A Ray-Tracing Algorithm for Spinning Compact Object Spacetimes with Arbitrary Quadrupole Moments. I. Quasi-Kerr Black Holes

TL;DR

This paper introduces a new ray-tracing algorithm for spinning compact objects with arbitrary quadrupole moments, enabling tests of the no-hair theorem and analysis of accretion disk emissions.

Contribution

It presents a novel numerical method for simulating photon trajectories in non-Kerr spacetimes with arbitrary quadrupole moments.

Findings

Quadrupole moments significantly affect iron line profiles.

The algorithm can distinguish between Kerr and non-Kerr spacetimes.

Observable effects in accretion disk spectra are demonstrated.

Abstract

We describe a new numerical algorithm for ray tracing in the external spacetimes of spinning compact objects characterized by arbitrary quadrupole moments. Such spacetimes describe non-Kerr vacuum solutions that can be used to test the no-hair theorem in conjunction with observations of accreting black holes. They are also appropriate for neutron stars with spin frequencies in the 300-600 Hz range, which are typical of the bursting sources in low-mass X-ray binaries. We use our algorithm to show that allowing for the quadrupole moment of the spacetime to take arbitrary values leads to observable effects in the profiles of relativistic broadened fluorescent iron lines from geometrically thin accretion disks.