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Approximate analytic model of the boundary layer around a low magnetic field neutron star at the disk accretion

G. S. Bisnovatyi-Kogan

TL;DR

This paper develops a semi-analytic, one-dimensional model for the boundary layer around a weakly magnetized, rapidly accreting neutron star in which radiation pressure dominates in both the inner disk and the BL. It constructs a matching framework between an outer radiation-dominated disk solution and an inner BL solution, deriving closed-form expressions for pressure, density, temperature, angular velocity, and BL thickness as functions of radius or angular velocity, and identifies a disk–BL boundary via thickness criteria. The analysis yields the BL luminosity $\Phi_b = \frac{GM\dot M}{2r_*}(1-\omega_*)^2$ and shows how the BL structure evolves as the stellar rotation nears a critical value, including BL disappearance and possible jet formation at high luminosities. The results provide a tractable analytic baseline for understanding BL physics and motivate future multi-dimensional simulations to capture complex dynamics and jet production.

Abstract

An approximate analytic one-dimensional model is constructed, for the accretion disk boundary layer surrounding a neutron star whose low magnetic field does not affect the process of accretion. A high luminosity model is considered, with radiation pressure dominant in the interior part of the disk.

Approximate analytic model of the boundary layer around a low magnetic field neutron star at the disk accretion

TL;DR

This paper develops a semi-analytic, one-dimensional model for the boundary layer around a weakly magnetized, rapidly accreting neutron star in which radiation pressure dominates in both the inner disk and the BL. It constructs a matching framework between an outer radiation-dominated disk solution and an inner BL solution, deriving closed-form expressions for pressure, density, temperature, angular velocity, and BL thickness as functions of radius or angular velocity, and identifies a disk–BL boundary via thickness criteria. The analysis yields the BL luminosity and shows how the BL structure evolves as the stellar rotation nears a critical value, including BL disappearance and possible jet formation at high luminosities. The results provide a tractable analytic baseline for understanding BL physics and motivate future multi-dimensional simulations to capture complex dynamics and jet production.

Abstract

An approximate analytic one-dimensional model is constructed, for the accretion disk boundary layer surrounding a neutron star whose low magnetic field does not affect the process of accretion. A high luminosity model is considered, with radiation pressure dominant in the interior part of the disk.

Paper Structure

This paper contains 7 sections, 59 equations, 1 figure.

Table of Contents

  1. Introduction
  2. One-dimensional model of accretion disk
  3. One-Dimensional Model of the Boundary Layer
  4. Thermal Processes in the Boundary Layer
  5. Global Model of the Accretion Disk with a Boundary Layer
  6. Analytical Model of the Accretion Disk and Boundary Layer around a Neutron Star
  7. Conclusion

Figures (1)

  • Figure 1: Qualitative picture of accretion onto a neutron star with a weak magnetic field in the boundary layer formation region. In the interior part of the disk, the plasma is assumed to be radiation dominated, which corresponds to high luminosity during accretion. The arrows indicate the direction of gas motion within the boundary layer, where part of the matter is incorporated into the neutron star. At high luminosities, the radiation force accelerates part of the plasma in the vertical direction, leading to the formation of collimated jets. The assumed edge of the boundary layer is located where the characteristic BL thickness, as given by equation (\ref{['4.4c']}), becomes comparable to the vertical thickness of the disk.