Measuring the Spins of Accreting Black Holes

TL;DR

This paper reviews methods for measuring black hole spins via X-ray emission modeling, focusing on the continuum-fitting technique for stellar-mass black holes, and discusses implications of spin measurements on black hole formation and jet production.

Contribution

It provides a comprehensive overview of spin measurement techniques and applies the continuum-fitting method to eight stellar-mass black holes, highlighting new insights into their natal spins and jet phenomena.

Findings

High spins are likely natal for some black holes.

Spin alone does not determine jet presence.

Continuum-fitting method effectively measures stellar-mass black hole spins.

Abstract

A typical galaxy is thought to contain tens of millions of stellar-mass black holes, the collapsed remnants of once massive stars, and a single nuclear supermassive black hole. Both classes of black holes accrete gas from their environments. The accreting gas forms a flattened orbiting structure known as an accretion disk. During the past several years, it has become possible to obtain measurements of the spins of the two classes of black holes by modeling the X-ray emission from their accretion disks. Two methods are employed, both of which depend upon identifying the inner radius of the accretion disk with the innermost stable circular orbit (ISCO), whose radius depends only on the mass and spin of the black hole. In the Fe K method, which applies to both classes of black holes, one models the profile of the relativistically-broadened iron line with a special focus on the gravitationally redshifted red wing of the line. In the continuum-fitting method, which has so far only been applied to stellar-mass black holes, one models the thermal X-ray continuum spectrum of the accretion disk. We discuss both methods, with a strong emphasis on the continuum-fitting method and its application to stellar-mass black holes. Spin results for eight stellar-mass black holes are summarized. These data are used to argue that the high spins of at least some of these black holes are natal, and that the presence or absence of relativistic jets in accreting black holes is not entirely determined by the spin of the black hole.