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The 'Cow' mystery strikes back: Two more rare, explosive events captured:

The 'Cow' is not alone; with the help of W. M. Keck Observatory on Maunakea in Hawaii, astronomers have discovered two more like it—the 'Koala' and a similar mysterious bright object called CSS161010. This trio of fast blue optical transients (FBOTs) appear to be relatives, all belonging to a highly-luminous family that has a track record for surprising astronomers with their fast, powerful bursts of energy.

The 'Koala,' which is a nickname derived from the tail end of its official name ZTF18abvkwla, suddenly appeared as a bright new source in the optical sky before disappearing within just a few nights. A team of astronomers at Caltech realized this behavior was similar to the 'Cow' and requested radio observations to see if the two were connected.

"When I reduced the data, I thought I made a mistake," said Anna Ho, graduate student of astronomy at Caltech and lead author of the study. "The 'Koala' resembled the 'Cow' but the radio emission was ten times brighter—as bright as a gamma-ray burst!"

Ho and her research team's paper is published in today's issue of The Astrophysical Journal.

Another cosmic explosion of this type, CSS161010, fascinated a team of astronomers led by Northwestern University. Based on radio observations, they calculated this transient launched material into space faster than 0.55 times the speed of light.

"This was unexpected," said Deanne Coppejans, postdoctoral associate at Northwestern University and lead author of the study on CSS161010. "We know of energetic stellar 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 to relativistic speeds—evidence that this is a new class of transient!"

Coppejans and her team's paper is published in today's issue of The Astrophysical Journal Letters.

These three strange events make up a new subtype of FBOTs, which first dazzled the world in the summer of 2018 when the 'Cow,' short for AT2018cow, exploded in the sky.

Three months later, Ho's team captured the 'Koala.' Though the 'Cow' was the first to make world headlines, CSS161010 was actually the first FBOT discovered with luminous radio and X-ray emission, but astronomers did not know how to interpret these findings yet.

"At that time, there was really no theoretical model that predicted bright radio emission from bright FBOTs," said Coppejans. "It wasn't until we conducted follow-up radio and X-ray observations that the true nature of CSS161010 revealed itself. Seeing it at these wavelengths is important because the data showed we were looking at something new and highly energetic."

What makes these luminous FBOTs strange is they look like supernova explosions, but flare up and vanish much faster. They're also extremely hot, making them appear bluer

D. L. Coppejans et al. A Mildly Relativistic Outflow from the Energetic, Fast-rising Blue Optical Transient CSS161010 in a Dwarf Galaxy, The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/ab8cc7

Original Submission