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Flickering of Visible light from Black Holes
Contrary to light never escaping black holes, flickering visible light observed from V404 Cygni is proposed caused by the frictional heating of nanoparticles and constituent atoms produced at high temperatures from macroscopic debris
Since Hawking, it is generally accepted that anything falling into black holes cannot escape, including light. Recently, however, astronomers have reported  flickers of visible light from V404 Cygni, a black hole of about nine solar masses and a companion star similar to our sun. Located about 7,800 light-years away from Earth, V404 Cygni was dormant, but a recent X-ray outburst made it one of the brightest X-ray sources in the Universe. The visible light fluctuations that followed had periods 100 seconds to 150 minutes. Although astronomers usually study black holes with X-ray or gamma-ray telescopes, the visible light fluctuations allow detection by the amateur astronomer with a 20-cm telescope as illustrated in the thumbnail.
Similar visible light fluctuations were observed in X-ray emissions from black hole system, GRS 1915+105, located about 35,900 light-years away from Earth. Since GRS 1915+105 experiences high levels of accretion, it was thought the light fluctuation was caused due to instabilities that can occur in accretion disks when they get very massive. However, the accretion rates at V404 Cygni are at least 10 times lower than those seen at other black hole systems suggesting high accretion rates are not the source of variable light flickering.
Because of this, the astronomers concluded both V404 Cygni and GRS 1915+105 have companion stars that are relatively far apart, which permits a large accretion disk to form where flow from the outer disk might not be steady suggesting accretion can become unstable and fluctuate wildly, thereby explaining the variable light flickering.
Instead of unstable flow in large accretion disks, the fluctuations of light may be simply explained by how frictional heat produces light from debris in accretion disks. By classical physics, macroscopic debris fragments having heat capacity do increase in temperature under frictional heat allowing thermal emission of visible light and even X-rays, e. g., the 1 keV X-rays require thermal temperatures T > 10 million K. But optical emissions are observed in V404 Cygni at the same time as X-rays. What this means is frictional heat is somehow mediated to not just produce 1 keV X-rays, but rather a broad ranged SED including visible frequencies as measured for V404 Cygni, see  Extended Data, Fig. 6. SED stands for spectral energy distribution.
To produce a broad ranged SED, the frictional heating of macroscopic debris requires high temperatures to form NPs and even higher temperatures to exceed atom binding energies and separate constituent atoms. NPs stand for nanoparticles. Unlike macroscopic debris, NPs and constituent atoms follow QM by the Planck law that requires the heat capacity of the atom to vanish thereby precluding the conservation of frictional heat by an increase in temperature. QM stands for quantum mechanics. Vanishing heat capacity of the atom by QM is not new, but the consequence of the Planck law formulated over a century ago. For heat capacity to vanish, however, the NP must be placed under high EM confinement. In NPs, the EM confinement is natural consequence of their high surface-to-volume ratios that require absorbed heat to be momentarily confined to their surface. Hence, NP atoms are under EM confinement over nanoscale wavelengths that by the Planck law requires their heat capacity to vanish. Since the NP atom temperatures cannot increase by QM, conservation proceeds by QED converting the surface heat into EM waves standing between diametrically opposite NP surfaces. QED stands for quantum electrodynamics.
But QED here differs from the complex light-matter interaction theory advanced by Feynman and others. Simply stated: Absent heat capacity, heat supplied to a NP of diameter d is conserved by creating standing EM waves having half-wavelength λ / 2 = n d, where n is the NP refractive index. Once the surface heat is expended in forming the standing waves, the EM confinement vanishes allowing non-thermal EM radiation to be emitted from optical to X-ray frequencies. Optical light requires high temperatures to reduce macroscopic debris to NPs while X-rays require even higher temperatures to form individual debris atoms. See diverse QED applications at http://www.nanoqed.org, 2010 - 2016.
The X-rays and fluctuations of visible light from V404 Cygni are produced by the frictional heating of macroscopic debris in the accretion disk to form NPs and constituent atoms.
QED induced optical emission occurs from debris NPs having diameters d < 500 nm. But X-rays require the debris to be reduced to bare atoms having d < 1 nm, the most likely debris being gaseous hydrogen common to accretion disks. Hydrogen having atom binding energy of 4.52 eV requires high temperatures to separate the H atoms. Once separated, the heated H-atoms having a diameter d = 106 pm and n = 1 emit 5.8 keV X-rays consistent  with the SED for V404 Cygni.
Variable light flickering in V404 Cygni has nothing to do with unstable flow in large accretion disks. Rather, the visible light is emitted depending on whether the frictional heat is locally available to be absorbed by NPs. If not, the light emission ceases. Similarly, X-ray emission ceases if the H-atoms are not heated. Because of the temporal non-uniformity of debris in the accretion disk, flickering of both X-ray and visible light are observed.
 M. Kimura, et al., "Repetitive patterns in rapid optical variations in the nearby black-hole binary V404 Cygni," Nature, Vol 52, 54-58, 7 January, 2016.