Black Holes in Astrophysics

TL;DR

Problem: establish robust observational evidence for astrophysical black holes and their horizons across mass scales. \nApproach: summarize mass measurements via dynamical methods (e.g., mass function $f(M)$, Newtonian/Kerr spacetime ISCO concepts) and spin inferences from spectral fitting, QPOs, and relativistic Fe lines, plus energy-transport diagnostics like ADAFs and GRMHD jet models. \nKey findings: BH candidates span $M \sim ${\rm few}$–$20 M_\odot$ in XRBs and $M \sim 10^6$–$10^{9.5} M_\odot$ in galactic nuclei; mean radiative efficiencies $\eta$ imply substantial spin; strong circumstantial evidence for event horizons comes from quiescent luminosities, absence of a boundary layer and Type I bursts, and prospects for shadow imaging. \nSignificance: provides tests of general relativity in the strong-gravity regime and clarifies energy-extraction mechanisms (e.g., magnetic fields in the ergosphere) that power relativistic jets and AGN.

Abstract

This article reviews the current status of black hole astrophysics, focusing on topics of interest to a physics audience. Astronomers have discovered dozens of compact objects with masses greater than 3 solar masses, the likely maximum mass of a neutron star. These objects are identified as black hole candidates. Some of the candidates have masses of 5 to 20 solar masses and are found in X-ray binaries, while the rest have masses from a million to a billion solar masses and are found in galactic nuclei. A variety of methods are being tried to estimate the spin parameters of the candidate black holes. There is strong circumstantial evidence that many of the objects have event horizons. Recent MHD simulations of magnetized plasma accreting on rotating black holes seem to hint that relativistic jets may be produced by a magnetic analog of the Penrose process.

Figures (2)

• Figure 1: The radius of the innermost stable circular orbit $R_{\rm ISCO}$, the Keplerian frequency at this radius $\Omega_{\rm K,ISCO}$, and the binding energy at this radius $\eta$, as functions of the BH spin parameter $a_*$. Positive values of $a_*$ imply that the BH corotates with the orbit and negative values mean that the BH counter-rotates. By measuring the quantity $R_{\rm ISCO}/M$ or $\Omega_{\rm K,ISCO}M$ or $\eta$, one could estimate $a_*$.
• Figure 2: Quiescent luminosities $L_{\rm min}$ of X-ray novae plotted against the orbital period $P_{\rm orb}$ of the binaries. The filled symbols correspond to BH candidates, the open symbols to NSs, and arrows represent upper limits. The shaded bands are to guide the eye. At any given orbital period, the NSs as a group are a factor of $\sim100$ brighter than the BH candidates. This difference may be interpreted as evidence that the BH candidates possess event horizons. (Taken from McClintock et al. 2004)