Astro Sat’s full multi-wavelength capabilities used to unravel secrets of X-ray binary hosting black hole

The research titled, ‘A multi-wavelength study of the hard and soft states of MAXI J1820+070 during its 2018 outburst’ presents a comprehensive analysis of the 2018 outburst of MAXI J1820+070 and is led by Dr Srimanta Banerjee and professor Gulab Dewangan of IUCAA, Pune

Astro Sat, India’s first Space-based multi-wavelength astronomy mission, has now been used by an international team of scientists to paint a comprehensive portrait of an X-ray binary system hosting a black hole. Astro Sat has provided unique insights into both the near and distant regions surrounding the black hole in the X-ray binary system, MAXI J1820+070, located about 9,800 light years away from us.

The same has been corroborated by the Las Cumbres Observatory and the soft X-ray data from NASA’s NICER mission.

The research titled, ‘A multi-wavelength study of the hard and soft states of MAXI J1820+070 during its 2018 outburst’ presents a comprehensive analysis of the 2018 outburst of MAXI J1820+070 and is led by Dr Srimanta Banerjee and professor Gulab Dewangan of IUCAA (Pune).

The team including researchers from India, the United Kingdom (UK), Abu Dhabi and Poland, utilised Astro Sat’s far UV, soft and hard X-ray data alongside quasi-simultaneous observations from the Las Cumbres Observatory (optical) and NICER (soft X-ray). By capturing soft and hard X-ray emissions with its three X-ray payloads and far UV radiation with its UV telescope, Astro Sat provided unique insights into both the near and distant regions surrounding the black hole in the X-ray binary system, MAXI J1820+070. This study marks a pivotal achievement in the history of Astro Sat as the first instance where its full multi-wavelength capabilities have been harnessed.

Banerjee said, “This study further revealed a captivating connection between the X-ray emission from the inner regions near the black hole and optical/UV emission from the outer region of the accretion disk. The researchers found that X-rays undergo substantial reprocessing in the outer accretion disk, representing the primary mechanism for generating optical/UV photons in this system. Importantly, the proportion of reprocessed radiation is notably higher in the hard state, suggesting the existence of a warped or convex outer disk during this phase. Previous studies of this source, as well as other X-ray binaries, have predominantly relied on photometric optical/UV data for multi-wavelength study. However, for a precise determination of the properties of the outer accretion disk, spectroscopic data play a crucial role. In this context, the Astro Sat mission assumes a pivotal role by providing a unique opportunity to acquire spectroscopic data across diverse wavelengths, spanning from far UV to hard X-rays. Additionally, the exceptional energy resolution of NICER data, along with the extensive coverage of Astro Sat’s three-pointing X-ray instruments, enables a detailed investigation of the region proximal to the source through reflection analyses.”

Professor Dewangan said, “Black hole X-ray binaries are composed of a stellar-mass black hole and a companion star engaged in an 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 9,800 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.