Bright X-ray Pulsars: how outflows influence beaming, pulsations and pulse phase lags by Alexander A. Mushtukov et al. on Wednesday 23 November

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.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.08952v2