the one in Table 7.7 can be used to run MAKE/KSTEST ks.

  
Table 7.7: Example of a parameter file for K-S test (BINNED)