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<accretion-disk tests of strong-field gravity> put radiating matter in a strong gravitational field at relativistic orbital speeds. <general relativity> predicts an <innermost stable circular orbit>, $r_{\rm ISCO}=6GM/c^2$ for a nonrotating <black hole>, with a spin-dependent radius in the <Kerr metric>. If the radiating inner edge tracks that orbit, its fitted radius and binding-energy release probe the geometry. For a thin disk with negligible inner-edge torque, $L\simeq\eta\dot Mc^2$; the Schwarzschild efficiency is $\eta=1-\sqrt{8/9}\simeq0.057$.
In <relativistic iron-line spectroscopy>, a fluorescent iron line supplies a rest-energy reference. The <relativistic Doppler effect> from orbital motion, <relativistic beaming>, <gravitational redshift> and ray bending produce an asymmetric broadened profile, including a red wing from the deepest emitting radii. The ASCA observation of MCG-6-30-15 showed a broad asymmetric iron feature relative to the approximately $6.4\ \mathrm{keV}$ neutral-iron <energy>, interpreted through relativistic disk-line models.<ASCA: MCG-6-30-15 by NASA HEASARC>[Mission result description: broad iron line, low-energy asymmetry and relativistic interpretation]{ref} The observed line must be separated from the illuminating continuum, <absorption> and other <reflection> features; inclination, ionization and the radial illumination law affect the inferred radius.
<thermal-continuum fitting of an accretion disk> provides an independent method for a geometrically thin, optically thick disk. Its spectrum and normalization constrain a characteristic inner area, provided <mass>, distance, inclination and atmospheric spectral hardening are controlled. A pre-2013 application selected thermal-state observations of GRS 1915+105 at relatively low disk <luminosity> to make the thin-disk approximation appropriate.<The Spin of the Near-Extreme Kerr Black Hole GRS 1915+105 by Jeffrey E. McClintock and others>[Abstract: relativistic continuum fit, thermal-state selection, luminosity cut and inner-edge assumptions]{ref}
\b[Consistency among inner radius, line transfer and thermal emission tests the strong-field disk picture.] A <black-hole spin> estimate obtained by assuming a <Kerr metric> is not by itself a model-independent verification of <general relativity>. Stress across the nominal inner edge, finite disk thickness, coronal geometry, calibration and competing <absorption> models must be constrained; a gravity test compares alternative geometries while allowing for those astrophysical uncertainties. An emitting material surface or boundary layer can further complicate disks around <neutron stars>.
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