For the first time, researchers witness X-rays leaking from a nova explosion

The observation confirms a 30-year-old hypothesis.
Grant Currin
The same view of the sky separated by just four hours.König et al

Astronomers are swimming in data, but they still rely on something stargazers have always needed: a little bit of luck.

It was lucky that on July 7, 2020, a space telescope happened to be making its 23rd scan of a patch of sky in the constellation Reticulum when X-rays from a brief thermonuclear explosion rolled past the instrument.

"[Postdoctoral researcher] Riccardo Arcodia was the first to notice this very
bright source at a point in the sky where there wasn't a bright source known before," astrophysicist Joern Wilms, a co-author on the paper describing the observation, tells IE.

The instrument, eROSITA, hadn't registered anything out of the ordinary when it scanned the same spot just four hours before, and no trace of the explosion was left when its gaze returned to the source of the explosion four hours later.

"So, imagine a flurry of excited emails...," Wilms says. The researchers eventually realized they'd detected never-before-seen X-rays from a nova explosion. The finding was described in a paper published Wednesday in the peer-reviewed journal Nature

The observation confirms a 30-year-old hypothesis 

As it turns out, the researchers hadn't just observed something cool. They'd collected data to confirm a three-decade-old hypothesis that speculated what would happen when dead star turned itself into a hydrogen bomb in one last blaze of glory.

When a small star burns through all its fuel (the way the Sun will billions of years from now) it shrinks into what's called a white dwarf. Imagine the mass of the Sun compressed into a dense ball no bigger than Earth. When a white dwarf and another star orbit each other (it's actually a fairly common arrangement) the white dwarf's tremendous gravity can pull matter from the star and trigger a thermonuclear reaction called a nova.

Researchers already knew that a nova releases visible light for hours or even days after the explosion. That light can't escape the nova right away because a layer of material collected by the white dwarf, called the accretion disk, is too dense for light to pass through. In 1990, researchers predicted that an invisible form of light — certain X-rays — could leak through the accretion disk right after the explosion occurred.

That's what the researchers found.

It took time for researchers to "squeeze the maximum science" out of the observation

Finding the explosion was a stroke of luck, but interpreting the data required skill and focus. After making the observation, the researchers quickly realized that the flash coincided with a nova explosion that had been observed by other instruments. Making sense of the X-ray data was a tall order.  It "took more than a year because we had to invent the methods to analyze such a bright source," Wilms says. Observing the X-ray flash was one thing. "Squeez[ing] the maximum science out of this data set" required a lot more time, thought, and computer power, he says.

This observation didn't just confirm a hypothesis from 1990. The measurement also provides key data about the explosion that will help researchers fine-tune their understanding of these dramatic cosmic explosions. The observations shed light on exactly when the thermonuclear reaction happened. The data will also help researchers determine the temperature on the surface of the white dwarf, which makes it possible to infer its mass.  

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