Fast blue optical transients (FBOTs) share some characteristics with supernova explosions of massive stars and with explosions that generate gamma-ray bursts (GRBs), but still have distinctive differences from each. Three FBOTs are currently known: AT2018cow (‘The Cow’ or SN 2018cow), CSS161010 and ZTF18abvkwla (‘The Koala’).
An artist’s conception illustrates the phenomena that make up the new class of cosmic explosions called fast blue optical transients. Image credit: Bill Saxton / NRAO / AUI / NSF.
The most famous FBOT is AT2018cow, a very powerful astronomical explosion spotted in June 2018 in a galaxy 200 million light-years away in the constellation of Hercules.
Two additional blasts, CSS161010 detected in a galaxy about 500 million light-years from Earth in 2016 and ZTF18abvkwla spotted in a galaxy about 3.4 billion light-years away in 2018, also showed unusual characteristics and were being observed and analyzed.
Two teams of astronomers used the Karl G. Jansky Very Large Array, the Giant Metrewave Radio Telescope, and NASA’s Chandra X-ray Observatory to observe these two events.
The first team, led by Caltech astronomer Anna Ho, immediately noted that the radio emission from ZTF18abvkwla was as bright as that from a GRB.
The second team, headed by Northwestern University astronomer Deanne Coppejans, found that CSS161010 had launched an unexpected amount of material into interstellar space at more than half the speed of light.
“It took almost two years to figure out what we were looking at just because it was so unusual,” said Northwestern University astronomer Raffaella Margutti, member of the CSS161010 team.
In both cases, the follow-up observations indicated that the objects shared features in common with AT2018cow.
An artist’s conception illustrates the differences in phenomena resulting from an ordinary core-collapse supernova explosion, an explosion creating a gamma-ray burst, and one creating a fast blue optical transient. Image credit: Bill Saxton / NRAO / AUI / NSF.
The scientists concluded that these events represent, along with AT2018cow, a type of stellar explosion significantly different from others.
“FBOTs probably begin the same way as certain supernovae and gamma-ray bursts — when a star much more massive than the Sun explodes at the end of its normal atomic fusion-powered life. The differences show up in the aftermath of the initial explosion,” they explained.
“In the ordinary supernova of this type, called a core-collapse supernova, the explosion sends a spherical blast wave of material into interstellar space.”
“If, in addition to this, a rotating disk of material briefly forms around the neutron star or black hole left after the explosion and propels narrow jets of material at nearly the speed of light outward in opposite directions, these jets can produce narrow beams of gamma rays, causing a GBR.”
“The rotating disk, called an accretion disk, and the jets it produces, are called an engine.”
“FBOTs also have such an engine. In their case, unlike in gamma-ray bursts, it is enshrouded by thick material. That material probably was shed by the star just before it exploded, and may have been pulled from it by a binary companion.”
“When the thick material near the star is struck by the blast wave, it causes the bright visible-light burst soon after the explosion that initially made these objects appear so unusual. This is one of the characteristics that distinguished them from ordinary supernovae.”
“As the blastwave from the explosion collides with the material around the star as it travels outwards, it produces radio emission. This very bright emission was the important clue that proved that the explosion was powered by an engine.”
“The shroud of dense material means that the progenitor star is different from those leading to GRBs,” Dr. Ho.
In AT2018cow and CSS161010, the dense material included hydrogen, something never seen in GRBs.
Using the W.M. Keck Observatory, the astronomers found that both CSS161010 and ZTF18abvkwla, like AT2018cow, are in small, dwarf galaxies.
“The dwarf galaxy properties might allow some very rare evolutionary paths of stars that lead to these distinctive explosions,” Dr. Coppejans said.
Although a common element of FBOTs is that all three have a ‘central engine,’ the researchers caution that the engine also could be the result of stars being shredded by black holes, though they consider supernova-type explosions to be the more likely candidate.
“Observations of more FBOTs and their environments will answer this question,” Dr. Margutti said.
The findings were published in two papers in the Astrophysical Journal and the Astrophysical Journal Letters.
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Anna Y.Q. Ho et al. 2020. The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at z = 0.27. ApJ 895, 49; doi: 10.3847/1538-4357/ab8bcf
D.L. Coppejans et al. 2020. A Mildly Relativistic Outflow from the Energetic, Fast-rising Blue Optical Transient CSS161010 in a Dwarf Galaxy. ApJL 895, L23; doi: 10.3847/2041-8213/ab8cc7
