The Future of Black Hole Exploration
By Hana Mohammed K
Cosmofluencer (Season 2)
Introduction
The discovery of black holes emerged from an idea centuries ago. In 1784, English astronomer John Michell proposed an idea of the existence of a big body in the universe from which even light couldn’t escape. He referred to these bodies as dark stars. In 1915, Albert Einstein came up with the General Relativity Theory which explored the relationship between space and time. This explains how space and time work with respect to gravity. This theory also gave a wider knowledge about the “Special Relativity Theory” proposed by him earlier. It stated that space and time are connected.
According to General Relativity, spacetime is four dimensional. This theory gives a clearer explanation for the existence of black holes. Black holes are the region of spacetime. The gravity is strong there and no light can pass through it. Although these theories explained black holes, science today is more focused on finding the proofs to support them.
In 2009, an international collaboration was launched to build an instrument to prove the Black Hole Theory using telescopes. But normal telescopes are not powerful enough to detect the presence of black holes. The project aimed to create a virtual earth-sized telescope array. This has a group of telescopes synchronized with the radio observatories around the world. Long radio waves with a frequency of 230 GHz, 86 GHz and 2.3+8.7 GHz were used in this telescope array to detect the black holes. This telescope is known as the Event Horizon Telescope (EHT)
In 2019, the EHT was able to detect the presence of a supermassive blackhole in the centre of galaxy Messier 87 (M87) which is 33 billion times more massive than the Sun. Since light cannot pass through the blackhole, the telescope was not able to give an image of the entire blackhole as the theory stated. But the accretion disk of the black hole was filled with gaseous or plasma matter, and very hot stars were revolving around the black hole emitting light, which was detected by the EHT. The image shows a black center with a hot shining light revolving around it. The inside portion is black since light cannot pass through the black hole.
In 2021, NASA, ESA and CSA launched the latest powerful infrared telescope known as James Webb Space Telescope (JWST) to detect the most distant active supermassive black hole about 13.2 billion light years away. As the technologies and developments are evolving in the field of science, new discoveries to detect the presence of black holes are becoming more and more successful.
The Electromagnetic Spectrum and the Black Hole
As capturing an image of a black hole is not possible due to the nature of light, the electromagnetic waves of light cannot help us in this situation But as studying and finding details about black holes is important for the people to explore in the future, the brighter and sharper image of the detection of the black holes is critical for Astronomy.
Therefore, finding the most suitable area of the electromagnetic spectrum which can give out the best results of black holes is essential. The electromagnetic spectrum of light contains seven different types of waves with different frequency and wavelength. They are: radio waves, microwaves, infra- red light, visible light, ultraviolet light, X-rays and gamma rays.
Radio waves are used in the Event Horizon Telescope to detect black holes. They produced the first image of a supermassive black hole in the galaxy Messier 87.
For microwaves, physicists Qiang Cheng and Tie Jun Cui of the State Key Laboratory of Millimeter Waves at Southeast University in Nanjing, China, invented an absorbing device known as an “omnidirectional electromagnetic absorber”. This device, made of a thin cylinder comprising 60 concentric rings of metamaterials, is capable of absorbing microwave radiation and acts as an astrophysical black hole.
For the visible light spectrum, astronomers in 2022 reported that a mysterious and ferociously bright flash of visible light that came towards Earth originated from a black hole that is directly pointing at us. These were detected using the long radio wave telescopes. In 1997, NASA’s Hubble Space Telescope provided a never-before-seen view of a warped disk flooded with a torrent of ultraviolet light from hot gas trapped around a suspected massive black hole. The Chandra X-ray Telescope was able to make a remarkable discovery of the detection of emission of X-rays from the matter outside of the black hole in Sagittarius A*. As all the electromagnetic waves did bring out different results while approaching the black hole, the real deal here is about obtaining the most precise image of the matter around the black hole. Here, only the long radio waves of EHT, the infrared rays in JWST and the detection of X-ray emission by Chandra X-ray Telescope gave us the hope for our goal.
The discovery of X-ray emission from the matter surrounding the black hole has created a big impact in the future exploration of black holes other than long radio waves. Chandra has studied the galaxy M87 many times during its 20-year mission and sees a much wider field-of-view than the EHT. This satellite was able to separate the emissions from the hot gas and dust surrounding the black hole. It was able to detect the variations in the emission of X-rays and came up with a conclusion that it is occurring near the event horizon of the black hole.
The low intensity of the X-ray emission showed that the blackhole was a starving one. One of the major recent studies from an international collaboration between Japan and Sweden stated that using X-rays they were able to find that gravity affects the shape of matter near the black hole in the binary system Cygnus X-1. This black hole is also one of the brightest sources of X-rays in the sky. The team behind the research found this using a new technique known as X-ray Polarimetry. The light from the matter is detected due to a star that closely orbits the black hole.
Polarization filters light so that it vibrates in one direction. This is how the X-rays around a black hole behave. But it was found that the X-rays and some gamma rays coming from the black hole did penetrate this filter. The team launched an X-ray Polarimeter known as PoGO+. They needed to understand the source of light and its scattering property. At the end, the fractions of hard X-rays reflected off the accretion disk were joined together to find the shape of the matter.
The Cygnus X-1 is a black hole in a binary system. Therefore, matter can exist in two ways near the black hole. The corona of a black hole is an area of hot plasma close to a black hole. The Lamp Post Geometry Model aims to provide a simple explanation for the origin of X-ray power-law continuum and the relative spectral features found in the blackhole. This model has a compact corona bounded closely to the photons of the blackhole. It bends towards the accretion disk. This results in more reflected light. The extended model has a larger corona and it is spread around the vicinity of the black hole. The reflected light from the disk is very less here. Since the light doesn’t bend much under the strong gravity of the black hole, the extended model is the most suitable one. This study using the X-ray Polarimeter resulted in a way for Astrophysicists to understand two major characteristics of black holes: the spin, and the evolution of the black hole.
In a groundbreaking exploration of X-ray Polarimetry studies about black holes, the recent launched X-ray Polarimetry satellite of ISRO called XPoSat for studying cosmic X-rays from black holes, and also the first X-ray Polarimetry satellite in the world launched by NASA in 2022 called Imaging X-ray Polarimetry Explorer (IXPE), shows the major benefits of using X-rays in the electromagnetic spectrum of light in the future.
Conclusion
The various analyses related to the study of black holes with respect to the electromagnetic spectrum have been able to prove that radio waves used in the Event Horizon Telescope (EHT) and the X-ray Polarimeter research study have opened up major opportunities to explore black holes. The X-ray Polarimeter study was able to find information about the two major characteristics of black holes, the spin and its evolution. By comparison, the EHT gives an image 4000 times sharper than the JWST. This shows that radio waves are more powerful in exploring the black hole than infrared rays.
We can conclude that the future exploration of black holes by the space organisations and astrophysicists should be more focused on unfolding the secrets of the black holes using the X-rays and radio waves. This can pave a new way of understanding the principle and the secrets of the blackholes in the future.
References
- X-ray technology reveals never-before-seen matter around black hole | Science Daily
- Black Holes | Wikipedia
- Researchers create portable black hole | Nature.com
- Polarization in lamp-post model of black-hole accretion discs | M Dovčiak et al 2012 J. Phys.: Conf. Ser. 372 012056