Modeling accretion columns in accretion-powered pulsars: I. Accretion column emission and geometry

Falkner S, Sokolova-Lapa E, Schwarm FW, Dauser T, Bissinger M, Ballhausen R, Falanga M, Ferrigno C, Postnov KA, Schönherr G, Wolff MT, Becker PA, Pottschmidt K, Fürst F, Kretschmar P, Rothschild RE, Hemphill PB, Staubert R, Kreykenbohm I, Wilms J (2026)

Publication Type: Journal article

Publication year: 2026

Journal

Book Volume: 711

Article Number: A224

DOI: 10.1051/0004-6361/202453642

Abstract

Context. Accretion-powered X-ray pulsars show complex spectral and timing behavior that reflects the extreme conditions near the neutron star. Modeling these observables remains challenging, particularly at high mass-accretion rates, Ṁ ≳ 10¹⁷ g s⁻¹, where the extended accretion column plays a role in shaping anisotropic emission seen by a remote observer. Aims. Here, we present a modular framework that links radiation processes inside the column to the emission observed at infinity. The framework combines existing models for continuum and cyclotron resonant scattering feature (CRSF) formation in sidewall column emission with ray tracing, thereby bridging the gap between the rest-frame emission and the observable flux. Methods. We model the radiation from the accretion column using continuum emission from the column walls, calculated in the regime of saturated Comptonization in the presence of a radiation-dominated shock. This emission then passes through a thin outer layer with a fast-moving bulk flow, where CRSFs, derived from Monte Carlo simulations, are imprinted. Finally, a geometry-independent ray-tracing code accounts for light bending in the Schwarzschild metric, incorporating the shape and location of the emitting regions and neutron star rotation to compute the observed phase- and energy-dependent flux. Results. We present the height-dependent anisotropic emission in the column rest frame. For the chosen model, approximately 70% of the radiation is emitted within 1 km of the base of the column, predominantly at large angles to the magnetic field. Relativistic boosting in the bulk flow affects the appearance of CRSFs in the spectra, leading to a strong dependence of the line locations on the angle relative to the magnetic field. Conclusions. The proposed theoretical framework allows a straightforward combination of internal emission models with ray tracing to study the observed flux. Here, we focus on the joint treatment of the two-dimensional column structure and CRSF formation in the neutron star rest frame. The observed phase and spectral variations of the flux are highly sensitive to the location of the emitting regions and the observer’s viewing angle, which is explored in an accompanying paper.

Authors with CRIS profile

Involved external institutions

National Aeronautics and Space Administration (NASA) United States (USA) (US) International Space Science Institute (ISSI) Switzerland (CH) University of Geneva / Université de Genève (UNIGE) Switzerland (CH) Lomonosov Moscow State University / Московский государственный университет имени М.В.Ломоносова Russian Federation (RU) U.S. Naval Research Laboratory United States (USA) (US) George Mason University United States (USA) (US) European Space Astronomy Centre (ESAC) Spain (ES) University of California, San Diego (UC San Diego, UCSD) United States (USA) (US) Boston Fusion Corp United States (USA) (US) Eberhard Karls Universität Tübingen Germany (DE)

How to cite

APA:

Falkner, S., Sokolova-Lapa, E., Schwarm, F.-W., Dauser, T., Bissinger, M., Ballhausen, R.,... Wilms, J. (2026). Modeling accretion columns in accretion-powered pulsars: I. Accretion column emission and geometry. Astronomy & Astrophysics, 711. https://doi.org/10.1051/0004-6361/202453642

MLA:

Falkner, Sebastian, et al. "Modeling accretion columns in accretion-powered pulsars: I. Accretion column emission and geometry." Astronomy & Astrophysics 711 (2026).

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