Cycles of activity and dormancy in black holes, including awakening and feedback effects.Black holes are not perpetually active; they cycle through phases of intense accretion and relative dormancy. When matter falls into a black hole, it heats up and emits radiation, making the black hole 'active' — often as an active galactic nucleus or a microquasar. Over time, the supply of nearby matter diminishes, and the black hole becomes quiet, only to be reawakened when fresh material, such as a passing star or gas cloud, comes within its gravitational reach. These activity cycles are crucial for understanding how black holes grow and how they influence their host galaxies.
The idea of episodic black hole activity emerged in the 1970s with the discovery of X-ray binaries and the realization that active galactic nuclei must be powered by recurring accretion events. The term 'activity cycle' became standard in the 1980s as observations of radio galaxies revealed multiple epochs of jet activity.
The cycle is driven by the availability of matter. In the active phase, gas spirals into an accretion disk, where friction and magnetic fields heat it to millions of degrees, producing X-rays and jets. As the disk is consumed or blown away by radiation pressure, accretion slows and the black hole enters a quiescent state. A new influx of matter — from a stellar tidal disruption, a galaxy merger, or instabilities in the interstellar medium — can restart the cycle. Feedback from jets and outflows can also regulate the process by pushing gas away, creating a self-limiting loop.
Studying activity cycles helps astronomers interpret variability in quasars and X-ray binaries, and informs models of galaxy evolution where black hole feedback regulates star formation.
Our own galaxy's supermassive black hole, Sagittarius A*, is currently in a dormant phase, but it shows occasional flares when it consumes small clumps of gas — and it may have been far more active in the past, as evidenced by the Fermi bubbles extending above the galactic plane.
| r | variable | radial coordinate — radial coordinate metre |
| M | variable | black hole mass — mass kilogram |
| a | variable | specific angular momentum (spin parameter) — angular momentum per unit mass metre |
| Q | variable | electric charge — electric charge coulomb |
| r_± | variable | Inner and outer event horizon radii — horizon radius metre |
| Q | variable | Electric charge of the black hole — electric charge coulomb |
| M | variable | Mass of the black hole — mass kilogram |
| G | constant | Newtonian constant of gravitation — gravitational constant = 6.67430e-11 cubic metre per kilogram per second squared |
| c | constant | Speed of light in vacuum — speed of light = 299792458 metre per second |
| k_e | constant | Electrostatic force constant in Coulomb's law — coulomb constant = 8.9875517923e9 newton metre squared per coulomb squared |
| M_BH | variable | Black hole mass — mass kilogram |
| L | variable | Continuum luminosity (e.g., at 5100 Å) — luminosity watt |
| FWHM | variable | Full width at half maximum of emission line — line width metre per second |
| α | variable | Power-law index for luminosity — power law index |
| β | variable | Power-law index for line width — power law index |
| A | variable | Area of the event horizon — area square metre |
| M | variable | Mass of the black hole — mass kilogram |
| a | variable | Specific angular momentum (angular momentum per unit mass) — specific angular momentum metre |
| π | constant | Ratio of a circle's circumference to its diameter — pi = 3.14159 |
| \Delta(r) | variable | Horizon function of the Kerr metric — horizon function square metre |
| r | variable | Radial coordinate (Boyer-Lindquist) — radial coordinate metre |
| M | variable | Black hole mass — mass kilogram |
| a | variable | Specific angular momentum (spin parameter) — specific angular momentum metre |
| r_{\text{ISCO}} | variable | radius of the innermost stable circular orbit — radius metre |
| r_g | variable | gravitational radius (GM/c^2) — radius metre |
| Z_1 | variable | auxiliary function of spin parameter a |
| Z_2 | variable | auxiliary function of spin parameter a |
| \mp | operator | indicates two possible solutions depending on orbital direction — minus plus sign |
| \sqrt{} | operator | takes the square root of the argument — square root |
