Radiation from rapidly rotating oblate neutron stars

TL;DR

This paper develops a detailed relativistic model for emission from rapidly rotating oblate neutron stars, including effects like light bending and frame-dragging, and assesses their impact on observable spectral and pulse profiles.

Contribution

It introduces an open-source numerical framework for modeling emission from rotating neutron stars, incorporating second-order quadrupole effects and providing detailed predictions for spectral and pulse observations.

Findings

Second order quadrupole effects are weak and unlikely to produce narrow spectral features.

The framework accurately predicts spectral line smearing and pulse profiles for fast-spinning neutron stars.

Quantitative estimates of rotational smearing effects on observed spectra are provided.

Abstract

A theoretical framework for emission originating from rapidly rotating oblate compact objects is described in detail. By using a Hamilton-Jacobi formalism, we show how the special relativistic rotational effects such as aberration of angles, Doppler boosting, and time dilatation naturally emerge from the general relativistic treatment of rotating compact objects. We use the Butterworth-Ipser metric expanded up to the second order in rotation and hence include effects of light bending, frame-dragging, and quadrupole deviations to our geodesic calculations. We also give detailed descriptions of the numerical algorithms used and provide an open source implementation of the numerical framework called bender. As an application, we study spectral line profiles (i.e., smearing kernels) from rapidly rotating oblate neutron stars. We find that in this metric description the second order quadrupole effects are not strong enough to produce narrow observable features in the spectral energy distribution for almost any physically realistic parameter combination, and hence, actually detecting them is unlikely. The full width at tenth-maximum and full width at half-maximum of the rotation smearing kernels are also reported for all viewing angles. These can be then used to quantitatively estimate the effects of rotational smearing on the observed spectra. We also calculate accurate pulse profiles and observer skymaps of emission from hot spots on rapidly rotating accreting millisecond pulsars. These allow us to quantify the strength of the pulse fractions one expects to observe from typical fast spinning millisecond pulsars.