A few years ago, researchers discovered a cosmic explosion like no other. It looked like a supernova but it was brighter and shorter-lived compared to previously seen events. Its designation AT2018cow inevitably got shortened and it got nicknamed the "Cow". As it turns out, the Cow has a herd. Researchers have now found two more of these rare transient events.
The two new events continue to defy an exact explanation but do confirm that we are seeing a class of objects unfamiliar to us. The first one, reported in The Astrophysical Journal Letters, is an event called CSS161010. While similar to the Cow, it dwarfs that event when it comes to the material outflow it produced. CSS161010's ejection was much more significant in mass and that material was thrown out at 55 percent of the speed of light.
“This was unexpected,” lead author Deanne Coppejans from Northwestern’s University said in a statement. “We know of energetic explosions that can eject material at almost the speed of light, specifically gamma-ray bursts, but they only launch a small amount of mass — about 1 millionth the mass of the Sun. CSS161010 launched 1 to 10 percent the mass of the Sun at more than half the speed of light — evidence that this is a new class of transient.”
“We thought we knew what produced the fastest outflows in nature,” added Northwestern’s Raffaella Margutti, senior author of the study. “We thought there were only two ways to produce them — by collapsing a massive star with a gamma-ray burst or two neutron stars merging. We thought that was it. With this study, we are introducing a third way to launch these outflows. There is a new beast out there, and it’s able to produce the same energetic phenomenon.”
The second event, reported in The Astrophysical Journal, is ZTF18abvkwla, nicknamed the "Koala", based on the last four letters of the automatically generated name. Again, the Koala looks similar to the Cow, but if it was on steroids.
“When I reduced the data, I thought I made a mistake,” lead author of the second study Anna Ho, a graduate student of astronomy at Caltech, said in a statement. “The ‘Koala’ resembled the ‘Cow’ but the radio emission was 10 times brighter – as bright as a gamma-ray burst!”
The Cow, CSS161010, and the Koala have been classified as new transient events called "fast blue optical transients," or FBOTs. The name is because they disappear quicker than a supernova, and are so hot they glow blue.
Their origin is still a matter of debate. The most likely explanation is that they are explosions of massive stars, whose specific properties create these rare events. But another possibility is that we are seeing the quick and tempestuous demise of a star at the hand of a medium-sized black hole, bigger than what you get from a supernova but not supermassive like the ones at the core of galaxies.
“The Cow and CSS161010 were very different in how fast they were able to speed up these outflows,” Margutti said. “But they do share one thing — this presence of a black hole or neutron star inside. That’s the key ingredient.”
All three events originate in dwarf galaxies, smaller galaxies like the companions of the Milky Way. These galaxies tend to have stars that have less heavy elements and this might allow for the formation of peculiar stars we have not seen in our own galaxy.
"These observations of the 'Koala' and CSS161010 show how much we can learn from radio and X-ray observations of FBOTs," said Ho. "The challenge going forward is to delineate different FBOT subtypes and to develop more precise vocabulary. It's exciting to help investigate a new and unexpected phenomenon. In science, you sometimes don't find what you were expecting to find, but along the way you uncover new directions."