Black holes can produce oscillating outbursts of radiation that were thought to be associated with high rates of infalling matter. The observation of pulses of visible light from a black hole complicates this picture. See Letter p.54
Accretion of matter onto black holes is an efficient way of converting mass into energy, much more so than the process of nuclear fusion, which powers the light from stars. But unlike fusion, the physics behind accretion is still not understood, more than 40 years after the identification of accreting black holes in the Milky Way1,2. On page 54 of this issue, Kimura et al.3 present exquisite observations made during a black-hole accretion episode. They show that the visible radiation from the black hole's vicinity oscillates dramatically — sometimes regularly, other times not — in a manner not predicted by models. Such oscillations were previously associated with high rates of infalling matter, but the authors report that the observed oscillations can occur even when the rate of infall is low. Understanding this behaviour could help astronomers to better understand violent accretion episodes onto black holes.
The researchers studied the black hole V404 Cygni, which is 2.4 kiloparsecs from Earth. The Cygnus constellation is a popular area of the sky for black-hole specialists because it hosts several other bright, accreting black holes and neutron stars. In June 2015, V404 Cygni underwent a short-lived accretion 'outburst' that lasted for about two weeks, causing it briefly to become one of the brightest cosmic X-ray sources beyond the Solar System. The black hole's gravity is strong enough to strip matter off the surface of an orbiting companion star, and the potential energy of this infalling matter is released, in part, as the observed electromagnetic radiation.
The infalling matter is thought to be hot, magnetized plasma. But if this material were to plunge directly into the black hole, its energy would be lost immediately without any brightening. The standard picture of accretion is that the plasma instead acts as a viscous fluid that spirals towards the black hole in the form of a disk, and that its energy is liberated as a result of friction in the disk. Any plasma that cannot be accreted is expelled in the form of a fast narrow stream (a jet) or as an outflowing wind.
If there is a balance between the accreting plasma and frictional energy losses, then the mass is steadily accreted. But naturally occurring changes in the rate of mass accretion can upset this balance and cause an unstable see-saw-like behaviour: periods of enhanced accretion that empty parts of the disk are followed by quieter periods when the parts are refilled, after which the cycle begins again. An approximate analogy is the repetitive filling and emptying of a Japanese bamboo fountain.
Such behaviour has been observed in one other black-hole system, GRS 1915+105 in the Aquila constellation, which undergoes high levels of mass accretion. Several classes of repetitive oscillation occur in this system, but only in its observed X-ray emission4. Kimura and collaborators draw parallels between GRS 1915+105 and the visible-light oscillations in V404 Cygni, but make the crucial distinction that the latter oscillations occur at a much lower rate of mass accretion than the former ones. In other words, the repetitive behaviour is not strictly associated with episodes of high mass accretion.
V404 Cygni is an important study target for several reasons. It was the first Galactic object to have its mass (nine solar masses) firmly placed within the range of masses associated with black holes5,6. Its distance from Earth is also known with higher accuracy than those of other black holes7. Moreover, it looks extremely bright when it accretes matter, despite being partly veiled behind interstellar gas and dust. In the absence of this veil, V404 Cygni would have been one of the most distant objects in the Milky Way visible in dark skies to the unaided eye in June 2015. Because V404 Cygni is so well characterized, Kimura et al. are able to propose a mechanism to explain the visible-light oscillations.
The authors suggest that in systems such as V404 Cygni and GRS 1915+105 there is a relatively large volume of space between the companion star and the black hole, which allows a large disk to form. But the supply of infalling matter from the companion star is insufficient to fill such a large disk with a steady flow. Without a steady flow, the accretion rate becomes unstable and can fluctuate violently (Fig. 1). These fluctuations, in turn, trigger oscillating emissions of energetic X-ray photons near the black hole, which then light up the whole disk with the observed pulsating visible effects.
But the authors show that this explanation requires the disk to be very large, close to its maximum possible size. Moreover, the X-ray oscillations that they observed from V404 Cygni are much stronger than the visible-light ones. These puzzling facts will need to be accounted for. How, and whether, the jet of the black hole tracks these oscillations is also yet to be determined. The proposed parallels between the observed oscillations and those of GRS 1915+105 will undoubtedly be investigated in detail in the future. This will help researchers to understand the above issues in light of the wealth of supporting observations currently being analysed by astronomers the world over.
Black-hole outbursts are unpredictable and some can be two weeks or even shorter in duration, so worldwide coordination and round-the-clock monitoring is essential if we are to understand the physics of these extreme events. This becomes particularly challenging when coordinating observations between space telescopes and those on the ground. The outburst of V404 Cygni last year invigorated the efforts of black-hole astronomers to tackle these challenges, with at least one conference dedicated entirely to this theme. Amateurs can also play a key part in this effort. Kimura and colleagues gathered data from many small telescopes, some with optical elements only 20 centimetres in diameter, showing that, in astronomy, size is not necessarily what matters; collaboration does.Footnote 1
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