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.

The electromagnetic radiation emitted by accreting black holes in transient X-ray binaries spans a wide energy spectrum, ranging from radio waves to X-rays, encompassing a myriad of physical processes. X-ray radiation originates near the black hole through two primary mechanisms: soft X-ray emission from the inner portion of the accretion disk and high-energy X-rays resulting from the interaction of soft X-ray disk photons with a hot (100s of million kelvin) electron plasma, known as the 'corona', located somewhere between the black hole and the accretion disk. Additionally, a portion of hard X-ray photons can be reflected from the disk, leading to reflected X-ray emission. Black hole X-ray binaries, such as MAXI J1820+070, often exhibit multiple accretion states throughout an outburst. The two primary states are the soft state, characterized by predominant emission from the disk, and the hard state, where X-ray radiation from the corona dominates. However, the inner accretion geometry of the hard state remains a long-standing challenge in high-energy astrophysics. Conversely, optical/UV photons are emitted from the outer region of the accretion disk, the geometry of which remains one of the least understood aspects of accretion physics. Moreover, X-ray photons from the inner region of the accretion flow can also illuminate the outer accretion disk, resulting in the generation of optical/UV photons through reprocessing.

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. Led by Dr. Srimanta Banerjee and Prof. Gulab Dewangan of IUCAA (Pune), the research team utilized AstroSat's far UV, soft and hard X-ray data alongside quasi-simultaneous observations from Las Cumbres Observatory (optical) and NICER (soft X-ray). Their findings indicate that, in the hard state, the accretion disk recedes significantly from the black hole, making way for a structured corona comprising two distinct components with unique physical properties. Conversely, during the soft state, the disk moves closer to the black hole, while the corona's emission diminishes. Notably, the study reveals a perplexing emission component in the soft state, identified as residual emission cascading into the black hole. AstroSat’s Soft X-ray Telescope (SXT) played a crucial role in underpinning the inner accretion disk, while the Large Area X-ray Proportional Counter (LAXPC) and the Cadmium Zinc Telluride Imager (CZTI) instruments provided high quality high energy X-ray spectra down to 150 keV, and helped to infer a two component corona geometry in the hard state. Figure 2 depicts a plausible global accretion geometry for the hard and soft states of MAXI J1820+070 inferred from this study. Moreover, the researchers employed advanced techniques to measure the black hole's spin, one of its two fundamental properties (alongside mass), revealing the black hole to be moderately to highly spinning.

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 AstroSat 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 AstroSat's three-pointing X-ray instruments, enables a detailed investigation of the region proximal to the source through reflection analyses.

Due to the limited availability of such multi-wavelength spectroscopic data for X-ray binaries, often hindered by significant reddening resulting from their proximity to the Galactic plane, this work, focusing on MAXI J1820+070, underscores its importance. The study not only advances our understanding of this specific object but also lays the foundation for future endeavors, demonstrating the comprehensive capabilities of AstroSat in facilitating exploration across a wide array of astrophysical phenomena.

Prof. Gulab Dewangan, a faculty member at IUCAA (Pune), says “AstroSat provides unique capability for multi-wavelength observations of X-ray binaries and other cosmic sources, and studies like this are indispensable for unraveling the complexities of these cosmic systems.”

Prof. Dipankar Bhattacharya, Chairperson of the AstroSat Science Working Group and a co-author of this study, remarked: “This is the first time the full capability of all the co-pointed instruments in AstroSat have been used in unison, supplemented by ground-based observations, and the results are fascinating. I am happy to be a part of this unique investigation of one of the most interesting Black Hole sources discovered recently”.

Prof. Poshak Gandhi, a faculty member at University of Southampton (UK) and an adjunct faculty member at IUCAA, who submitted a Target of Opportunity proposal leading to this study, says: “Coordinating multiple observatories that can observe across the electromagnetic spectrum has always been challenging. AstroSat's unique strength is beautifully demonstrated by this study which (together with the Las Campanas Observatory) covers a range of energies almost 1,000 wider than what our eyes can see, all at exactly the same time.”

Dr Fraser Lewis, Research Director of the Faulkes Telescope Project, said “As an educational project with research interests, we were only too pleased to work with a group such as this in providing data on this exciting object. We can already see the spin-offs into education from this work as our teachers and students are always interested to understand more about these highly energetic and enigmatic objects”.

AstroSat is India’s first dedicated multi-wavelength space observatory developed and operated by the Indian Space Research Organisation (ISRO). The UVIT was built in collaboration between IIA (Bangalore), IUCAA (Pune), TIFR (Mumbai), ISRO (Bengaluru), and CSA (Canada), with its Payload Operation Centre (POC) at IIA, Bengaluru. The SXT was built by TIFR (Mumbai) in collaboration with University of Leicester (UK) and ISRO (Bengaluru) with its POC at TIFR (Mumbai). The LAXPC was built by TIFR (Mumbai) in collaboration with RRI (Bengaluru) and ISRO (Bengaluru) with its POC at TIFR, Mumbai. The CZT-Imager was built by TIFR (Mumbai) in collaboration with ISRO (Bengaluru), IUCAA (Pune), PRL (Ahmadabad) with its POC at IUCAA (Pune). This work uses data from the Faulkes Telescope Project, which is an education partner of LCO. The Faulkes Telescopes are maintained and operated by LCO.

Research contacts:

	Prof. Gulab Dewangan IUCAA, Pune E-mail: gulabd_at_iucaa.in Phone: +91-20-96737 44462
	Dr. Srimanta Banerjee IUCAA, Pune E-mail: srimanta_at_iucaa.in Phone: +91-77384 37325

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