Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), an der Sternwarte 16, 14482 Potsdam, Germany
e-mail: g.schoenherr@aip.de
² Dr. Remeis-Sternwarte & ECAP, Sternwartstr. 7, 96049 Bamberg, Germany
³ INTEGRAL Science Data Centre, Université de Genève, chemin d’Écogia 16, 1290 Versoix, Switzerland
⁴ Institut für Astronomie und Astrophysik, Abt. Astronomie, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ European Space Astronomy Centre (ESA/ESAC), Science Operations Dept., PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁶ George Mason University, 4400 University Drive, Fairfax VA 22030, USA
⁷ High Energy Space Environment Branch, Space Science Division, Naval Research Laboratory, Washington DC 20375, USA
⁸ CRESST, University of Maryland Baltimore County and NASA’s Goddard Space Flight Center, Greenbelt MD 20771, USA
⁹ International Space Science Institute (ISSI), Hallerstr. 6, 3012 Bern, Switzerland
¹⁰ California Institute of Technology, Pasadena CA 91125, USA

Received: 6 August 2013
Accepted: 9 March 2014

Abstract

Context. Accretion-powered X-ray pulsars show highly energy-dependent and complex pulse-profile morphologies. Significant deviations from the average pulse profile can appear, in particular close to the cyclotron line energies. These deviations can be described as energy-dependent phase lags, that is, as energy-dependent shifts of main features in the pulse profile.

Aims. Using a numerical study we explore the effect of cyclotron resonant scattering on observable, energy-resolved pulse profiles.

Methods. We generated the observable emission as a function of spin phase, using Monte Carlo simulations for cyclotron resonant scattering and a numerical ray-tracing routine accounting for general relativistic light-bending effects on the intrinsic emission from the accretion columns.

Results. We find strong changes in the pulse profile coincident with the cyclotron line energies. Features in the pulse profile vary strongly with respect to the average pulse profile with the observing geometry and shift and smear out in energy additionally when assuming a non-static plasma.

Conclusions. We demonstrate how phase lags at the cyclotron energies arise as a consequence of the effects of angular redistribution of X-rays by cyclotron resonance scattering in a strong magnetic field combined with relativistic effects. We also show that phase lags are strongly dependent on the accretion geometry. These intrinsic effects will in principle allow us to constrain a system’s accretion geometry.