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

TL;DR

This paper introduces a new energy-conserving fitting strategy for accretion column models in neutron star binaries, demonstrated on Cen X-3, improving the physical interpretation of X-ray spectra.

Contribution

It proposes a novel fitting approach that enforces energy conservation, simplifying the application of physically motivated accretion column models to observed spectra.

Findings

Accretion column radius estimated at ~63 meters.

Spectrum dominated by bulk-Comptonization of bremsstrahlung photons.

Model consistent with previous studies on Cen X-3.

Abstract

Due to the complexity of modeling the radiative transfer inside the accretion columns of neutron star binaries, their X-ray spectra are still commonly described with phenomenological models, for example, a cutoff power law. While the behavior of these models is well understood and they allow for a comparison of different sources and studying source behavior, the extent to which the underlying physics can be derived from the model parameters is very limited. During recent years, several physically motivated spectral models have been developed to overcome these limitations. Their application, however, is generally computationally much more expensive and they require a high number of parameters which are difficult to constrain. Previous works have presented an analytical solution to the radiative transfer equation inside the accretion column assuming a velocity profile that is linear in the optical depth. An implementation of this solution that is both fast and accurate enough to be fitted to observed spectra is available as a model in XSPEC. The main difficulty of this implementation is that some solutions violate energy conservation and therefore have to be rejected by the user. We propose a novel fitting strategy that ensures energy conservation during the $\chi^2$-minimization which simplifies the application of the model considerably. We demonstrate this approach as well a study of possible parameter degeneracies with a comprehensive Markov-chain Monte Carlo analysis of the complete parameter space for a combined NuSTAR and Swift/XRT dataset of Cen X-3. The derived accretion-flow structure features a small column radius of $\sim$63 m and a spectrum dominated by bulk-Comptonization of bremsstrahlung seed photons, in agreement with previous studies.