BXA
BXA connects the Xspec and Sherpa X-ray spectral analysis environments with UltraNest for Bayesian parameter estimation and model comparison.
What BXA is
BXA connects the Xspec and Sherpa X-ray spectral analysis environments with UltraNest for Bayesian parameter estimation and model comparison. It can estimate parameters across arbitrary dimensions, find best fits, calculate error bars and marginal probability distributions, and compute model evidence for Bayes factors, including when comparing models that are not nested. Quantile-Quantile plots help visualize differences between a model and data without binning. Its fitting algorithm explores parameter spaces without requiring good starting points and continues until convergence. BXA is intended for systematic analysis of large datasets, comparisons among models, and low-count datasets analyzed with realistic models. It is free, open-source software under GNU GPL v3 or later, with Linux listed as its platform. Install it with pip or conda; it requires Sherpa or Xspec to be installed and configured. MPI parallelisation can extend inference from laptops to computing clusters when mpi4py is installed. Release testing is stated to cover Ubuntu Linux only.
Good
- Estimates parameters and computes error bars and distributions.
- Compares models that are not nested.
- Fitting does not require good starting points.
- MPI can scale inference to computing clusters.
- Free software under GNU GPL v3 or later.
Watch out
- Requires Sherpa or Xspec to be installed and configured.
- Release testing covers Ubuntu Linux only.
- Version 5 changes Xspec priors to soft parameter limits.
Who it suits
BXA suits researchers using Xspec or Sherpa who need Bayesian fitting and model comparison, including work with large or low-count datasets. MPI support may suit users scaling inference to computing clusters.
Verdict
BXA brings Bayesian estimation and model comparison to Xspec and Sherpa workflows. Check its software requirements and the version 5 prior change before installing.
Plans and prices
From BXA's own pricing page, checked 5 Oct 2026.
| Plan | Price | What it gives you |
|---|---|---|
| BXA | Free | Free software under GNU GPL v3 or later |
The facts
Each one read from BXA's own pages, with the date it was checked.
What it does
- Automated fitting
- The fitting algorithm explores parameter spaces without requiring good starting points and runs until convergence.checked 5 Oct 2026
- Installation
- BXA can be installed with pip or conda and requires either Sherpa or Xspec to be installed and configured.checked 5 Oct 2026
- Intended use
- The documentation highlights systematic analysis of large datasets, comparing multiple models, and analyzing low-count datasets with realistic models.checked 5 Oct 2026
- Model comparison
- It computes model evidence for Bayes factors and supports comparing models that are not nested.checked 5 Oct 2026
- Model discovery
- It provides Quantile-Quantile plots to visualize deviations between a model and data without binning.checked 5 Oct 2026
- Notable change
- In version 5, Xspec priors use soft parameter limits instead of hard parameter limits, which the release notes flag as a potentially breaking change.checked 5 Oct 2026
- Parallel processing
- MPI parallelisation can scale inference from laptops to computing clusters when mpi4py is installed.checked 5 Oct 2026
- Parameter estimation
- It estimates parameters in arbitrary dimensions, finds best fits, computes error bars and marginal probability distributions, and supports MPI parallelisation.checked 5 Oct 2026
- Purpose
- BXA connects Xspec and Sherpa X-ray spectral analysis environments to UltraNest for Bayesian parameter estimation and model comparison.checked 5 Oct 2026
- Security and warranty
- The repository publishes a GNU GPL license whose text disclaims warranties for the program.checked 5 Oct 2026
- Support
- The project directs users to file bug reports and feedback as GitHub issues and asks bug reports to include operating system, Sherpa or Xspec version, setup details, and reproduction steps.checked 5 Oct 2026
- Supported systems
- BXA runs on operating systems supported by CIAO/Sherpa or HEASoft/Xspec, while release testing is stated to cover Ubuntu Linux only.checked 5 Oct 2026
- Xspec compatibility
- When run from pyxspec, BXA provides chain FITS files compatible with Xspec/pyxspec MCMC features and error propagation tools.checked 5 Oct 2026
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