Fluorescent iron lines as a probe of astrophysical black hole systems

By Christopher S.Reynolds, Michael A.Nowak. 2002

272 citations recorded by the source corpus.

Abstract With most physicists and astrophysicists in agreement that black holes do indeed exist, the focus of astrophysical black hole research has shifted to the detailed properties of these systems. Nature has provided us with an extremely useful probe of the region very close to an accreting black hole—X-ray irradiation of relatively cold material in the vicinity of the black hole can imprint characteristic features into the X-ray spectra of black hole systems, most notably the Kα fluorescent line of iron. Detailed X-ray spectroscopy of these features can be used to study Doppler and gravitational redshifts, thereby providing key information on the location and kinematics of the cold material. This is a powerful tool that allows us to probe within a few gravitational radii, or less, of the event horizon. Here, we present a comprehensive review of relativistic iron line studies for both accreting stellar mass black holes (i.e., galactic black hole candidate systems, GBHCs), and accreting supermassive black holes (i.e., active galactic nuclei, AGN). We begin with a pedagogical introduction to astrophysical black holes, GBHCs, AGN, and accretion disks (including a brief discussion of recent work on the magnetohydrodynamical properties of accretion disks). We then discuss studies of relativistic iron lines in the AGN context, and show how differences between classes of AGN can be diagnosed using X-ray spectroscopy. Furthermore, through a detailed discussion of one particular object (MCG–6-30-15), we illustrate how the exotic physics of black hole spin, such as the Penrose and Blandford-Znajek processes, are now open to observational study. We proceed to discuss GBHCs, which turn out to possess rather more complicated X-ray spectra, making robust conclusions more difficult to draw. However, even in these cases, modern X-ray observatories are now providing convincing evidence for relativistic effects. We conclude by discussing the science that can be addressed by future X-ray observatories.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/59cc38181e0a0f4a1f80771d2e7ce4a050171f53

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Citations in the corpus

X-Ray Properties of Black-Hole Binaries (2006, methodology): “Relativistic beaming and gravitational redshifts in the inner disk region can serve to create an asymmetric line profile (for a review, see Reynolds & Nowak 2003).”
Constraining Black Hole Spin via X-Ray Spectroscopy (2006, result): “The line profiles shown in this figure have the same parameter settings as those shown in the review by Reynolds & Nowak (2003) to enable easy comparison.”
Irradiation of an accretion disc by a jet: general properties and implications for spin measurements of black holes (2013, background): “Such effects were first seen in the skew symmetric shape of the fluorescent Fe Kα line from these objects (Tanaka et al. 1995; Reynolds & Nowak 2003, and references therein).”
Black holes in Einstein-aether and Hořava-Lifshitz gravity (2011, background): “The deviation of this quantity away from its GR value (3/ √ 2 − 1), together with the value of the combination ω ISCO rg mentioned above, is a measure of how easily a black hole in æ-theory (or HL gravity) could be distinguished from a Schwarzschild black hole using iron-Kα lines [38, 39] (see also ”
Suzaku Observations of the Hard X-ray Variability of MCG-6-30-15: the Effects of Strong Gravity Around a Kerr Black Hole (2006, background): “The low energy extent of the line can reveal the inner radius of the accretion disc and thus the black hole spin (for reviews see Fabian et al 2000; Reynolds & Nowak 2003 and Fabian & Miniutti 2006).”
The Masses and Spins of Neutron Stars and Stellar-Mass Black Holes (2014, methodology): “[318] M. A. Nowak, R. V. Wagoner, M. C. Begelman, D. E. Lehr, The 67 HZ Feature in the Black Hole Candidate GRS 1915+105 as a Possible “Diskoseismic” Mode, ApJL 477 (1997) L91.”
THE SIZES OF THE X-RAY AND OPTICAL EMISSION REGIONS OF RXJ 1131–1231 (2009, background): “…continuum emission is due to unsaturated inverse Compton scattering of soft photons by hot electronsin a corona surrounding the inner parts of the accretion disk (see the review by Reynolds & Nowak 2003), but the extent and geometrical configuration of the X-ray emission region is an open question.”
A long hard look at MCG–6-30-15 with XMM-Newton (2002, unknown): “The broad Fe Kα line therefore potentially offers a powerful diagnostic of the physical conditions in the immediate environment of the black hole (see Fabian et al. 2000; Nandra 2001and Reynolds & Nowak 2003 for recent reviews).”
An Extended Scheme for Fitting X-Ray Data with Accretion Disk Spectra in the Strong Gravity Regime (2004, unknown): “For a recent review for AGNs, see Fabian et al. (2000), Reynolds & Nowak (2003), and references cited therein.”
Alignment of the Spins of Supermassive Black Holes Prior to Coalescence (2007, background): “X-ray observations of several active galactic nuclei reveal relativistically broadened Fe Kα fluorescence lines indicative of spins â > 0.9 (Iwasawa et al. 1996; Fabian et al. 2002; Reynolds & Nowak 2003; Brenneman & Reynolds 2006).”
Measuring the spins of accreting black holes (2011, methodology): “In the Fe Kα method, one determines rISCO by modeling the profile of the broad and skewed iron line, which is formed in the inner disk by Doppler effects, light bending, relativistic beaming, and gravitational redshift (Fabian et al 2000, Reynolds and Nowak 2003, Miller 2007).”
Intermediate - mass black holes (2003, background): “…stable circular orbit, as might interaction with the spin of the black hole: for theoretical work see Krolik (1999); Gammie (1999); Agol & Krolik (2000); Reynolds & Armitage (2001), and for possible observational evidence see Wilms et al. (2001); Miller et al. (2002b); Reynolds & Nowak (2003).”
Observational evidence for a correlation between jet power and black hole spin (2011, unknown): “In addition to the CF method, a second method based on fitting the profile of the relativistically broadened Fe Kα line has been used to estimate BH spins (Reynolds & Nowak 2003; Miller 2007).”
Black Hole Spin Evolution (2003, background): “Some SMBHs and GBHCs show broad, skewed Fe K lines, for example in MCG 6-30-15 (Tanaka et al. 1995; Fabian et al. 2002), Cyg X-1 (Miller et al. 2002b), and XTE J1650 500 (Miller et al. 2002a; for a review see Reynolds & Nowak 2003).”