Mergers of black holes

Mergers of black holes

Subject – Science and Tech

Context – Mergers of black holes and ‘kicks’ that hold a key to puzzles

Concept –

• Black holes form when a massive star undergoes a supernova explosion towards the end of its lifetime.
• The black hole forms from the remnants of the explosion. However, there are factors that place limits on the mass of a black hole so formed.
• Black holes with masses between approximately 45-135 times the solar mass are unlikely to be produced by standard stellar evolution as the pair-instability process either limits the max mass of the black hole or completely disrupts the star during the supernova explosion.
• The two detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) made the first observation of a pair of binary black holes on September 19, 2014.
• Since then, with other gravitational wave observatories about 40 mergers have been detected, of which nearly five have masses above 100 times solar mass.
• One of the theories of intermediate mass black hole formation has to do with ‘hierarchical growth’.
  • That is, if the black holes exist among a dense cluster of stars, the remnant (black hole) of a merger can pair up with another black hole close by to form a binary.
  • This can eventually merge to form a second remnant which is more massive. This process, happening in a hierarchical manner, can explain intermediate mass black hole formation.
• During the mergers, gravitational waves take away energy and linear momentum, as a reaction, the remnant black hole acquires an opposite momentum. This is the “kick” it receives.
  • These kicks can be quite large, giving it a velocity of up to 1000 kilometres per second.
  • If this kick velocity is above the escape velocity of the star cluster in which the black hole is formed, it literally escapes from the environment and moves out.
  • This prevents it from undergoing further hierarchical mergers.
  • The extent of the kick received by the remnant can be calculated from the masses of the merging black holes and their spin
  • The kick estimates help understand which mergers have the possibility of undergoing further hierarchical mergers and forming into intermediate mass black holes

To know more about Black Holes, please click here.

To know about LIGO and VIRGO, please click here.