For decades, scientists have been trying to unravel the source of the mysterious X-rays that seem to permeate space within our Solar System.
Now, thanks to a NASA-funded study, researchers have finally solved part of the puzzle—but in doing so, they have uncovered an even deeper mystery.
Using data from the DXL (Diffuse X-ray emission from the Local galaxy) rocket mission, which launched in 2012, scientists confirmed that much of the diffuse soft X-ray background comes from a long-theorized region of hot gas known as the Local Hot Bubble.
However, the same data also revealed the presence of high-energy X-rays that have no known source.
Unraveling the Mystery of X-ray Emissions
Before this study, there were two competing hypotheses about the origins of these low-energy X-rays.
Some researchers believed that they were the result of interactions between solar wind and neutral gases in space, while others proposed that they originated from the Local Hot Bubble, a massive region of ionized gas that envelops our Solar System.
According to astrophysicist Massimiliano Galeazzi from the University of Miami, the new findings confirm that both sources contribute to the X-ray emissions, with solar wind accounting for about 40% of the total emissions in the galactic plane and even less elsewhere.
This means the remaining X-rays must be coming from the Local Hot Bubble, proving its existence.
But this resolution has led to another perplexing question: what is causing the additional, higher-energy X-ray emissions detected by DXL?
The Unexpected Discovery: High-Energy X-rays
When the DXL payload launched aboard a NASA Black Brant IX sounding rocket in 2012, it had a limited mission time—just five minutes above Earth’s atmosphere.
But even within this short window, its instruments detected high-energy X-ray emissions that neither solar wind charge exchange (SWCX) nor the Local Hot Bubble could explain.
“At higher energies, [the solar wind and the Local Hot Bubble] contribute less than a quarter of the X-ray emission,” said Youaraj Uprety, a researcher at Middle Tennessee State University.
“So there’s an unknown source of X-rays in this energy range.”
A Paradigm Shift in Our Understanding of Space X-rays
For decades, scientists assumed that any X-ray emissions detected in our Solar System had to come from local sources.
This assumption stemmed from the fact that neutral gas within our Solar System would absorb X-rays originating from distant sources, making it nearly impossible for X-rays from the rest of the galaxy to reach us.
Initially, the Local Hot Bubble hypothesis was proposed to explain the phenomenon.
This bubble, thought to be about 300 light-years long, is believed to have been formed by multiple supernova explosions millions of years ago.
“We think that around 10 million years ago, a supernova exploded and ionized the gas of the Local Hot Bubble,” Galeazzi explained.
“But one supernova wouldn’t be enough to create such a large cavity and reach these temperatures—so it was probably two or three supernovae over time, one inside the other.”
Later, scientists discovered another potential source of X-rays: solar wind charge exchange (SWCX).
This process occurs when charged particles from the Sun’s solar wind collide with neutral gases in space, stealing electrons.
When the electrons return to a stable state, they emit X-rays.
But the latest findings from the DXL mission suggest that neither the Local Hot Bubble nor SWCX can account for all the X-ray activity in our region of space.
A New X-ray Source Emerges
The DXL data show that solar wind contributes less than 25% of X-ray emissions at higher energy levels.
That means that even with the Local Hot Bubble factored in, a significant fraction of X-rays remain unaccounted for.
“The temperature of the Local Hot Bubble is not high enough to produce X-rays in this energy range,” Uprety stated.
“So we’re left with an open question on the source of these X-rays.”
So, what could be producing these high-energy X-rays? Scientists have proposed a few possibilities:
- Undetected supernova remnants that are closer to us than previously thought.
- Exotic cosmic phenomena, such as interactions between dark matter and normal matter.
- A previously unknown astrophysical mechanism occurring within our Solar System.
Looking Ahead: The DXL-2 Mission
The mystery is far from solved, but researchers are already taking steps to uncover more information.
In December of last year, NASA launched DXL-2, a follow-up mission designed to build on the discoveries of its predecessor.
The hope is that by gathering additional data, scientists will be able to narrow down the possibilities and finally determine the source of these unexplained X-ray emissions.
“This is why we explore,” said NASA planetary science director Jim Green. “For years, scientists believed that these X-ray emissions were fully understood.
Now, we’ve found something entirely unexpected, and it’s forcing us to rethink our models. That’s what makes space exploration so exciting.”
As the team sifts through the latest data, one thing is certain: space is far more mysterious than we ever imagined. The next big discovery might be just around the corner.
