Astronomers have found a pair of objects that have been added to a strange object discovered in 2018 that combined to constitute a new class of cosmic explosions. The new type of explosion shares some of the characteristics with supernova explosions of massive stars and with the explosions that generate gamma-ray bursts (GRBs). Despite the similarities, the new class of explosion has distinctive differences from each of the other classes. These three objects are currently known as AT2018cow (“The Cow” or SN 2018cow), CSS161010 and ZTF18abvkwla (“The Koala”).
The Cow was discovered in June 2018 and immediately drew the worldwide attention from astronomers with its surprising characteristics and behavior. This object shared some attributes with supernova explosions, though it differed in some key characteristics and, in particular, its unusual initial brightness and how rapidly it brightened and faded in just a few days. The other two new explosions, CSS161010 in 2016 and The Koala in 2018, were almost 500 million and 3.4 billion light-years away from Earth, respectively. They were discovered by automated sky surveys (Catalina Real-time Transient Survey, All-Sky Automated Survey for Supernovae, and Zwicky Transient Facility) using visible-like telescopes to scan large areas on the night sky. The Koala had radio emission with similar brightness to GRBs, while CSS161010 ejected unusual large amount of material into space with speeds that exceeds half the speed of light!
Deanne Coppejans, of Northwestern University, led the study on CSS161010, which found that the object had launched an “unexpected” amount of material into interstellar space at more than half the speed of light. Her Northwestern co-author Raffaella Margutti, said, “It took almost two years to figure out what we were looking at just because it was so unusual.”
Follow-up observations of these two explosions with the National Science Foundation’s Karl G. Jansky Very Large Array (VLA), the Giant Metrewave Radio Telescope in India and the NASA’s Chandra X-ray Observatory showed that they shared common features with The Cow and that these events represent a type of stellar explosion significantly different from others and they called them Fast Blue Optical Transients.
Anna Ho, of Caltech, lead author of the study on ZTF18abvkwla, immediately noted that the object’s radio emission was as bright as that from a gamma-ray burst. “When I reduced the data, I thought I had made a mistake,” she said.
FBOTs are triggered similarly to a certain type of supernovae and GRBs when a star much more massive than the Sun explodes at the end of its life. The difference shows up after the initial explosion. The FBOTs had the so-called “engine” seen in other types of explosions, but it’s enshrouded by thick material that was likely shed by the star before explosion. When the thick material near the star is struck by the blast wave after the explosion, a bright visible-light burst occurs, which is what makes the objects so unusual. Scientists say that the shroud of dense material means that the progenitor star is different from those that lead to gamma-ray bursts. Using the W.M. Keck Observatory, the astronomers found that both CSS 161010 and ZTF18abvkwla, like the Cow, are in small, dwarf galaxies. Deanne Coppejans said that the dwarf galaxy properties “might allow some very rare evolutionary paths of stars” that lead to these distinctive explosions.
Related scientific papers: Coppejans et al. 2020, ApJL, 895, L23 (DOI: 10.3847/2041-8213/ab8cc) and Ho et al. 2020, ApJ, 895, 49 (DOI:10.3847/1538-4357/ab8bcf)