The first supernova discovered by its X-rays has an unusual engine at its core.

Somewhere in our universe, a stellar cataclysm is occurring, bringing the life of a massive star to an end. The most common type of cataclysm is a core-collapse supernova. That’s where a massive star’s interior implodes, leading to a runaway fusion reaction and a tremendous explosion, where the energy emitted by the star can briefly shine brighter than billions of stars.

And yet, it’s the rarer types of stellar cataclysms — superluminous supernovae, hypernovae, tidal disruptions events, and even stranger cosmic explosions — that can shine brighter than anything else we have ever observed.

In 2018, a new class of explosions was seen for the first time: the “Cow” class.

The star was detected automatically by a telescope in Hawaii that monitors the sky for unexpected brightening (or dimming) events, its randomly generated name came out as “AT2018cow.” In this case the last three letters happen to spell an actual word. Astronomers call it the Cow class. Today, it’s the prototype for a novel class of explosions that occur throughout the Universe.

Another event in the same “Cow” class of objects was recently discovered: the first one detected not by its visible light signatures, but by its spectacular X-ray brightening. Known as AT2020mrf, it literally bathed the Universe in X-rays across billions of light-years, including us.

Astronomers also offered evidence that either feeding black holes or neutron stars could be the source behind these unusual supernovas.

The event was originally spotted in July 2020 using X-ray data from the orbiting Spektrum-Roentgen-Gamma (SRG) telescope. They checked observations taken in optical light by the Palomar Observatory, and found that they had also spotted the event.

Yuan Yao, a graduate student at Caltech and study author, said, “Cow-like events have at their cores very active, and most exposed, compact objects that emit high-energy X-rays. We still don’t know why the central engine is so active, but it probably has something to do with the type of the progenitor star being different from normal explosions.”

How bright was it? The original “Cow” event had a large and significant X-ray brightness for a supernova. The AT2020mrf explosion was 20 times brighter in that X-ray light.

It possessed other features that made it a remarkably interesting object in its own right, including:

• a very high temperature of 20,000 K
• a very high speed, of around 10% the speed of light (much faster than a normal supernova’s of 2-3% the speed of light)
• intense radio emissions
  

Perhaps most interesting is the fact that it is found in a very small, low-mass, dwarf galaxy: one with a mass of only 100 million stars, or less than 0.1% the mass of our Milky Way. Sometimes smaller galaxies have more supernovas.

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