Bright X-ray pulsars: how outflows influence beaming, pulsations and pulse phase lags

Monthly Notices of the Royal Astronomical Society, vol. 518, pp. 5457–5464

Abstract

ABSTRACT Extreme accretion in X-ray pulsars (XRPs) results in radiation-driven outflows launched from the inner parts of the accretion disc. The outflows affect the apparent luminosity of the XRPs and their pulsations through the geometrical beaming. We model processes of geometrical beaming and pulse formation using Monte Carlo simulations. We confirm our earlier statement that strong amplification of luminosity due to the collimation of X-ray photons is inconsistent with a large pulsed fraction. Accounting for relativistic aberration due to possibly high outflow velocity (∼0.2c) does not affect this conclusion. We demonstrate that the beaming causes phase lags of pulsations. Within the opening angle of the accretion cavity formed by the outflows, phase lags tend to be sensitive to observers viewing angles. Variations in outflow geometry and corresponding changes of the phase lags might influence the detectability of pulsation in bright X-ray pulsars and ULXs. We speculate that the strong geometrical beaming is associated with large radiation pressure on the walls of accretion cavity due to multiple photons reflections. We expect that the mass-loss rate limits geometrical beaming: strong beaming becomes possible only under sufficiently large fractional mass-loss rate from the disc.

Authors 2

1. Alexander A. Mushtukov corresponding Aachen Leiden Observatory

   Leiden University · University of Oxford · Leiden Observatory

   Affiliation as printed

   Astrophysics, Department of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

   Leiden Observatory, Leiden University , NL-2300 RA Leiden, the Netherlands

2. Simon Portegies Zwart Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University , NL-2300 RA Leiden, the Netherlands