Convenience features

Fixing FITS keywords

Source, background, ARF and RMF files should reference each other by FITS header keywords.

The fixkeywords.py script adjusts the keywords:

fixkeywords.py src.pi bkg.pi rmf.rmf arf.arf

This also fixes ARF/RMF that start the energy bounds at zero (which is invalid) instead of a small number.

Galactic absorption

The galactic column density in the direction of the source is often needed. https://www.swift.ac.uk/analysis/nhtot/donhtot.php provides a look-up service.

The gal.py script fetches the value from there:

gal.py src.pi

and stores it in src.pi.nh.

You can also give this script multiple spectral files and it avoids duplicate requests.

Accelerating slow models

Some models are very slow (such as convolutions). It is worthwhile to cache them or produce interpolation grids.

For Sherpa, caching of arbitrary models is provided by CachedModel, which you can use as a wrapper:

Automatic production of an interpolation model is possible with the RebinnedModel:

Xspec chain files

BXA, when run from pyxspec, also provides chain fits file compatible with the mcmc feature in xspec/pyxspec. xspec error propagation tools can thus be used after a BXA fit. In xspec, one can load it with:

XSPEC12> chain load path/to/mychain.fits

Parallelisation

BXA supports parallelisation with MPI (Message Passing Interface). This allows scaling the inference from laptops all the way to computing clusters.

To use it, install mpi4py and run your python script with mpiexec:

$ mpiexec -np 4 python3 myscript.py

No modifications of your scripts are needed. However, you may want to run plotting and other post-analysis only on rank 0.

Analysis of many data sets, or of many models are trivial to parallelise. If your script accepts a command line argument, unix tools such as “make -j 10” and “xargs –max-args 1 –max-procs 10” can help run your code in parallel.

Verbosity

If you want to make the fitting more quiet, set verbose=False when calling run().

You can find more instructions how to reduce the output of the UltraNest fitting engine here.

Code inside a XSilence container disables Xspec chatter:

from bxa.xspec.solver import XSilence

with XSilence():
        # do something here