get_prior

Synopsis

Access the prior function associated with a Sherpa model parameter.

Syntax

get_prior(par)

The Sherpa prior functions constitute a subset of the pyBLoCXS suite of functions; pyBLoCXS is a sophisticated Markov chain Monte Carlo (MCMC) based algorithm designed to carry out Bayesian Low-Count X-ray Spectral (BLoCXS) analysis in the Sherpa environment. The algorithm explores parameter space at a suspected minimum using a predefined Sherpa model to high-energy X-ray spectral data.

By default, pyBLoCXS uses a flat prior defined between the hardcoded parameter minima and maxima. The get_prior() function may be used to access the name of the prior function ('prior') which has been assigned to a thawed fit parameter ('par'), using set_prior() (see 'ahelp set_prior').

par - Sherpa model parameter for which the associated prior function shouldbe returned

The list of currently set prior-parameter pairs is returned by the list_priors command.

Refer to the pyBLoCXS documentation for additional information about the algorithm.

Example 1

print get_prior(therm.kT)

Access the prior function assigned to the temperature parameter of the Sherpa model 'xsapec.therm'.

sherpa> normgauss1d.g1
sherpa> xsapec.therm
sherpa> g1.pos=2.5; g1.fwhm=0.5
sherpa> set_prior(therm.kT,g1)

sherpa> get_prior(therm.kT)

normgauss1d.g1
   Param        Type          Value          Min          Max      Units
   -----        ----          -----          ---          ---      -----
   g1.fwhm      thawed          0.5  1.17549e-38  3.40282e+38
   g1.pos       thawed          2.5 -3.40282e+38  3.40282e+38
   g1.ampl      thawed            1 -3.40282e+38  3.40282e+38

Example 2

get_prior(abs1.nH)

Access the name of the user-defined function which was asisgned as the prior for the 'nH' parameter of a Sherpa absorption model.

sherpa> def lognorm(x, sigma=0.5, norm=1.0, x0=20.):
   xl=numpy.log10(x)+22.
   return (norm/numpy.sqrt(2*numpy.pi)/sigma)*numpy.exp(-0.5*(xl-x0)*(xl-x0)/sigma/sigma)


sherpa> set_prior(abs1.nH,lognorm)

sherpa> print get_prior(abs1.nH)
<function lognorm at 0xc857230>

Bugs

See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.