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A wild find on Covid’s origin; and Black Hole’s leftovers pack a punch

Evidence was found of the virus in samples from the Wuhan wet market, and astronomers solve a mystery about the source of a type of X-rays from space.

Published : Oct 29, 2024 21:08 IST - 0 MINS READ

A raccoon dog in its cage at a fur farm in Zhangjiakou, China, July 2015. Raccoon dogs are now linked to early Covid-19 transmission at a Wuhan market.

A raccoon dog in its cage at a fur farm in Zhangjiakou, China, July 2015. Raccoon dogs are now linked to early Covid-19 transmission at a Wuhan market. | Photo Credit: GREG BAKER

Wuhan market most likely source of COVID-19

WHILE the laboratory origin hypothesis of the COVID-19 virus SARS-CoV-2 has been held as legitimate and still holds some currency in the medical community, work published in a recent issue of the journal Cell points more towards an animal origin at the Wuhan market in China. The paper is the result of research carried out as part of a large international collaboration.

The work studied data from environmental samples taken at the Huanan wildlife market in Wuhan shortly after its closure in early 2020, according to Florence Débarre, one of the authors of the study and research director at the Institute of Ecology and Environmental Sciences, French National Centre for Scientific Research, Paris.

The data had been shared by the Chinese Center for Disease Control and Prevention on open and public databases. They included the raw genetic sequences of more than 800 samples collected at the Huanan market, on cages and carts, on the floors and walls of the stalls, and in the pipes and sewers.

The data allowed researchers to highlight the co-presence at this location of genetic material from the SARS-CoV-2 virus and certain wild animals, such as masked palm civets (wild canids similar to foxes, with a dark facial mask similar to that of raccoons), civets (small carnivorous mammals close to mongooses), raccoon dogs, and bamboo rats, that had previously been identified as possible intermediate hosts (between horseshoe bats, the reservoir of SARS-CoV-2-like coronaviruses, and humans) but still remain unconfirmed for SARS-CoV-2. These mammalian wildlife species, Débarre pointed out, were already involved in the emergence of the severe acute respiratory syndrome (SARS) epidemic in the early 2000s and considered as agents facilitating the transmission of the virus from animals to humans. Their work informs this open question, the authors wrote in the paper.

These animals were identified on the basis of their DNA and located in the south-west part of the market, which was also a hot spot where many samples tested positive for SARS-CoV-2. The researchers combined metagenomic and phylogenetic approaches to recover genotypes of market animals and compared them with those from farms and other markets. “There is a particular stall where the virus and the animals were found,” Débarre added.

Since the data were based on environmental samples, it was not possible for the researchers to conclusively demonstrate that the animals were infected. But the discovery of virus samples located in the same place as the genetic material of these animals suggests that they were.

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An artist’s impression of an orbiting star crashing through the accretion disc around a supermassive black hole and causing a burst of X-rays. Inset: Detection of X-rays (purple) overlaid on a visible light image of the host galaxy.

An artist’s impression of an orbiting star crashing through the accretion disc around a supermassive black hole and causing a burst of X-rays. Inset: Detection of X-rays (purple) overlaid on a visible light image of the host galaxy. | Photo Credit: X-ray: NASA/CXC/QUB/M. Nicholl et al.; Optical/IR: PanSTARRS, NSF/Legacy Survey/SDSS; Illustration: Soheb Mandhai/The Astro Phoenix; Image Processing: NASA/CXC/SAO/N. Wolk

An X-ray from outer space with a black hole link

A STAR that passes close to a supermassive black hole (SMBH) is ripped apart by the black hole’s tidal force, bit by bit. Much of the resultant stellar material loops around the black hole while some portion of it is captured and forms an accretion disk around the black hole. This messy process is called a tidal disruption event (TDE). Astronomers occasionally catch a glimpse of TDEs, and a recent one has helped solve a mystery about the source of a type of transient X-ray.

Known as quasi-periodic eruptions (QPEs), these transient flashes are soft X-rays that emanate from the centres of galaxies every few hours or few weeks. Being rare, QPEs are difficult to study, and astronomers do not really know what causes them. One prevalent idea is that they are caused by a large star or stellar-mass black hole orbiting the SMBH when its orbit intersects the accretion disk of the SMBH. Each time the smaller object passes through the disk, it triggers superheated plasma to release X-rays.

Recently, a collaborative effort by researchers of four NASA space observatories—Chandra X-ray Observatory, Hubble Space Telescope (HST), Neutron star Interior Composition Explorer (NICER), and Neil Gehrels Swift Observatory (Swift)—and ISRO’s astrophysics research telescope AstroSat resulted in the discovery of an SMBH having torn apart a star now using the stellar wreckage to pummel another nearby star (or a smaller stellar-mass black hole) that was in the clear earlier. This discovery provides astronomers with valuable insights, linking two mysteries where there were only hints of a plausible connection earlier.

In 2019, astronomers working with the wide-field optical telescope at the Palomar Observatory in California observed the signal of a TDE, designated AT2019qiz, when a star got too close to an SMBH and was being destroyed. Once shredded, the star’s remains had formed an orbiting disc around the black hole. Over a few years, however, the disc expanded outwards and is now directly in the path of an “orbiting” star (or possibly a black hole), which is now repeatedly crashing through the debris disc. When it does so, the collisions cause bursts of X-rays to be released, which astronomers captured with Chandra.

“There had been feverish speculation that these phenomena were connected, and now we’ve discovered the proof that they are,” said Dheeraj Pasham of the Massachusetts Institute of Technology, one of the authors of the paper, which was published in the latest issue of Nature.

This observation was followed up by observations with Chandra and the HST in 2023 to study the debris from the TDE. M. Nicholl and collaborators, authors of the present paper, followed it up using NICER and observed that AT2019qiz erupts roughly every 48 hours. Swift and AstroSat confirmed the finding. This result has implications for the search for more QPEs associated with TDEs, which would enable a study of the prevalence and distances of objects in close orbits around SBMHs.

“AstroSat’s Soft X-ray Telescope and the Ultra-Violet Imaging Telescope (UVIT) both detected the source AT2019qiz, but the eruptions were only seen in X-rays,” noted Gulab Dewangan of the Pune-based Inter-University Centre for Astronomy and Astrophysics and one of the co-authors.

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