The Black Hole Spin Controversy

Abstract

Astrophysical black holes are the simplest possible objects. They are completely described by only two parameters in general relativity, their mass and the specific angular momentum, spin. The spin is a fundamental characteristic of a black hole, but unlike mass it leaves a mark on space-time only very close to the event horizon, and is therefore more difficult to measure. The observational relevance of the spin comes from the fact that it sets the size scale of the last stable orbit around the accreting black hole, dragging the accretion disc farther in the faster it spins.

Black hole spin is a very controversial topic in both stellar and supermassive black holes. There are two methods to determine spin in binary systems, using either the direct disc continuum or the reflected emission. These can give significantly different values, and understanding the source of this discrepancy is important especially for interpreting AGN spin, where only the reflection method can be used.

We have strived to understand and carefully model the time-averaged energy spectra from several Galactic black hole binaries, in order to ultimately measure the black hole spin. Achieving this has required in-depth understanding of the instruments as well as the data analysis methods. The spin leaves only minor traces in the data, and these can be affected by calibration uncertainties, uncertainties in the models and systematic uncertainties. This knowledge is absolutely essential for a detailed analysis of all relativistically smeared atomic features seen in the time-averaged energy spectra of compact objects.