Xspec with BXA
=======================================
This documentation shows how to use BXA with `PyXSpec`_.
Begin by importing bxa in a python session with heasoft loaded:
.. literalinclude:: ../examples/xspec/example_simplest.py
:lines: 4-5
Load your data, define your background model and source model as usual
with `PyXSpec`_.
.. _PyXspec: https://heasarc.gsfc.nasa.gov/xanadu/xspec/python/html/
.. _xspec-priors:
Defining priors
---------------------
Define your background model and source model as usual in xspec.
Then define the priors over the free parameters, for example:
.. literalinclude:: ../examples/xspec/example_simplest.py
:lines: 13-27
The above is taken from
`examples/xspec/example_simplest.py `_.
Make sure you set the parameter minimum and maximum values to appropriate (a priori reasonable) values.
The limits are used to define the uniform and loguniform priors.
You can freeze the parameters you do not want to investigate, but BXA only modifies the parameters specified.
You can also define your own prior functions, which transform
unit variables unto the values needed for each parameter.
See the `UltraNest documentation on priors `_
for more details about this concept.
The script `examples/xspec/example_advanced_priors.py `_
gives an example of such a custom prior function (`my_custom_prior`).
API information:
.. autofunction:: bxa.xspec.create_uniform_prior_for
.. autofunction:: bxa.xspec.create_loguniform_prior_for
.. autofunction:: bxa.xspec.create_gaussian_prior_for
.. autofunction:: bxa.xspec.create_custom_prior_for
Prior Predictive Checks
------------------------
To check that your priors and model is okay and working,
create a flipbook of prior sample predictions.
1) Pick a random sample from the prior::
import numpy
from bxa.xspec.solver import set_parameters
prior_function = bxa.create_prior_function(transformations)
values = prior_function(numpy.random.uniform(size=len(transformations)))
set_parameters(transformations, values)
print("set to parameters:", values)
2) make a plot
Repeat this 20 times and look at the plots.
Do the shapes and number of counts expected
look like a reasonable representation of your prior expectation?
.. _xspec-run:
Running the analysis
---------------------
This runs the fit and stores the result in the specified output folder:
.. literalinclude:: ../examples/xspec/example_simplest.py
:lines: 30-32
.. autoclass:: bxa.xspec.BXASolver
:noindex:
The returned results contain posterior samples and the Bayesian evidence.
These are also reported on the screen for you.
.. _xspec-analyse:
Parameter posterior plots
--------------------------
Credible intervals of the model parameters, and histograms
(1d and 2d) of the marginal parameter distributions are plotted
in 'myoutputs/plots/corner.pdf' for you.
.. figure:: absorbed-corner.*
:scale: 50%
You can also plot them yourself using corner, triangle and getdist, by
passing `results['samples']` to them.
For more information on the corner library used here,
see https://corner.readthedocs.io/en/latest/.
Model checking
-----------------------
For this functionality, you also need scipy installed.
The following code creates a plot of the convolved posterior model:
.. literalinclude:: ../examples/xspec/example_simplest.py
:lines: 37-60
.. figure:: absorbed-convolved_posterior.*
Example of the convolved spectrum with data.
For each posterior sample (solution), the parameters are taken and put
through the model. All such lines are plotted. Where the region is darker,
more lines ended up, and thus it is more likely.
The data points are adaptively binned to contain at least 20 counts.
The error bars are created by asking: which model count rate can produce
this amount of counts.
In a Poisson process, the inverse incomplete gamma
function provides this answer. The 10%-90% probability range is used.
On the colors of the data points:
For all intents and purposes, you can ignore the colors.
The colors are intended to aid the discovery of discrepancies, by using
a custom Goodness of Fit measure. In this procedure (gof module),
a tree of the bins is built, i.e. in the first layer, every 2 bins
are merged, in the second, every 4 bins are merged, etc.
Then, the counts in the bins are compared against with
the poisson process of the model. The worst case, i.e. the least likely
probability over the whole tree is considered. That is, for each bin,
the lowest probability of all its merges is kept. Finally, this is multiplied
by the number of nodes in the tree (as more comparisons lead to more
random chances).
Then, if the probability for the bin is below :math:`10^{-2}`, the point is marked orange,
and if it reaches below :math:`10^{-6}`, it is marked red.
It is ok to ignore the colors, this computation is not used otherwise.
The following code creates a plot of the unconvolved posterior:
.. literalinclude:: ../examples/xspec/example_simplest.py
:lines: 65-79
.. figure:: absorbed-unconvolved_posterior.*
Example of the unconvolved spectrum.
For each posterior sample (solution), the parameters are taken and put
through the model. All such lines are plotted. Where the region is darker,
more lines ended up, and thus it is more likely.
For plotting the model parameters found against the data, use these functions.
.. automethod:: bxa.xspec.BXASolver.posterior_predictions_unconvolved
.. automethod:: bxa.xspec.BXASolver.posterior_predictions_convolved
.. automethod:: bxa.xspec.sinning.binning
:noindex:
Error propagation
---------------------
`results['samples']` provides access to the posterior samples (similar to a Markov Chain).
Use these to propagate errors:
* For every row in the chain, compute the quantity of interest
* Then, make a histogram of the results, or compute mean and standard deviations.
This preserves the structure of the uncertainty (multiple modes, degeneracies, etc.)
*Continuing in Xspec*: A chain file, compatible with Xspec chain commands is
written for you into *chain.fits*. In Xspec, load it using `"chain load" `_.
This should set parameters, and compute flux estimates.
.. _xspec-models:
.. include:: model_comparison.rst
.. _xspec-design:
.. include:: experiment_design.rst
.. _xspec-qq:
.. include:: model_discovery.rst
For Xspec, the *qq* function in the *qq* module allows you to create such plots easily:
.. literalinclude:: ../examples/xspec/example_simplest.py
:lines: 83-90
.. autofunction:: bxa.xspec.qq.qq
Refer to the :ref:`accompaning paper `, which gives an introduction and
detailed discussion on the methodology.