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Due to the complexity of modeling the radiative transfer inside the accretioncolumns of neutron star binaries, their X-ray spectra are still commonlydescribed with phenomenological models, for example, a cutoff power law. Whilethe behavior of these models is well understood and they allow for a comparisonof different sources and studying source behavior, the extent to which theunderlying physics can be derived from the model parameters is very limited.During recent years, several physically motivated spectral models have beendeveloped to overcome these limitations. Their application, however, isgenerally computationally much more expensive and they require a high number ofparameters which are difficult to constrain. Previous works have presented ananalytical solution to the radiative transfer equation inside the accretioncolumn assuming a velocity profile that is linear in the optical depth. Animplementation of this solution that is both fast and accurate enough to befitted to observed spectra is available as a model in XSPEC. The maindifficulty of this implementation is that some solutions violate energyconservation and therefore have to be rejected by the user. We propose a novelfitting strategy that ensures energy conservation during the$\chi^2$-minimization which simplifies the application of the modelconsiderably. We demonstrate this approach as well a study of possibleparameter degeneracies with a comprehensive Markov-chain Monte Carlo analysisof the complete parameter space for a combined NuSTAR and Swift/XRT dataset ofCen X-3. The derived accretion-flow structure features a small column radius of$\sim$63 m and a spectrum dominated by bulk-Comptonization of bremsstrahlungseed photons, in agreement with previous studies.

Fitting strategies of accretion column models and application to the broadband spectrum of Cen X-3