On the illumination of neutron star accretion discs

TL;DR

This study uses relativistic ray tracing to analyze how neutron star accretion discs are illuminated by X-ray sources, evaluating existing models and deriving emissivity profiles that match observations of specific systems.

Contribution

It assesses the applicability of black hole emission line models to neutron star discs and computes theoretical emissivity profiles for various illumination scenarios.

Findings

Relativistically broadened emission line models are valid for neutron star discs under certain conditions.

Emissivity profiles are well described by a single power law with index ~3.

Observed emissivity in Serpens X-1 is consistent with boundary layer illumination.

Abstract

The illumination of the accretion disc in a neutron star X-ray binary by X-rays emitted from (or close to) the neutron star surface is explored through general relativistic ray tracing simulations. The applicability of the canonical suite of relativistically broadened emission line models (developed for black holes) to discs around neutron stars is evaluated. These models were found to describe well emission lines from neutron star accretion discs unless the neutron star radius is larger than the innermost stable orbit of the accretion disc at 6rg or the disc is viewed at high inclination, above 60deg where shadowing of the back side of the disc becomes important. Theoretical emissivity profiles were computed for accretion discs illuminated by hotspots on the neutron star surfaces, bands of emission and emission by the entirety of the hot, spherical star surface and in all cases, the emissivity profile of the accretion disc was found to be well represented by a single power law falling off slightly steeper than r^-3. Steepening of the emissivity index was found where the emission is close to the disc plane and the disc can appear truncated when illuminated by a hotspot at high latitude. The emissivity profile of the accretion disc in Serpens X-1 was measured and found to be consistent with a single unbroken power law with index q=3.5 (-0.4,+0.3), suggestive of illumination by the boundary layer between the disc and neutron star surface.