A multi-model approach to X-ray pulsars

G Schönherr,I Kreykenbohm,K Wolfram,The Magnet Collaboration ,T Dauser,S Falkner,I Caballero,K Pottschmidt,F Schwarm,R Staubert,O Nishimura,M T Wolff,J Wilms,D Klochkov,C Ferrigno,P Becker,B West,P Kretschmar

doi:10.1051/epjconf/20136402003

Abstract

The emission characteristics of X-ray pulsars are governed by magnetospheric accretion within the Alfven 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.

Highlights

The neutron stars in accreting X-ray pulsars possess very strong magnetic fields of the order of 1012 Gauss that dominate the accretion process: For all accretion types, the magnetospheric coupling of the accretion flow to the field lines governs the formation of the accretion columns and determines the geometrical and physical setting for the regions in which the observed X-ray radiation is generated
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
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

Summary

Introduction

The neutron stars in accreting X-ray pulsars possess very strong magnetic fields of the order of 1012 Gauss that dominate the accretion process: For all accretion types (e.g. wind accretion, Be accretion, or Roche lobe overflow), the magnetospheric coupling of the accretion flow to the field lines governs the formation of the accretion columns and determines the geometrical and physical setting for the regions in which the observed X-ray radiation is generated. Progress has been made during the past years in developing fitting routines based on physical models with a focus on either the continuum or on cyclotron line features (cyclomc: Schönherr & Wilms [1], bwmod: Ferrigno, Becker, Wolff [2, 3], compmag: Farinelli et al [4], cyclomod: Schwarm, Schönherr, Wilms).

Physical setup and parameters

Go multi model

Results and Conclusions