Pulse profiles of highly compact pulsars in general relativity

TL;DR

This paper investigates how extreme gravitational light bending in highly compact neutron stars affects their pulse profiles, revealing distinctive features that could help constrain neutron star equations of state.

Contribution

It provides a classification of photon paths and models pulse profiles for highly compact neutron stars with M/R > 0.284, highlighting differences from standard neutron stars.

Findings

Pulse profiles are qualitatively different for highly compact stars.

The flux ratio between maximum and minimum is significantly larger.

Pulse profile features can help constrain neutron star equations of state.

Abstract

Gravitational light bending by compact stars is an important astrophysical phenomenon. The bending angle depends on the stellar compactness, which is the ratio of stellar mass $M$ to radius $R$. In this paper, we investigate the pulse profile of highly compact rotating neutron stars for which the bending angle exceeds $\pi/2$. When $M/R > 0.284$ (the bending angle becomes equal to $\pi/2$ for the stellar model with $M/R=0.284$), such a large bending happens, resulting in that a photon emitted from any position on the stellar surface can reach an observer. First, we classify the parameter plane of inclination angle $i$ and angle $\Theta$ between the rotation axis and the normal on the hot spot by the number of photon paths reaching the observer. Then, we estimate the time-dependent flux of photons emitted from two hot spots on the rotating neutron star, associated with the magnetic polar caps, for various combinations of $i$ and $\Theta$, and for two values of compactness, assuming that the stellar rotation is not so fast that the frame dragging and the stellar deformation are negligible. As the result, we find that the pulse profiles of highly compact neutron stars are qualitatively different from those for the standard neutron stars. In particular, the ratio of the maximum observed flux to the minimum one is significantly larger than that for the standard neutron stars. This study suggests that one would be able to constrain the equation of state for neutron stars through the observation of pulse profile with angles $i$ and $\Theta$ determined by other methods.