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

¹ Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: philipp.thalhammer@fau.de
² Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Erwin-Rommel-Strae 1, 91058 Erlangen, Germany
³ Department of Physics and Center for Space Science and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁵ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
⁶ Quasar Science Resources SL for ESA, European Space Astronomy Centre (ESAC), Science Operations Departement, 28692 Villanueva de la Cañada, Madrid, Spain
⁷ NASA Marshall Space Flight Center, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
⁸ Universities Space Research Association, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA
⁹ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
¹⁰ Physics and Astronomy, George Mason University, Fairfax, VA, USA
¹¹ Sternberg Astronomical Institute, M. V. Lomonosov Moscow State University, Universitetskij Pr., 13, Moscow 119992, Russia
¹² Southeastern Universities Research Association, Washington, DC 20005, USA

Received: 16 February 2021
Accepted: 18 August 2021

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 χ²-minimization which simplifies the application of the model considerably. We demonstrate this approach as well as 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 ∼63 m and a spectrum dominated by bulk-Comptonization of bremsstrahlung seed photons, in agreement with previous studies.