On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

Abstract

The luminosity of accreting magnetised neutron stars can largely exceed the Eddington value due to appearance of accretion columns. The height of the columns can be comparable to the neutron star radius. The columns produce the X-rays detected by the observer directly and illuminate the stellar surface, which reprocesses the X-rays and causes additional component of the observed flux. The geometry of the column and the illuminated part of the surface determines the radiation beaming. Curved spacetime affects the angular flux distribution. We construct a simple model of the beam patterns formed by direct and reflected flux from the column. We take into account the possibility of appearance of accretion columns, whose height is comparable to the neutron star radius. We argue that depending on the compactness of the star the flux from the column can be either strongly amplified due to gravitational lensing, or significantly reduced due to column eclipse by the star. The eclipses of high accretion columns result in specific features in pulse profiles. Their detection can put constraints on the neutron star radius. We speculate that column eclipses are observed in X-ray pulsar V 0332+53, leading us to the conclusion of large neutron star radius in this system (∼ 15 km if M ∼ 1.4M ). We point out that the beam pattern can be strongly affected by scattering in the accretion channel at high luminosity, which has to be taken into account in the models reproducing the pulse profiles.

Anticorrelation between X-ray luminosity and pulsed fraction in the Small Magellanic Cloud pulsar SXP 1323

“…Fan-beam emission becomes dominant, which reduces the eclipse of the accretion column. Furthermore, the contribution of the flux reflected by the NS surface is significant (Mushtukov et al 2018). It raises the un-pulsed flux, therefore we see the luminosity increasing and the pulsed fraction decreasing.…”

Discovery of a Pulse-phase-transient Cyclotron Line in the X-Ray pulsar GRO J2058+42

“…Due to a large gradient of the B-field strength over the visible column height (see, e.g., Nishimura 2015) or latitudes on the stellar surface, the scattering feature can vanish from the observed energy spectra. However, a situation where the accretion column is partially eclipsed by the neutron star at certain phases of the pulse and the observer sees only a fraction of the accretion column is possible (Mushtukov et al 2018). In this case, the dispersion of the magnetic field strength over the visible part of the column is relatively small, and the cyclotron line can appear at some phases of pulsations as it is observed in GRO J2058+42.…”

On the Geometry of the X-ray emission from pulsars a consistent inclination and beaming solution for the Be/X-ray pulsar SXP 1062

“…A third theory involves reflection of energetic photons off the accretion mound that forms at the base of the column (Mushtukov et al 2018). Again the accretion column would diminish as accretion lessens, leading to fewer photons being reflected from the mound.…”