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*search_grid* - Grid of $\chi ^{2}$ - default mode

After the best fit is obtained, the confidence limits on two parameters can be obtained by running search_grid. This task calculates a grid of $\chi ^{2}$ values for a given input model. The task will also calculate the absolute minimum [within the limits of the grid or the limits from the input model spectrum, whichever is larger], and use it to draw the contours ( $\chi^2_{min}$+ $\Delta\chi^{2}$).

In the simplest case the user can hit <cr> at the first few prompts. Then search_grid will select the last dataset used (from pkgpars), will use the best fit from the last run of fit (the null string ``'' at the model prompt will always select the best fit from the _prd.tab file), and will place the results in a _grd.tab file with the same root as the observed file. A grid will be built around those values, incrementing out to $\pm$ delta (provided by the user) in nx steps in the x dimension and ny steps in the y dimension (nx and ny provided by the user). search_grid will return $n_x \times n_y$$\chi ^{2}$ values, a $\chi^2_{min}$ on the grid points, and a true minimum (which will coincide with the central grid point in this simple case).


input files: rp90_prd.tab; rp90_obs.tab

output file: rp90_a_grd.tab

  xs> search_grid
  Performing a grid search.
  observed spectrum [root_obs.tab] = (rp90):  <cr>
                                                    # input data from rp90_obs.tab
  model descriptor [null --> last best fit]:  <cr>                  # The best fit
                                                # parameters are from rp90_prd.tab
  root name for output file [root_grd.tab]:  rp90_a
                                         # output is rp90_a_grd.tab.  Root changed
         # because you will probably want to repeat this with different parameters


The best-fit model parameters (from previous fit) will be listed on the screen.

Please choose the grid axes:

  model component number for the first axis (1): <cr>
  first axis parameter (norm|index|temp|galacticNh|intrinsicNh):  temp
  half range of first axis parameter:  0.5
  number of steps in the first axis (1:):  10
  model component number for the second axis (1):  2
  second axis parameter (norm|index|temp|galacticNh|intrinsicNh):  temp
  half range of second axis parameter:  0.5
  number of steps in the second axis (1:): 10

The best-fit parameters from this current grid of fits will be listed on the screen.


The output file is in table format, and can be listed with the tables.ttools task tprint.



 
next up previous contents
Next: *search_grid* - Customized grid Up: Spectral Analysis Previous: *counts_plot* - Inspection of
rsdc@cfa.harvard.edu
1998-06-10