India’s first dedicated multi-wavelength space observatory, AstroSat, has been exploring the secrets of the universe since its launch in 2015. One of its main objectives is to study black holes, the mysterious objects that have such a strong gravitational pull that not even light can escape. Black holes are formed by the collapse of massive stars or the merger of neutron stars, and they emit powerful bursts of X-ray radiation that can be detected by AstroSat.
How AstroSat studies black holes
It is equipped with five instruments that can simultaneously observe the same celestial object in different wavelengths, from ultraviolet to X-ray. This gives AstroSat an advantage over other space telescopes that can only observe one wavelength at a time. By combining the data from different instruments, AstroSat can paint a comprehensive picture of the black hole and its surroundings.
One of the instruments on board AstroSat is the Cadmium Zinc Telluride Imager (CZTI), which has detected the birth of a black hole for the 500th time in May 2022. CZTI is sensitive to high-energy X-rays and gamma rays, which are emitted by black holes during their formation or accretion process. CZTI can also measure the polarization of X-rays, which reveals information about the shape and structure of the black hole’s environment.
Another instrument on board AstroSat is the Soft X-ray Telescope (SXT), which can observe lower-energy X-rays from black holes. SXT can measure the temperature and luminosity of the accretion disk, the swirling mass of gas and dust that surrounds the black hole and feeds it. SXT can also detect changes in the brightness and spectrum of the X-ray emission, which indicate variations in the accretion rate and state of the black hole.
AstroSat’s achievements and challenges
It has made several groundbreaking discoveries about black holes using its multi-wavelength capabilities. For example, in 2018, AstroSat observed a transient black hole X-ray binary system called MAXI J1820+070, which became the second brightest object in the X-ray sky during its outburst. AstroSat captured soft and hard X-ray emissions and far ultraviolet radiation from this system, revealing the inner accretion disk’s significant recession from the black hole during the hard state, forming a structured corona with distinct components.
In 2024, AstroSat measured X-ray polarization from a well-known black hole system called Cygnus X-1 for the first time. Cygnus X-1 consists of a stellar-mass black hole and a massive companion star that orbit each other. AstroSat detected polarized high-energy X-rays from this system, indicating that they originated from a jet of plasma that was ejected from near the black hole at relativistic speeds.
AstroSat has also contributed to global efforts to study black holes by collaborating with other space missions and ground-based observatories. For instance, AstroSat participated in a coordinated multi-wavelength campaign to observe a supermassive black hole at the center of a galaxy called M87, which was imaged by the Event Horizon Telescope in 2019. AstroSat provided valuable data on the X-ray emission from this black hole, which helped to understand its properties and behavior.
However, AstroSat also faces some challenges in its mission to study black holes. One of them is the limited lifetime of its instruments, which are expected to degrade over time due to exposure to radiation and cosmic rays. Another challenge is the competition from other space missions that have similar or better capabilities to observe black holes, such as NASA’s NICER and Fermi, or ESA’s upcoming Athena. AstroSat also has to deal with data analysis and dissemination issues, such as ensuring data quality, accessibility, and usability for researchers and students.
Conclusion
Despite these challenges, AstroSat continues to be India’s key to unraveling black hole mysteries. AstroSat has demonstrated its unique potential to study black holes in multiple wavelengths and polarization modes, providing new insights into their formation, evolution, and environment. AstroSat has also showcased India’s scientific excellence and leadership in space astronomy, inspiring future generations of astronomers and space enthusiasts.
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