paper

The Spin of Supermassive Black Holes

arXiv:1307.3246 · doi:10.1088/0264-9381/30/24/244004

Abstract

Black hole spin is a quantity of great interest to both physicists and astrophysicists. We review the current status of spin measurements in supermassive black holes (SMBH). To date, every robust SMBH spin measurement uses X-ray reflection spectroscopy, so we focus almost exclusively on this technique as applied to moderately-luminous active galactic nuclei (AGN). After describing the foundations and uncertainties of the method, we summarize the current status of the field. At the time of writing, observations by XMM-Newton, Suzaku and NuSTAR have given robust spin constraints on 22 SMBHs. We find a significant number of rapidly-rotating SMBHs (with dimensionless spin parameters a>0.9) although, especially at the higher masses (M>4*10^7Msun), there are also some SMBHs with intermediate spin parameters. This may be giving us our first hint at a mass-dependent spin distribution which would, in turn, provide interesting constraints on models for SMBH growth. We also discuss the recent discovery of relativistic X-ray reverberation which we can use to "echo map" the innermost regions of the accretion disk. The ultimate development of these reverberation techniques, when applied to data from future high-throughput X-ray observatories such as LOFT, ATHENA+, and AXSIO, will permit the measurement of black hole spin by a characterization of strong-field Shapiro delays. We conclude with a brief discussion of other electromagnetic methods that have been attempted or are being developed to constrain SMBH spin.

Invited article accepted for publication in the focus issue on astrophysical black holes in Classical and Quantum Gravity, guest editors: D. Merritt and L. Rezzolla. Minor revisions based on referee report and other feedback

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