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Astronomers puzzled by ‘cow’ in the sky

An image of AT2018cow, taken using the 2-metre Himalayan Chandra Telescope.  

Astronomers across the world are puzzled by a new phenomenon — a very bright celestial object seen close to the very small galaxy CGCG 137-068 — reported in The Astronomer’s Telegram on June 17. Since then, the journal has been receiving a spate of communications, from Indian astrophysicists, too. A fast-brightening spot, initially thought to be a bright transient astronomical event lying close to the galaxy CGCG 137-068, was spotted by the ATLAS telescope. Named AT2018cow, the transient was soon given the nickname ‘cow’. This is not due to any resemblance to the quadruped but a mere coincidence of letters in the way of naming such transients.

The cow proved to be suitably interesting, because though it was initially thought to be a nova, later analysis showed that it was more like a broadline supernova of the type Ic. Transient astronomical events last from a few seconds to several weeks and may have several causes. Specifically, a Type Ic supernova is caused by the explosion of an extremely massive star which has lost its outer layers of hydrogen and helium.

“As part of the GROWTH collaboration, we are keeping an eye on the mysterious transient AT2018cow using the 2-metre Himalayan Chandra Telescope (HCT) at Hanle, Ladakh,” says Shubham Srivastav, a post-doctoral fellow at IIT Bombay, in an email to The Hindu. The HCT observations indicate that the transient is steadily fading in all optical bands. Although spectroscopic features indicate a tentative similarity with broad line Ic supernovae, its true nature remains a puzzle, he adds.

A.J. Nayana, a research scholar at National Centre for Radio Astrophysics, Pune (NCRA) says: “We observed AT2018cow with the Giant Metrewave Radio Telescope (GMRT) on June 28 and reported our first results on June 29 in The Astronomers Telegram.” Since there are many transients being discovered each day, the researchers had to wait and decide if this object was worth the effort. Also, as Prof Poonam Chandra at NCRA, Pune, explains, “Radio frequencies are the lowest energy of the electromagnetic spectrum and the object evolves much later at radio frequencies. The GMRT results tell us that the radio-frequency radiation is still absorbed, most likely, by the relativistic electrons themselves which are responsible for the radio emission.”

Astronomers are in a frenzy trying to figure out what this is. “Even if it is finally a Type IC supernova, it will not be a conventional one,” says Varun Bhalerao from the Indian Institute of Technology Bombay, who is involved in analysing data received from the GMRT as well as the Himalayan Chandra Telescope. “The images and spectrum were obtained by the Hanle facility and GMRT. We have requested Astrosat to observe and it will start on July 3,” he adds.

In particular, AT2018cow’s fast rise time and high luminosity are unprecedented for a supernova. Hence it seems like nothing that was known before. “Anything in the unknown territory becomes exciting, and hence currently difficult to explain. However, I am confident that as astronomy community collects more data in multiple wavelengths, we will be able to nail down this exotic supernova,” says Prof. Chandra.

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Printable version | Jan 19, 2021 9:18:46 PM | https://www.thehindu.com/sci-tech/science/astronomers-puzzled-by-cow-in-the-sky/article24300319.ece

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