A brief and unusual flash spotted in the night sky on June 16, 2018, puzzled astronomers and astrophysicists across the globe. The event ‚Äď called AT2018cow and nicknamed ‚Äúthe Cow‚ÄĚ after the coincidental final letters in its official name ‚Äď 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. 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‚Äôs 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 ‚Äď a stellar explosion ‚Äď that gave birth to a black hole or a neutron star.
Watch what scientists think happens when a black hole tears apart a hot, dense white dwarf star. A team working with observations from NASA‚Äôs Neil Gehrels Swift Observatory suggests this process explains a mysterious outburst known as AT2018cow, or ‚Äúthe Cow.‚ÄĚ Credit: NASA‚Äôs Goddard Space Flight Center
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 ‚Äútidal disruption event.‚ÄĚ Just as the Moon‚Äôs gravity causes Earth‚Äôs 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‚Äôs behavior.
‚ÄúWe‚Äôve never seen anything exactly like the Cow, which is very exciting,‚ÄĚ said Amy Lien, an assistant research scientist at the University of Maryland, Baltimore County and NASA‚Äôs Goddard Space Flight Center in Greenbelt, Maryland. ‚ÄúWe think a tidal disruption created the quick, really unusual burst of light at the beginning of the event and best explains Swift‚Äôs multiwavelength observations as it faded over the next few months.‚ÄĚ
Lien and her colleagues think the shredded star was a white dwarf ‚Äď a hot, roughly Earth-sized stellar remnant marking the final state of stars like our Sun. They also calculated that the black hole‚Äôs mass ranges from 100,000 to 1 million times the Sun‚Äôs, almost as large as the central black hole of its host galaxy. It‚Äôs unusual to see black holes of this scale outside the center of a galaxy, but it‚Äôs 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.
‚ÄúThe Cow produced a large cloud of debris in a very short time,‚ÄĚ said lead author Paul Kuin, an astrophysicist at University College London (UCL). ‚ÄúShredding 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.‚ÄĚ
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.
‚ÄúWe saw features in the Cow that we have never seen before in a transient, or rapidly changing, object,‚ÄĚ 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. ‚ÄúOur 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 ‚Äď for the first time ‚Äď the creation of a compact body in real time.‚ÄĚ
Margutti‚Äôs team analyzed data from multiple observatories, including NASA‚Äôs NuSTAR, ESA‚Äôs (the European Space Agency‚Äôs) XMM-Newton and INTEGRAL satellites, and the National Science Foundation‚Äôs 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‚Äôs 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.
‚ÄúIf we‚Äôre 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,‚ÄĚ said Brian Grefenstette, a NuSTAR instrument scientist at Caltech and a co-author of Margutti‚Äôs paper. ‚ÄúWe 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‚Äôre seeing with the Cow, that doesn‚Äôt necessarily mean the solution will be simple.‚ÄĚ
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