XSPEC

XSPEC is a command-driven interactive spectral fitting program for astronomical X-ray data. It is part of the HEASoft software suite and is developed and maintained by the High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA, with contributions from researchers worldwide. XSPEC is used to fit physical models to energy spectra recorded by X-ray detectors, accounting for the instrument response and observational effects.

XSPEC employs forward-folding, where a user-defined model spectrum is folded through a detector’s response matrix and ancillary response files to compare with observed counts. It provides a large library of built-in spectral components, including power laws, blackbodies, thermal plasmas, accretion-disk models, and Comptonization, as well as numerous line and reflection components. Components can be combined multiplicatively or additively, and convolution with response and line-spread functions is supported.

Fitting and statistics: XSPEC supports different statistics depending on data quality, including chi-squared for binned, well-counted data and Cash statistics (C-stat) for Poisson data with low counts. It accommodates background modeling, data from multiple detectors or observations, and simultaneous fitting. The package offers error estimation, confidence intervals, and goodness-of-fit measures, and can generate simulated spectra and diagnostic plots for assessment.

Data I/O and workflow: XSPEC reads PHA files along with corresponding RMF and ARF response files and optional background files, producing fit results, plots, and tables. It runs on Unix-like systems as part of HEASoft, with Windows ports available. The program includes a scripting interface for batch processing and has an active user community, extensive documentation, and a broad model library contributed by users.