Narrow Atomic Features from Rapidly Spinning Neutron Stars

TL;DR

This paper demonstrates that atomic line profiles from moderately spinning neutron stars have narrow cores due to oblateness and quadrupole effects, enhancing the prospects for detecting such lines and measuring neutron star radii.

Contribution

It introduces a ray-tracing method in the Hartle-Thorne approximation to accurately model atomic features from oblate, spinning neutron stars, highlighting the importance of these effects for observations.

Findings

Line profiles have narrow cores narrower than pure Doppler broadening.

Oblateness and quadrupole moments significantly affect atomic line shapes.

Narrow cores increase the likelihood of detecting atomic lines from rapid rotators.

Abstract

Neutron stars spinning at moderate rates (~300-600Hz) become oblate in shape and acquire a nonzero quadrupole moment. In this paper, we calculate profiles of atomic features from such neutron stars using a ray-tracing algorithm in the Hartle-Thorne approximation. We show that line profiles acquire cores that are much narrower than the widths expected from pure Doppler effects for a large range of observer inclinations. As a result, the effects of both the oblateness and the quadrupole moments of neutron stars need to be taken into account when aiming to measure neutron star radii from rotationally broadened lines. Moreover, the presence of these narrow cores substantially increases the likelihood of detecting atomic lines from rapidly spinning neutron stars.