The Cow Event

[James Kerman]

On June 16th 2018 the Asteroid Terrestrial-impact Last Alert System telescope in Hawaii detected a flash of light at least 10 times brighter than a typical supernova.  The event was dubbed AT2018cow and has been puzzling astronomers for the past 6 months.

Image Credit: https://www.northwestern.edu/

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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 Thursday, Jan. 10, at the 233rd American Astronomical Society meeting in Seattle.

A Black Hole Shredding a Compact Star?

 

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, an assistant research scientist at the University of Maryland, Baltimore County and NASA's Goddard Space Flight Center in Greenbelt, 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 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. They also calculated that the black hole's mass ranges from 100,000 to 1 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.

"The Cow produced a large cloud of debris in a very short time," said lead author Paul Kuin, an astrophysicist at University College London (UCL). "Shredding a bigger star to produce a cloud like this would take a bigger black hole, result in a slower brightness increase and take longer for the debris to be consumed."

Or a New View of a Supernova?

A different team of scientists was able to gather data on the Cow over an even broader range of wavelengths, spanning from radio waves to gamma rays. Based on those observations, the team suggests that a supernova could be the source of the Cow. When a massive star dies, it explodes as a supernova and leaves behind either a black hole or an incredibly dense object called a neutron star. The Cow could represent the birth of one of these stellar remnants.

"We saw features in the Cow that we have never seen before in a transient, or rapidly changing, object," said Raffaella Margutti, an astrophysicist at Northwestern University in Evanston, Illinois, and lead author of a study about the Cow to be published in The Astrophysical Journal. "Our team used high-energy X-ray data to show that the Cow has characteristics similar to a compact body like a black hole or neutron star consuming material. But based on what we saw in other wavelengths, we think this was a special case and that we may have observed - for the first time - the creation of a compact body in real time."

Margutti's team analyzed data from multiple observatories, including NASA's NuSTAR, ESA's (the European Space Agency's) XMM-Newton and INTEGRAL satellites, and the National Science Foundation's Very Large Array. The team proposes that the bright optical and ultraviolet flash from the Cow signaled a supernova and that the X-ray emissions that followed shortly after the outburst arose from gas radiating energy as it fell onto a compact object.

Typically, a supernova's expanding debris cloud blocks any light from the compact object at the center of the blast. Because of the X-ray emissions, Margutti and her colleagues suggest the original star in this scenario may have been relatively low in mass, producing a comparatively thinner debris cloud through which X-rays from the central source could escape.

"If we're seeing the birth of a compact object in real time, this could be the start of a new chapter in our understanding of stellar evolution," said Brian Grefenstette, a NuSTAR instrument scientist at Caltech and a co-author of Margutti's paper. "We looked at this object with many different observatories, and of course the more windows you open onto an object, the more you can learn about it. But, as we're seeing with the Cow, that doesn't necessarily mean the solution will be simple."

https://www.jpl.nasa.gov/news/news.php?feature=7314

 

Edited January 11, 2019 by James Kerman
Thread title changed

[razark]

This is not what I expected from the thread title.

(Which is probably a good thing.)

 

Still interesting, though.

[Jaelommiss]

I know just enough to take wild guesses, but not enough to identify why I'm wrong. If someone could point out where my guess falls apart (I'm loathe to call my spitballing a hypothesis) I'd appreciate it.

If a star's orbit was deflected by another star such that it was within a blackhole's Rosche limit, would that explain the appearance of the gas cloud? The subsequent accretion disk could also explain the dramatic rise in luminosity. If a single star is insufficient for this amount of gas and luminosity, how likely would it be that multiple stars would suffer the same fate in such a short time span (my gut feeling is that this is so unlikely as to be almost impossible, but I don't know the numbers for it)? The article mentions that a blackhole of this size could not destroy a larger star from tidal forces, but if a larger star passed so close to the black hole that a portion of it passed within the Schwarschild radius (or close enough that mass was lost) could we see a larger star get disintegrated? This would require such precision that it seems incredibly unlikely, but, again, I don't have any of the numbers on it.

The second explanation provided by the article, that a supernova was not obscured by debris, seems more probable than any of my wild imaginings. 

Edited January 11, 2019 by Jaelommiss

[Scotius]

A black hole shredding a star? I think i've seen this before...

Ah, yes. I did:

 

[Green Baron]

This is not as much scifi as one might think: link

 

 

[Val]

Removed a post that was very off-topic from the intended topic, and a response.

[p1t1o]

Fair, apologies

 

[Nuke]

i have to say i liked the original title better.

[James Kerman]

2 hours ago, Nuke said:

i have to say i liked the original title better.

Sorry about that, Mate, It was just a little "tounge in cheek" humor however I changed it because I got to thinking someone might post a video and I don't want to encourage that.

