A multi-model approach to X-ray pulsars

Connecting spectral and timing models to pin down the intrinsic emission characteristics of magnetized, accreting neutron stars

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Dr. Remeis-Sternwarte & ECAP, Sternwartstr. 7, 96049 Bamberg, Germany
³ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
⁴ High Energy Space Environment Branch, Space Science Division, Naval Research Laboratory, Washington DC 20375, USA
⁵ CRESST, University of Maryland Baltimore County and NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ European Space Astronomy Centre (ESA/ESAC), Science Operations Dept., P.O. Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁷ INTEGRAL Science Data Centre, Université de Genève, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁸ Institut für Astronomie und Astrophysik, Sand 1, Abt. Astronomie, Universität Tübingen, Germany
⁹ Department of Electronics and Computer Science, Nagano National College of Technology, 716 Tokuma, 381-8550 Nagano, Japan
¹⁰ CEA Saclay, CNRS/CEA/Université P. Diderot, F-91191 Gif sur Yvette, France

^a e-mail: g.schoenherr@aip.de

Published online: 8 January 2014

Abstract

The emission characteristics of X-ray pulsars are governed by magnetospheric accretion within the Alfvén radius, leading to a direct coupling of accretion column properties and interactions at the magnetosphere. The complexity of the physical processes governing the formation of radiation within the accreted, strongly magnetized plasma has led to several sophisticated theoretical modelling efforts over the last decade, dedicated to either the formation of the broad band continuum, the formation of cyclotron resonance scattering features (CRSFs) or the formation of pulse profiles. While these individual approaches are powerful in themselves, they quickly reach their limits when aiming at a quantitative comparison to observational data. Too many fundamental parameters, describing the formation of the accretion columns and the systems’ overall geometry are unconstrained and different models are often based on different fundamental assumptions, while everything is intertwined in the observed, highly phase-dependent spectra and energy-dependent pulse profiles. To name just one example: the (phase variable) line width of the CRSFs is highly dependent on the plasma temperature, the existence of B-field gradients (geometry) and observation angle, parameters which, in turn, drive the continuum radiation and are driven by the overall two-pole geometry for the light bending model respectively. This renders a parallel assessment of all available spectral and timing information by a compatible across-models-approach indispensable. In a collaboration of theoreticians and observers, we have been working on a model unification project over the last years, bringing together theoretical calculations of the Comptonized continuum, Monte Carlo simulations and Radiation Transfer calculations of CRSFs as well as a General Relativity (GR) light bending model for ray tracing of the incident emission pattern from both magnetic poles. The ultimate goal is to implement a unified fitting model for phase-resolved spectral and timing data analysis. We present the current status of this project.