Tidal Disruption Events

In the case a star, or stellar system, enters the ‘sphere of influence’ of a massive black hole (MBH), it will experience a gravitational pull from which it will be unable to escape for the MBH will be dominating the dynamics of all objects in its vicinity. As such, the star will have crossed the critical distance from the black hole and will be subject to disruption. This distance is called the tidal radius.

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Image credit: NASA/CXC/U. Michigan/J. Miller et al.; Illustration: NASA/CXC/M. Weiss

However, within this tidal radius, due to the extremely large size of the bodies interacting and the immense gravitational pull, the force felt across the star cannot be considered uniform from one end to the other. As such, it is experiencing what we call a tidal force, which will not pull the whole stellar body towards the black hole, but rather stretch it gradually until eventually, the star’s surface situated closer to the black hole will start getting ripped apart from its main body.

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In some extreme cases, the tidal force is so strong, that the stellar body gradually gets stretched out horizontally, and compressed vertically into a long thin object, a process known as spaghettification.

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An illustration of spaghettification (Image: Black Hole Cam)

When these tidal disruption events, or TDEs, occur, the mass ripped away from the stellar body starts to accumulate around the black hole. This is referred to as an accretion process, where the mass starts to form a disk, an accretion disk, whose mass, and hence gravitational pull, causes even more matter from the star to get pulled away. As this diffuse material starts spiraling inwards towards the black hole, the temperature of this often gaseous matter is raised due to increased friction and gravitational pull, causing a flare, an eruption, of electromagnetic radiation.

Speculations and discovery

The first mentions of TDEs like events came from physicist John Wheeler who speculated that the gas released by a star breaking up in the vicinity of a black hole could be accelerated to relativistic speeds. However, his models restricted themselves to phenomena of small tidal amplitudes. Later on, in 1976, the idea of a critical radius under which stars would be disrupted by the tidal forces generated by the mass of the black hole was introduced by two astronomers from the Cambridge Institute of Astronomy. However, still, no concrete model had been introduced.

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It was in the 1980s at the Paris Observatory, that the first mention of TDE like events was made. But only a decade later were the first TDE candidates detected by the ROSAT satellite confirming the theory that the accretion of stellar debris could result in massive eruptions: the concretization of tidal disruption events.

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The Paris Observatory

To this date, all tidal disruption events are listed under ‘The Open TDE Catalog’ moderated by the Harvard Center for Astrophysics and whose latest addition was an event recorded in July 2020 located near the heart of the galaxy NGC 6297. There are 93 entries since 1999.

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