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Distinguishing Black Holes and Neutron Stars via Optical Images Illuminated by Thick Accretion Disks

Chen-Yu Yang, Xiao-Xiong Zeng

Abstract

This paper investigates the optical images of neutron stars within the framework of the radiatively inefficient accretion flow model, taking into account a polytropic equation of state. After obtaining the numerical solutions of the neutron star, we solved numerically the geodesic equations together with the radiative transfer equation. We mainly examine the effects of the polytropic index $N$ and the observer inclination angle $θ_o$ on the image morphology. The obtained images are also compared with the shadow of a Schwarzschild black hole. It is shown that, under the assumption that photon trajectories are terminated at the neutron star surface, the image exhibits a bright higher order structure surrounding an inner dark region. As $N$ increases, the size of the higher-order image gradually expands. As $θ_o$ increases, the obscuration of the neutron star silhouette by radiation originating outside the equatorial plane becomes more pronounced. Compared with the black hole shadow obtained under the same parameter configuration, the neutron star exhibits a larger higher order image and a more extended obscured inner dark region, whereas the higher order image of the black hole is more readily distinguishable. These results indicate significant differences in the optical appearance of neutron stars and black holes, and thus provide a theoretical basis for distinguishing between them through high resolution imaging.

Distinguishing Black Holes and Neutron Stars via Optical Images Illuminated by Thick Accretion Disks

Abstract

This paper investigates the optical images of neutron stars within the framework of the radiatively inefficient accretion flow model, taking into account a polytropic equation of state. After obtaining the numerical solutions of the neutron star, we solved numerically the geodesic equations together with the radiative transfer equation. We mainly examine the effects of the polytropic index and the observer inclination angle on the image morphology. The obtained images are also compared with the shadow of a Schwarzschild black hole. It is shown that, under the assumption that photon trajectories are terminated at the neutron star surface, the image exhibits a bright higher order structure surrounding an inner dark region. As increases, the size of the higher-order image gradually expands. As increases, the obscuration of the neutron star silhouette by radiation originating outside the equatorial plane becomes more pronounced. Compared with the black hole shadow obtained under the same parameter configuration, the neutron star exhibits a larger higher order image and a more extended obscured inner dark region, whereas the higher order image of the black hole is more readily distinguishable. These results indicate significant differences in the optical appearance of neutron stars and black holes, and thus provide a theoretical basis for distinguishing between them through high resolution imaging.

Paper Structure

This paper contains 7 sections, 43 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Equilibrium Structure and Ray-Tracing Framework
  3. Geometrically Thick Accretion Flow Model
  4. Flow Structure and Dynamics
  5. Radiative Transfer and Synchrotron Emission
  6. Numerical Results
  7. Conclusions

Figures (3)

  • Figure 1: Effects of the polytropic index $N$ and the observer inclination angle $\theta_o$ on the optical images of neutron stars in the case of isotropic radiation.
  • Figure 2: Effects of the polytropic index $N$ and the observer inclination angle $\theta_o$ on the optical images of neutron stars in the case of anisotropic radiation.
  • Figure 3: Comparison of the optical images of a neutron star (NS) and a Schwarzschild black hole (BH) in the RIAF model for different radiation mechanisms. The first and second rows correspond to observer inclination angles of $\theta_o=0^\circ$ and $80^\circ$, respectively. The polytropic index of the neutron star is fixed at $N=1.4$, while all other plotting parameters are kept the same.