The Australian Broadcasting Corporation has released some scientific commentary about The Cow (I believe from the 233rd meeting of the American Astronomical Society in Seattle):

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Dr Raffaella Margutti of Northwestern University in Chicago and Her  team (responsible for the image above) were also following the Cow, using 16 telescopes around the world to study radio waves, gamma rays, and a spectrum not usually used to study supernovae: hard X-rays, which are 10 times more powerful than normal X-rays.  Unlike a regular supernova, the Cow has very little material swirling around it, which it enabled the team to peer straight into its heart.
"The most striking feature in the Cow is what we find in the hard X-rays," Dr Margutti said.  These powerful X-rays indicated that lying deep within the core of the Cow was a central engine that continued to pump out energy for at least 100 days after the blast.  This kind of long-lasting energy source is not known in normal supernovae or gamma rays, but could occur as material swirls around a compact object such as a newly forming black hole or fast-spinning magnetar.  "The big question we're trying to find out is why in the first place we have a little amount of mass."  Dr Margutti said the blast could have been created by the death of a massive blue supergiant star, many times the mass of the Sun."They should explode, but we don't see them," Dr Margutti said.  "There is a possibility that those types of stars just go directly into a collapse without producing even a little bit of an explosion."Dr Margutti said the radio data indicated the Cow was not created by a pre-existing black hole stripping a star.

Dr Tara Murphy and her PhD student Dougal Dobie of the University of Sydney used the CSIRO's Australia Telescope Compact Array at Narrabri in New South Wales to observe radio emissions created by the blast's shockwave.  "What we can see is that the shockwaves were travelling about a 10th the speed of light and we saw the brightness of this source in radio keep increasing over time," Dr Murphy said.

The increasing brightness was also observed in shorter wavelengths by team leader and Caltech astronomer Anna Ho and her colleagues, using telescopes in Hawaii and Chile.   "The fact it was getting brighter with time and particularly in higher frequencies shows there must be something that was still powering the explosion pumping energy into this material,"  Dr Murphy said.  "It wasn't just this explosion that happened and then was fading away, there must be something that was still there that makes it different from the typical supernova we see."  She said the radio data pointed to the presence of a magnetar at the core of the supernova.

Whatever it is, it does not fit any of the models, said Dr Daniel Perley, an astronomer from Liverpool John Moores University in the UK who led another of the international teams.  "It has very different properties from anything we've seen before," Dr Perley said.  When word got out that a strangely bright object had been spotted in our cosmic neighbourhood by the ATLAS telescopes on June 16, astronomers scrambled to observe its rise and fall using a global network of telescopes.  "It may well be the most intensely observed astronomical source ever," Dr Perley said.  "The only thing that competes would be the gravitational wave detection from a year-and-a-half ago," he said, referring to the observation of fireworks created by the collision of two ancient stars.  Dr Perley and his team used optical telescopes to follow the Cow every night for more than a month.  The focus now is to find more objects like the Cow to definitely work out how they were formed, Dr Perley said.  "This shouldn't be one of a kind," he said.  "There were some events that had similar properties that were seen in previous surveys, but no-one really recognised them ... and they were never followed up in real time or even discovered in real time.  "So, now we know what to look for, we're pushing forward to find more and to get more data."

Dr Brad Tucker, of the Australian National University, did not study the Cow but has investigated supernova events detected by Kepler and now, the TESS telescope.  Of all the hypotheses put forward by the teams, he leans towards the creation of a black hole by a failed supernova.  "I don't know if the data favours this one way or the other. But the reason I like that is it's simple — it doesn't require any special physical mechanism.  "We know that black holes exist, but we've never been able to pinpoint the birth of a black hole.  "We know blue supergiants exist and explode. We know they explode at different regimes. What's not to say that this could create a black hole?  "Every time we see a blue supergiant, something is weird about it."  Less likely, he thought, was the idea of a tidal disruption event.  "If it's not a supernova we always say, 'Is this a tidal disruption event?' and you take a look at it and generally the answer is 'no'," Dr Tucker said.  "But again, these are things we're just finding more and more of and just trying to understand.  "We're narrowing down and ruling things out, but what we can definitively say is there is another type of explosion or type of star that goes bang in the night that we're just starting to uncover." 

https://www.abc.net.au/news/science/2019-01-11/cow-cosmic-blast-supernova-black-hole-magnetar/10703320 *Quotes have been reorganized.

 

Edited January 12, 2019 by James Kerman
spelling and additional data

[Green Baron]

Thanks !

Interesting that somebody considers a failed supernova of a blue supergiant as being less improbable than a tidal disruption, which there is another examples of (Arp299). The discussion is on ...

[YNM]

At least thank god it's not the talking cow in The Restaurant At The End Of The Universe.