ctlike

Perform maximum likelihood model fitting.

Synopsis

This tool performs a maximum likelihood fitting of a model to unbinned or binned data. All model parameters marked with the attribute free="1" in the input model XML file will be adjusted to the value that maximises the probability that the observed data have been drawn from the model. The tool can mix the analysis of unbinned and binned data, and is also able to perform a joint maximum likelihood analysis of data collected in separate observations or with different instruments.

:ref:`ctlike` will automatically switch between unbinned, binned and joint analysis on basis of the file provided on input (parameter inobs). Providing an event list will lead to an unbinned analysis while providing a counts cube will lead to a binned analysis. If several observations have been stacked into a single counts cube, an exposure cube, a point spread function cube and a background cube need also to be provided. :ref:`ctlike` will automatically prompt for these parameters in case that a counts cube is given. Answering NONE will ignore the exposure cube, point spread function cube and background cube and handle the data as a single observation using the specified instrument response function (parameters caldb and irf).

Multiple observations, including data collected with different instruments, can be handled by specifying an observation definition file on input. Each of the observations will be kept separately and associated with its appropriate instrument response functions, as opposed to a stacked analysis where average response functions, computed using :doc:`ctexpcube`, :doc:`ctpsfcube` and :doc:`ctbkgcube`, are used. If an observation definition file is provided, :ref:`ctlike` will use the joint likelihood of all the observations for parameter optimisation.

By default, :ref:`ctlike` will use the Poisson statistics for likelihood computation, but for binned analysis also Gaussian statistics can be specified (parameter stat). Optionally, a refit can be requested after an initial fit (parameter refit), but normally this is not needed. For all model components that have the tscalc="1" attribute set, :ref:`ctlike` will also compute the Test Statistics value that is a measure of the source significance. Optionally, the spatial model parameters can be kept fixed during that computation (parameter fix_spat_for_ts).

:ref:`ctlike` generates an output model XML file that contains the values of the best fitting model parameters. For all free parameters, an error attribute is added that provides the statistical uncertainty in the parameter estimate. If computation of the Test Statistics has been requested for a model component, a ts attribute providing the Test Statistics value is added. The output model can be used as an input model for other ctools.

In addition, the covariance matrix can be saved into a file by specifying a file name for the outcovmat parameter. If the file name ends with .fits or .fit a FITS file is written, otherwise a CSV file is written.

General parameters

inobs [file]

Input event list, counts cube or observation definition XML file.

inmodel [string]

Input model XML file.

expcube [file]

Input exposure cube file.

psfcube [file]

Input PSF cube file.

bkgcube [file]

Input background cube file.

edispcube [file]

Input energy dispersion cube file.

caldb [string]

Calibration database.

irf [string]

Instrument response function.

(edisp = no) [boolean]

Applies energy dispersion to response computation.

outmodel [string]

Output model XML file with values and uncertainties updated by the maximum likelihood fit.

(outcovmat = NONE) [string]

Output FITS or CSV file to store covariance matrix.

(statistic = DEFAULT) <DEFAULT|CSTAT|WSTAT|CHI2> [string]

Optimization statistic. DEFAULT uses the default statistic for all observations, which is CSTAT or the statistic specified in the observation definition XML file. CSTAT uses the C statistic for all observations, WSTAT uses the W statistic for all On/Off observations, and CHI2 uses the Chi squared statistic for all binned or stacked observations.

(refit = no) [boolean]

Perform refitting of solution after initial fit.

(like_accuracy = 0.005) [real]

Absolute accuracy of maximum likelihood value. Reducing this value will increase the number of iterations and provide a more accurate maximum log likelihood value. Converserly, decreasing the value will result in less iterations at the expense of a less accurate maximum likelihood value.

(max_iter = 50) [integer]

Maximum number of fit iterations.

(fix_spat_for_ts = no) [boolean]

Fix spatial parameters for TS computation.

Standard parameters

(nthreads = 0) [integer]

Number of parallel processes (0=use all available CPUs).

(chatter = 2) [integer]

Verbosity of the executable:

chatter = 0: no information will be logged

chatter = 1: only errors will be logged

chatter = 2: errors and actions will be logged

chatter = 3: report about the task execution

chatter = 4: detailed report about the task execution

(clobber = yes) [boolean]

Specifies whether an existing output model file should be overwritten.

(debug = no) [boolean]

Enables debug mode. In debug mode the executable will dump any log file output to the console.

(mode = ql) [string]

Mode of automatic parameters (default is ql, i.e. "query and learn").

(logfile = ctlike.log) [string]

Name of log file.

Related tools or scripts

None