Flickering pulsations in bright X-ray pulsars: the evidence of gravitationally lensed and eclipsed accretion column

• Main Authors: Mushtukov, Alexander A, Weng, Albert, Tsygankov, Sergey S, Mereminskiy, Ilya A
• Format: Journal Article
• Language: English
• Published: 05-04-2024
• Subjects: Physics - High Energy Astrophysical Phenomena; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2404.04137

It is expected that extreme mass accretion rate onto strongly magnetised
neutron star results in appearance of accretion columns above stellar surface.
For a distant observer, rotation of a star results in periodic variations of
X-ray flux. Because the mass accretion rate fluctuates around the average
value, the pulse profiles are not stable and demonstrate fluctuations as well.
In the case of bright X-ray pulsars, however, pulse fluctuations are not solely
attributed to variations in the mass accretion rate. They are also influenced
by the variable height of the columns, which is dependent on the mass accretion
rate. This study delves into the process of pulse profile formation in bright
X-ray pulsars, taking into account stochastic fluctuations in the mass
accretion rate, the corresponding variations in accretion column geometry and
gravitational bending. Our analysis reveals that potential eclipses of
accretion columns by a neutron star during their spin period should manifest
specific features in pulse profile variability. Applying a novel pulse profile
analysis technique, we successfully detect these features in the bright X-ray
transient V0332+53 at luminosities $\gtrsim 2\times 10^{38}\,{\rm
erg\,s^{-1}}$. This detection serves as compelling evidence for the eclipse of
an accretion column by a neutron star. Detection of the eclipse places
constraints on the relation between neutron star mass, radius and accretion
column height. Specifically, we can establish an upper limit on the accretion
column height, which is crucial for refining theoretical models of extreme
accretion.