Origin of mysterious cosmic X-rays found

Scientists have confirmed that some of the X-ray emissions in our universe originate from a huge bubble of hot ionised gas enveloping our solar system, a finding that strengthens our understanding of the solar neighbourhood's early history.

They also detected an entire group of X-rays that do not come from any known source - unveiling a new cosmic mystery.

In the last century, humans realised that space is filled with types of light we can not see – from infrared signals released by hot stars and galaxies, to the cosmic microwave background that comes from every corner of the universe.

Some of this invisible light that fills space takes the form of X-rays, the source of which has been hotly contended over the past few decades.

Researchers from University of Miami in the US confirmed some of our ideas about where these X-rays come from, using data from the DXL (Diffuse X-ray emission from the Local galaxy) sounding rocket, which was launche in 2012.

However, the data also unveiled a new mystery – an entire group of X-rays that do not come from any known source.

The two known sources of X-ray emission are the solar wind, the sea of solar material that fills the solar system, and the Local Hot Bubble, a theorised area of hot interstellar material that surrounds our solar system, researchers said.

"We show that the X-ray contribution from the solar wind charge exchange is about forty per cent in the galactic plane, and even less elsewhere," said Massimiliano Galeazzi, an astrophysicist at the University of Miami in the US.

"So the rest of the X-rays must come from the Local Hot Bubble, proving that it exists," said Galeazzi.

DXL also measured some high-energy X-rays that could not possibly come from the solar wind or the Local Hot Bubble.

"At higher energies, these sources contribute less than a quarter of the X-ray emission," said lead author Youaraj Uprety, who was at University of Miami during the research. "So there's an unknown source of X-rays in this energy range," Uprety said.

Scientists theorised that there was a huge bubble of hot ionised gas enveloping our solar system, with electrons energetic enough that they could release X-rays like this. They called this structure the Local Hot Bubble.

"We think that around 10 million years ago, a supernova exploded and ionised the gas of the Local Hot Bubble," said Galeazzi.

"But one supernova wouldn't be enough to create such a large cavity and reach these temperatures – so it was probably two or three supernova over time, one inside the other," he said.

"Identifying the X-ray contribution of the Local Hot Bubble is important for understanding the structure surrounding our solar system," said Uprety, who is now an astrophysicist at Middle Tennessee State University.

"It helps us build better models of the interstellar material in our solar neighbourhood," he said. The study was published in the Astrophysical Journal.