Surface geometry and shadow of a Schwarzschild black hole with a halo* *Supported by the National Natural Science Foundation of China (12105151), the Shandong Provincial Natural Science Foundation of China (ZR2020QA080), and partially supported by the National Natural Science Foundation of China (11875026, 11875025, 12035005)

Abstract

We studied the surface geometry and shadows of a Schwarzschild black hole with a halo containing quadrupolar and octopolar terms. It was found that the quadrupole term causes a Schwarzschild black hole to be prolate at the quadrupole strength and oblate at , and the octopole term causes to the shadow stretch upward at the octopolar strength ngth and downward at . The shadow of a Schwarzschild black hole with a halo stretches and squeezes along the horizontal direction at and , respectively. Meanwhile, the black hole shadow shifts upward at and downward at . We present the light rays that form the shadow boundary to explain the emergence of the extraordinary patterns of the black hole shadow with quadrupole and octopole terms. From the observable width W, height H, oblateness K, and distortion parameter of the black hole shadow, we can determine the quadrupole and octopolar strengths of a Schwarzschild black hole with a halo. Our results show that the quadrupolar and octopolar terms yield a series of interesting patterns for the shadow of a Schwarzschild black hole with a halo.