Determination of the spin parameter and the inclination angle by the relativistic images in black hole image

Abstract

We studied the relativistic images caused by strong gravitational lensing in Kerr black hole images, which carry some essential signatures about the black hole space-time. We defined a new celestial coordinates whose origin is the center of black hole shadow to locate the relativistic images. Under the influences of the dragging effect caused by rotating black hole and the inclination angle of observer, the relative positions between the primary and secondary images are different with the different Kerr spin parameter $a$ and the observer's inclination angle $i$, so it can be used to determine the value of $a$ and $i$. We provided the specific approach to measure the value of $a$ and $i$ by the relativistic images. The time delays between the primary and secondary images are different with the different $a$ and $i$. The time delays in conjunction with the relative positions between the primary and secondary images could allow us to measure the value of $a$ and $i$ more precisely. These relativistic images are as unique as fingerprints for black hole space-time, by which one can further determine other parameters of all kinds of compact objects and verify various theories of gravity. Our results provide a new method to implement parameter estimation in the study of black hole physics and astrophysics.