Influence of the external electromagnetic field on the properties of the Novikov-Thorne accretion disk in Kerr spacetime

TL;DR

This paper numerically investigates how external magnetic fields influence the properties and radiation spectra of accretion disks around Kerr black holes, extending the Novikov-Thorne model to magnetized, non-integrable spacetimes.

Contribution

It introduces a numerical method to compute orbital parameters in magnetized Kerr spacetime, linking magnetic field strength to disk radiation properties for the first time.

Findings

Magnetic fields increase disk radiation when aligned with black hole spin.

Identified a magnetic field threshold of approximately 1.06×10^{-9} T for detectability.

Extended the Novikov-Thorne model to include external magnetic field effects.

Abstract

The Novikov-Thorne accretion disk model is widely employed in astrophysics, yet computing its blackbody spectrum theoretically requires analytical expressions for the orbital parameters -- specific energy, angular momentum, and angular velocity -- of the constituent timelike particles, a task extremely challenging in non-integrable curved spacetimes. In this work, we numerically obtain these orbital parameters for quasi-Keplerian motion in Kerr spacetime with an asymptotically uniform magnetic field using iterative, finite-difference, and interpolation methods, enabling simulations of the disk's energy flux density, temperature, and blackbody spectra across diverse spin parameters, observational inclinations, and magnetic field strengths. We demonstrate that when the magnetic field aligns with the black hole's angular momentum, the disk's radiation positively correlates with field strength, while spectral analysis for our specific black hole mass and accretion rate reveals a conservative detectable threshold of $1.0638 \times 10^{-9}$ T for ambient magnetic fields. This study not only extends the Novikov-Thorne model to non-integrable axisymmetric spacetimes but also establishes the first direct relationship between external magnetic fields and disk properties, providing critical theoretical support for future magnetic environment studies through disk radiation observations.