Can we make black holes reveal themselves in echoes of light?

Context: The effects black holes have
on their surroundings are crucial to determining the structures of the galaxies
they occupy and how the stars around them evolved over time, the mass and
radius of a black hole impact the way light behaves. The study claims that
light echoes can be used as a new way to determine the masses and spins of
black holes. As researchers collected images of light travelling around the M87
black hole, they estimated the time light took to travel from the near end to
its far end, which depends on its mass and momentum. When light passes around a
black hole, its path bends. So some parts of the light take a direct route to
the viewer while others pass around the black hole a few times before getting
back on the original path. In this way, light emitted by a distant source in
the cosmos may reach the earth at different instances, creating light echoes.

Key points

·      
Black holes: Black
holes are extremely dense points in space that create deep gravity sinks from
which even light cannot escape. It can be formed by the death of a massive
star. A black hole takes up zero space but does have mass, that used to be a
star. And black holes get more massive as they consume matter near them. The
concept was given by Albert Einstein in 1915 but the term ‘black hole’ was
coined in the 1960s by American physicist John Archibald Wheeler.

·      
Categories of black holes: One
ranges between a few solar masses and tens of solar masses. These are thought
to form when massive stars die. The other is supermassive black holes. These
range from hundreds of thousands to billions of times that of the sun from the
Solar system to which Earth belongs.

Ø 
Supermassive black holes - At
the centre of most galaxies including our Milky Way, there is a supermassive
black hole. Sometimes these supermassive black holes collect disks of gas,
dust, and stellar debris around them- when they fall into the black hole. Its
gravitational energy can be converted into light.

·      
Observations on black holes: April
2019 - The scientists at the Event Horizon Telescope project released the
first-ever image of a black hole.

January
2021 - The Indian astronomers of ARIES institute reported
one of the strongest flares from a blazer called BL Lacertae from 10 million
light-years away.

August
2021 - Indian scientists have discovered the merger of three
supermassive black holes from as many galaxies to form a triple Active Galactic
Nucleus. This suggests that such group mergers also drive the growth of black
holes.

·      
Black hole triple system: A
black hole triple system is a rare and complex astronomical setup in which
three black holes orbit each other in a mutual gravitational dance. These
systems can form in several ways, such as through galaxy mergers, which lead to
the interaction and eventual binding of the supermassive black holes at the
centers of the merging galaxies.

Ø 
Hierarchical Structure -
Triple black hole systems often exhibit a hierarchical structure where two of
the black holes form a close binary, and the third orbits this binary at a
greater distance. This setup allows the outer black hole to exert periodic
gravitational influences on the inner binary, leading to unique dynamics like
the Lidov-Kozai effect, where the orbits oscillate in eccentricity and
inclination.

Ø 
Gravitational Wave Emissions -
The close interactions and gravitational pulls in a black hole triple system
can accelerate mergers and increase the production of gravitational waves.
These emissions offer valuable data for observatories like LIGO and Virgo,
which detect such waves and help researchers understand black hole dynamics in
multiple systems.

Ø 
Potential for High-energy Events -
Interactions among black holes in these systems can generate intense energy
outputs, including relativistic jets and high-energy radiation. These
interactions also provide clues about galaxy formation and the conditions in
dense star clusters.

·      
V404 Cygni System: The
V404 Cygni system is a well-known binary star system that consists of a black
hole and a companion star. It lies in the constellation Cygnus, approximately
7,800 light-years from Earth.

Ø 
Binary System with a Black Hole -
The system consists of a stellar-mass black hole with a mass estimated at
around 9-12 times that of the Sun and a companion star, likely a G or K-type
star. The companion star orbits the black hole every 6.5 days.

Ø 
Relativistic Jets -
The black hole occasionally ejects relativistic jets of particles during
outbursts. These jets are believed to form when matter near the event horizon
interacts with magnetic fields, resulting in streams of particles that are shot
into space at nearly the speed of light.

Ø 
Significance for Black Hole Studies -
V404 Cygni’s behaviour offers clues about black hole accretion mechanisms and
jet formation. Unlike many black hole systems, V404 Cygni’s accretion disk
shows extreme variability, which has helped scientists explore how accretion
dynamics can vary in response to environmental changes.