In a study recently accepted in The Astrophysical Journal, an international team of scientists has utilized AstroSat’s full multi-wavelength capability, featuring all four co-aligned payloads, to paint a comprehensive portrait of an X-ray binary system hosting a black hole. By capturing soft and hard X-ray emissions with its three X-ray payloads and far ultraviolet radiation with its UV telescope, AstroSat has unveiled a treasure trove of insights into both the near and distant regions surrounding the black hole in the X-ray binary system MAXI J1820+070. Additionally, optical data from Las Cumbres Observatory and soft X-ray data from NASA’s NICER mission further bolstered AstroSat’s findings. This study marks a pivotal achievement in the history of AstroSat as the first instance where its full multi-wavelength capabilities have been harnessed. The collaborative team includes researchers from India, the United Kingdom, Abu Dhabi, and Poland.

Black hole X-ray binaries are composed of a stellar-mass black hole and a companion star engaged in intricate gravitational interaction. The black hole, a gravitational monster, exerts an irresistible pull on its luminous counterpart, drawing in vast amounts of stellar material. As this material spirals towards the black hole, it releases an outpouring of energy, predominantly in X-rays but at other wavelengths too. Some of these systems, known as 'transients', remain dormant and under-luminous in X-rays for most of their lifespan, becoming episodically detectable during an 'outburst' phase. MAXI J1820+070, located approximately 9800 light-years away from us, is one such transient black hole X-ray binary, discovered during its outburst in 2018 with the MAXI instrument aboard the International Space Station (ISS). Due to its relatively close proximity to Earth and extreme brightness during its discovery (emerging as the second brightest object in the X-ray sky), it captured widespread attention in the astronomy community, prompting several observing campaigns across different electromagnetic bands. Figure 1 shows the location of MAXI J1820+070 above the Galactic Plane and the inset shows an X-ray image acquired with the Chandra X-ray observatory.

An international team of astronomers have been successful in detecting ionizing photons from a rare type of galaxies known as Lyman Continuum Leakers using the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat. “Detecting ionizing UV radiation from such galaxies is extremely challenging and was possible only because of the unique capabilities and high sensitivity of UVIT”, said Suraj Dhiwar, the lead author of the research published in 'The Astrophysical Journal Letters'.

Within the first billion years after the Big Bang, our Universe went through a major phase transition known as reionization, a process in which neutral hydrogen atoms dissociated into protons and electrons when they are struck by high energy UV radiation below wavelengths 912 Å known as Lyman Continuum emission. Understanding the cosmic reionization and the sources responsible for this process remains one of the frontier problems in astronomy.

“The Lyman continuum emission can be easily absorbed or scattered by the interstellar medium or the cir-cumgalactic medium of their host galaxies. Even when some of these ionizing photons manage to come out of the galaxy’s environment, they maybe absorbed by the vast intergalactic medium between us and the galaxy. This is what makes their discovery a rare event in astrophysics. Thanks to UVIT’s resolution and sensitivity that allowed us to create UV deep field in the far-ultraviolet filter.” says Prof. Kanak Saha.

In the current discovery, astronomers detected 10 Lyman continuum emitting galaxies from the peak era of cosmic star formation history, making it the first coherent sample of Lyman continuum leakers at this epoch. More interestingly, these Lyman Continuum photons have wavelength ~600 Å, falling in the Extreme Ultraviolet regime, the shortest ultraviolet wavelength with which a galaxy has been imaged so far. These galaxies are about 8 - 9 billion light years away from the Earth and have intense star formation rates, with some of them forming massive young stars at a rate 100 times higher than our Milky Way Galaxy. Prof. Rogier Windhorst said “the discovery would fill an important niche in understanding the evolution of these rare objects, which are at an epoch when the star formation was at its peak in the cosmic star formation history.”

Beside the UV observation from Astrosat, the Hubble Space Telescope was used to obtain the optical/infrared imaging and spectroscopy for these rare galaxies. The research utilized the deep ultraviolet observations made by Prof. Kanak Saha in 2018 using UVIT onboard Astrosat and was funded by the Indian Space Research Organisation (ISRO). The team of researchers includes Suraj Dhiwar, a research scholar at Inter-University Centre for Astronomy and Astrophysics (IUCAA) and Ph.D. student at the Savitribai Phule Pune University, Prof. Kanak Saha, Soumil Maulick and Dr. Chayan Mondal from IUCAA, Dr. Brent Smith and Prof. Rogier Windhorst from Arizona State University, USA, Prof. Marc Rafelski of NASA’s Space Telescope Science Institute, USA and Prof. Harry Teplitz from Caltech, USA.

Research contacts:

	Prof. Kanak Saha IUCAA, Pune E-mail: kanak_at_iucaa.in Phone: +91-20-2560 4124
	Mr. Suraj Dhiwar IUCAA, Pune E-mail: suraj_at_iucaa.in Phone: +91-020-2560 4306 Mobile: +91-90112 92643

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