Figure 2 - uploaded by Brent West
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2: Basic features of accretion onto pulsar magnetic pole are shown. To the right is a close-up view near the stellar surface. The "hot spot" near the surface produces X-rays that escape through the walls of the accretion column (Lamb, Pethick, and Pines 1973).
Source publication
Previous research to investigate the dynamics of luminous X-ray pulsars
and the observed spectra has largely been confined to the single-fluid
model in which the higher luminosity permits the accreting flow to be
regarded as a radiation-dominated ideal fluid. In this regime, the
inflowing ionized gas held no special significance when investigating...
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Citations
... The observed spectrum typically resembles a power-law with energy at low energies, transitioning to an exponentially falling spectrum above 10−20 keV. Analytical and numerical treatments of this "Comptonization" process and the resulting X-ray spectra have been presented, (e.g., Becker and Wolff, 2007;Farinelli et al., 2012;Postnov et al., 2015;West et al., 2017b). The response of the flow to this cooling and braking, and to changes in the accretion rate,Ṁ, was studied by Becker et al. (2012), who found that the behavior of the column is determined by two main factors: is the infalling material moving supersonically? ...
Studying the physical processes occurring in the region just above the magnetic poles of strongly magnetized, accreting binary neutron stars is essential to our understanding of stellar and binary system evolution. Perhaps more importantly, it provides us with a natural laboratory for studying the physics of high temperature and high density plasmas exposed to extreme radiation, gravitational, and magnetic fields. Observations over the past decade have shed new light on the manner in which plasma falling at velocities near the speed of light onto a neutron star surface is halted. Recent advances in modeling these processes have resulted in direct measurement of the magnetic fields and plasma properties. On the other hand, numerous physical processes have been identified that challenge our current picture of how the accretion process onto neutron stars works. Observation and theory are our essential tools in this regime because the extreme conditions cannot be duplicated on Earth. This white paper gives an overview of the current theory, the outstanding theoretical and observational challenges, and the importance of addressing them in contemporary astrophysics research.
