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AstroSat’s full multi-wavelength capabilities have enabled an international team of scientists to unravel the mysteries surrounding the X-ray binary system MAXI J1820+070, hosting a black hole. The comprehensive study, titled “A Multi-Wavelength Spectral Study of MAXI J1820+070 in the Soft and Hard States,” accepted for publication in The Astrophysical Journal, presents unique insights into the behaviour of this transient black hole X-ray binary during its 2018 outburst. The team is led by researchers from IUCAA, Pune, and includes researchers from India, the United Kingdom, Abu Dhabi, and Poland.

MAXI J1820+070, positioned around 9800 light-years distant from Earth, is a transient black hole X-ray binary. It was first detected during its outburst in 2018 using the MAXI instrument aboard the International Space Station (ISS). Because of its proximity to Earth and its remarkable brightness upon discovery, emerging as the second brightest object in the X-ray sky, MAXI J1820+070 garnered significant attention within the astronomy community. This led to numerous observing campaigns across various electromagnetic bands.

AstroSat, equipped with three X-ray payloads and a UV telescope, captured soft and hard X-ray emissions and far ultraviolet radiation, painting a detailed portrait of the near and distant regions surrounding the black hole in MAXI J1820+070. Collaborating with optical data from Las Cumbres Observatory and soft X-ray data from NASA’s NICER mission, the team understood the system’s dynamics.

The study reveals captivating findings about the accretion states of MAXI J1820+070. Black hole X-ray binaries, such as MAXI J1820+070, often exhibit multiple accretion states throughout an outburst. In the hard state, the accretion disk recedes significantly from the black hole, giving rise to a structured corona with distinctive components. Conversely, the disk moves closer to the black hole during the soft state while the corona’s emission diminishes. The hard and soft states are characterised by the emission of predominantly higher-energy X-rays from the system typically originating from the corona surrounding the black hole and the emission of mostly softer X-rays emitted by the accretion disk itself, respectively. Advanced techniques employed by the researchers unveiled the black hole’s spin, shedding light on its fundamental properties.

Moreover, the study establishes a compelling connection between X-ray emission from the inner regions near the black hole and optical/UV emission from the outer region of the accretion disk. Reprocessed radiation in the outer accretion disk emerges as a primary mechanism for generating optical/UV photons, with a notably higher proportion observed during the hard state.

The significance of this study extends beyond MAXI J1820+070, highlighting the pivotal role of AstroSat in advancing the understanding of transient black hole X-ray binaries. With its unique multi-wavelength capabilities, AstroSat opens doors to further exploration of diverse astrophysical phenomena, laying the groundwork for future endeavours in the field.

This research underscores the importance of acquiring multi-wavelength spectroscopic data for X-ray binaries, offering novel insights into the intricate workings of these cosmic phenomena.

The schematic diagram illustrates the geometries associated with the hard state (upper panel) and soft state (lower panel) of MAXI J1820+070, as deduced in this study. Various physical processes contributing to X-ray emissions are denoted by blue-coloured arrows, while those responsible for optical/UV emissions are represented by red-coloured arrows.