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search.filters.applied.f.access: openAccess × Author: Klochkov, D. × Author: Wilms, J. × Has files: Yes × Type: Article ×

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    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
    (Les Ulis : EDP Sciences, 2014) Schönherr, G.; Schwarm, F.; Falkner, S.; Becker, P.; Wilms, J.; Dauser, T.; Wolff, M.T.; Wolfram, K.; West, B.; Pottschmidt, K.; Kretschmar, P.; Ferrigno, C.; Klochkov, D.; Nishimura, O.; Kreykenbohm, I.; Caballero, I.; Staubert, R.
    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.
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    Luminosity dependent accretion state change in GRO J1008-57
    (Les Ulis : EDP Sciences, 2014) Kühnel, M.; Müller, S.; Kreykenbohm, I.; Fürst, F.; Pottschmidt, K.; Rothschild, R.E.; Caballero, I.; Grinberg, V.; Schönherr, G.; Shrader, C.; Klochkov, D.; Staubert, R.; Ferrigno, C.; Torrejón, J.-M.; Martínez-Núñez, S.; Wilms, J.
    In a former publication, we have analyzed the transient neutron star X-ray binary GRO J1008-57 using all available RXTE-, Swift-, and Suzaku-data. As we have found, the source's spectral components, i.e., a power-law with high exponential cutoff and a black-body, are strongly correlated with the hard X-ray flux (15-50 keV). We update the analytical description of these dependence, including a change in the photon index behaviour from a flat to a logarithmic function. The flux, where the change occurs, is consistent with the onset of the black-body emission. Thus, a change of the accretion state always occurs in GRO J1008-57 at a particular flux level.
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    The 35-day cycle in the X-ray binary HZ Her/Her X-1
    (Tatranská Lomnica : Astronomický Ústav SAV, 2020) Kolesnikov, D.; Shakura, N.; Postnov, K.; Volkov, I.; Bikmaev, I.; Irsmambetova, T.; Staubert, R.; Wilms, J.; Irtuganov, E.; Shurygin, P.; Golysheva, P.; Shugarov, S.; Nikolenko, I.; Trunkovsky, E.; Schonherr, G.; Schwope, A.; Klochkov, D.
    We present the results of modelling the 35-day superorbital changes in the B and V lightcurves and X-ray flux of HZ Her/Her X-1. The model is implemented in a computer program written in the C programming language, with a module for parameter optimisation written in Python. The model in-cludes a tilted precessing and warped accretion disc around a freely precessing neutron star. The disc is warped near its inner edge due to interaction with the rotating neutron star magnetosphere. The magnetic torque depends on the precessional phase of the neutron star. The neutron star X-ray emission flux also depends on the free precession phase, which modulates the X-ray illumi-nation of the optical star's atmosphere and the intensity of gas streams. We demonstrate that this model is able to reproduce both the optical observations of HZ Her and the behaviour of the system's 35-day X-ray cycle. © 2020 Astronomical Institute, Slovak Academy of Sciences.