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

TL;DR

This paper models how outflows in X-ray pulsars influence their observed luminosity, pulsations, and phase lags, highlighting the role of geometrical beaming and relativistic effects in shaping observable properties.

Contribution

It introduces Monte Carlo simulations to analyze the impact of outflows on pulsation beaming and phase lags, emphasizing the limits set by mass loss rates on strong geometrical beaming.

Findings

Strong collimation reduces pulsed fraction, conflicting with observations.

Beaming causes phase lags sensitive to viewing angles.

High outflow velocities do not alter the beaming effects significantly.

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 ($\sim 0.2c$) 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.