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search_grid xray.xspectral


NAME · USAGE · DESCRIPTION · PARAMETERS · EXAMPLES · TIME_REQUIREMENTS
BUGS · SEE_ALSO

NAME

search_grid -- performs a chi-square grid search

USAGE

search_grid grid model x_axis_steps y_axis_steps x_model_for_axis y_model_for_axis x_axis_param y_axis_param x_parameter_delta y_parameter_delta

DESCRIPTION

This task computes the best fit to a spectrum by finding the minimum Chi-square in a two-dimensional grid. A Chi-squared is computed at each grid point by "fixing" the grid parameters, but allowing any "free" parameters to be fit using the Simplex minimization method. At the very end, when the minimum grid point is found, then a final Simplex fit is performed using all the "free" parameters.

PARAMETERS

observed = "" prompt = observed spectrum [root_obs.tab]

This is a package wide parameter, found in the pkgpars parameter file.

The file names of one or more observed data sets on which the fit is to be performed. Multiple observation data sets can be specified by entering the file names, separated by semicolons, e.g. "foo1; foo2; foo3", etc. Normally these files have a "_obs.tab" extension. This extension need not be input explicitly; if the files do not follow this convention the explicit name should be typed.

Channels to fit for a given data set may be selected by appending a bracket notation to the file name. Thus "foo[3:11,15]" will cause channels 3 to 11 (inclusive) and 15 to be fit. If channels are not explicitly specified, the default channels for the given instrument are retrieved from the pkgpars parameter file.

grid = "" prompt = root name for output file [root_grd.tab]

The name of the output grid search file. There is no default for this file name; however, if the extension is omitted, a "_grd.tab" is added. The grid fit output file is an STScI table file containing the chisquare values at each grid point. The names of the columns correspond to the value of the "x" free parameter at each grid point. In addition, an extra column is added to give the values of the "y" free parameters at each grid point. Finally, the table header contains information about the model used, the grid points, etc.

model = "" prompt = model descriptor (null -> last best fit)

The ASCII model descriptor. If "?" is input a short helpfile appears, and the model prompt reappears. A null string ("") will cause the previously determined best model to be taken from the _prd.tab file. Type help models_spectral from the CL for further information.

x_axis_steps = prompt = number of steps in the first axis

Number of grid points on X axis.

y_axis_steps = prompt = number of steps in the second axis

Number of grid points on Y axis.

x_model_for_axis = prompt = model number for the first axis

Model number of X axis parameter.

y_model_for_axis = prompt = model number for the second axis

Model number of Y axis parameter.

x_axis_param = prompt = first axis parameter

Name of parameter to be used for the X axis.

y_axis_param = prompt = second axis parameter

Name of parameter to be used for the Y axis.

x_parameter_delta = prompt = half-range of first axis parameter

Half-range of x-axis parameter value

y_parameter_delta = prompt = half-range of second axis parameter

Half-range of y-axis parameter value

(pkgpars = "") [pset]

The name of the file containing the xspectral package wide parameters. If the name is null ("") then the parameters found for the pkgpars task will be used.

(final = yes) [boolean]

Perform final fit to evaluate the minimum chisquare and best-fit model.

(x_axis_type = "linear") [string]

If set to "logarithmic", then logarithmic steps will be used. Note that normalizations and column-densities are parameterized as logarithmic. It is unlikely that anything other than "linear" is required here.

(y_axis_type = "linear") [string]

If set to "logarithmic", then logarithmic steps will be used. Note that normalizations and column-densities are parameterized as logarithmic. It is unlikely that anything other than "linear" is required here.

(max_iteratio = 400) [int]

Maximum number of iterations allowed in minimization search performed from the minimum grid point at the end of the grid-point calculations.

(tolerance = 1.0E-3) [real]

Convergence tolerance for the minimization search performed at the end of the grid-point calculations. This should be decreased for improved fit, e.g. 5.0E-6.

(verbose = no) [boolean]

If set to "yes", then intermediate computational results are printed out to standard output.

(rebin = no) [boolean]

This parameter is only for compatability, and will cause an error in search_grid.

EXAMPLES

Assume the fit example from the fit help file has been performed on the input observed data set snr_obs.tab using the model "abs(20:23) * pow(4:7)". Now perform a grid search using the best fit value of that fit, as found in the snr_prd.tab output file. (The latter is specified by the null string input to the model descriptor).


xs> search_grid
Performing a grid search.
observed spectrum [root_obs.tab] (snr):
model descriptor {null=>last best fit}:
model from file: abs(20.8773:23.8773)*pow(4.3449:7.3449)
root name for output file [root_grd.tab]: .

The chisquare grid will be built around the following model
parameters:

Model: abs(20.88:23.88)*pow(4.34:7.34)

Model Component 1: Power Law
          energy index = 5.845 (free)
    normalization (log) = 1.0000 (calculated)
    galactic Nh (log) = 22.377 (free)
    intrinsic Nh (log) = 0.000 (fixed)
          redshift = 0.000 (fixed)

Please choose the grid axes:
first axis parameter (norm|index|temp|width|galacticNh|intrinsicNh): index
half-range of first axis parameter: 3
number of steps in the first axis (1:): 10
second axis parameter (norm|index|temp|width|galacticNh|intrinsicNh): galacticNh
half-range of second axis parameter: 0.5
number of steps in the second axis (1:): 10
 Best grid fit found at grid point: 9 7
       First parameter value =     8.178E0
      Second parameter value =     2.254E1
      Chi-square = 8.878

The final fit will be performed using the following model:

Model: abs(20.8773:23.8773)*pow(2.8449:8.8449)

Model Component 1: Power Law
          energy index = 5.845 (free)
    normalization (log) = 2.1493 (calculated)
    galactic Nh (log) = 22.377 (free)
    intrinsic Nh (log) = 0.000 (fixed)
          redshift = 0.000 (fixed)

Simplex 1

The final best fit parameters are:

Model: abs(22.3773)*pow(0.0406 5.8449)

Model Component 1: Power Law
          energy index = 5.845 (free)
    normalization (log) = 0.0406 (calculated)
    galactic Nh (log) = 22.377 (free)
    intrinsic Nh (log) = 0.000 (fixed)
          redshift = 0.000 (fixed)

Output Grid file: ./snr_grd.tab
Minimum chi-squared is 6.133
Predicted Data file: snr_prd.tab
Intermediate Spectra file: ./snr_int.tab


The grid table can be displayed using tprint.

TIME REQUIREMENTS

The time required to complete a grid search is dependent upon the number of grid points and the number of "free" parameters in the models. Note that the parameters used for the grid axes are temporarily "fixed" at each grid point.

BUGS

The obs_tab file name must not begin with a numeral. The same is true for the model input file, if one is used. Beginning the file names of these files with a numeral causes the parser to attempt to process the numeral, resulting in an error.

When using search_grid and one of grid axis is the normalization of the first model the input model should have a range for this normalization and not a fixed value. That is, a model of abs(17)*(12 0.3 c 100) + ray(0.7 1:2 c 100) will give an incorrect final fit if the first normalization is a grid axis, while the model abs(17)*(ray(11:13 0.3 c 100) + ray(0.7 1:2 c 100) will give the correct final fit. NB: A null string ("") should not be used for the search_model if you wish to use the first normalization on an axis.

SEE ALSO

Documentation on spectral models (help spectral_models ) for a description of the spectral model user interface.

singlefit, fit, pspc_fitting.


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