Holy cow! Mysterious blast studied with NASA telescopes leaves science puzzled

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The yellow cross shows the location of AT2018cow or 'the Cow'. (Image: NASA/Sloan Digital Sky Survey)

A brief and unusual flash spotted in the night sky on June 16, 2018, puzzled astronomers and astrophysicists across the globe, reports NASA. The event — called AT2018cow and nicknamed ‘the Cow’ after the coincidental final letters in its official name — is unlike any celestial outburst ever seen before, prompting multiple theories about its source. Over three days, the Cow produced a sudden explosion of light at least 10 times brighter than a typical supernova, and then it faded over the next few months.

Where did the Cow occur?

This unusual event occurred inside or near a star-forming galaxy known as CGCG 137-068, located about 200 million light-years away in the constellation Hercules.

The Cow was first observed by the NASA-funded Asteroid Terrestrial-impact Last Alert System telescope in Hawaii.

So, exactly WHAT is the Cow?

Using data from multiple NASA missions, including the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array (NuSTAR), two groups are publishing papers that provide possible explanations for the Cow’s origins.
One paper argues that the Cow is a monster black hole shredding a passing star.
The second paper hypothesizes that it is a supernova — a stellar explosion — that gave birth to a black hole or a neutron star.
Researchers from both teams shared their interpretations at a panel discussion on January 10 at the 233rd American Astronomical Society meeting in Seattle.

1) Is the Cow a black hole shredding a compact star?

Tidal disruption event

One potential explanation of the Cow is that a star has been ripped apart in what astronomers call a ‘tidal disruption event’.

Just as the Moon’s gravity causes Earth’s oceans to bulge, creating tides, a black hole has a similar but more powerful effect on an approaching star, ultimately breaking it apart into a stream of gas.

The tail of the gas stream is flung out of the system, but the leading edge swings back around the black hole, collides with itself, and creates an elliptical cloud of material.

According to one research team using data spanning from infrared radiation to gamma rays from Swift and other observatories, this transformation best explains the Cow’s behavior.

“We’ve never seen anything exactly like the Cow, which is very exciting,” said Amy Lien, assistant research scientist, University of Maryland and NASA’s Goddard Space Flight Center, Maryland.

We think a tidal disruption created the quick, really unusual burst of light at the beginning of the event and best explains Swift’s multiwavelength observations as it faded over the next few months,” Lien added.

Shredded star was a white dwarf

Lien and her colleagues think the shredded star was a white dwarf — a hot, roughly Earth-sized stellar remnant marking the final state of stars like our Sun.

Mass of the hole

They also calculated that the black hole’s mass ranges from 1,00,000 to one million times the Sun’s, almost as large as the central black hole of its host galaxy.

It’s unusual to see black holes of this scale outside the center of a galaxy, but it’s possible the Cow occurred in a nearby satellite galaxy or a globular star cluster whose older stellar populations could have a higher proportion of white dwarfs than average galaxies.

A paper describing the findings, co-authored by Lien, will appear in a future edition of the journal Monthly Notices of the Royal Astronomical Society.

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