Sudden increases in activity and emission from previously quiescent black holes.A black hole outburst is a sudden, dramatic increase in the emission of radiation from a black hole that was previously quiet or dormant. These events can occur across the electromagnetic spectrum, from radio waves to X-rays and gamma rays, and are often triggered by the sudden infall of matter, such as a star being torn apart by tidal forces or an instability in the accretion disk. Outbursts provide a unique window into the physics of black holes, allowing astronomers to study how matter behaves under extreme gravity and how energy is released during accretion episodes.
The concept of black hole outbursts emerged in the 1970s with the discovery of X-ray binaries like Cygnus X-1, but the term became widely used after observations of the first tidal disruption events in the 1990s. The specific phenomenon of a black hole 'waking up' after a long period of silence was highlighted by the 2018 observation of the supermassive black hole in the galaxy SDSS1335+0728, which began flaring after 25 years of inactivity.
The outburst typically begins when a reservoir of gas, often from a companion star or a passing cloud, is disrupted and starts spiraling toward the black hole. As the material falls inward, it forms a hot, rapidly rotating accretion disk. Friction and magnetic stresses within the disk heat the gas to millions of degrees, causing it to emit intense X-rays and other radiation. In some cases, part of the infalling matter is redirected into relativistic jets that shoot out from the black hole's poles, producing bright radio and gamma-ray emission. The sudden onset of these processes marks the outburst, which can last from weeks to years before the system returns to quiescence.
Outbursts are used to probe the demographics of black holes, including their masses and spin rates, and to test models of accretion disk physics. They also serve as natural laboratories for studying relativistic jet formation and particle acceleration.
The 2018 outburst of a supermassive black hole in SDSS1335+0728 was so powerful that it increased the galaxy's ultraviolet and X-ray brightness by factors of 4 and 10, respectively, and the emission is still ongoing years later.


















