In the last century, humans realized that space is filled with types of light we can't see. 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.
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It wasn't until the flight of the DXL sounding rocket, short for Diffuse X-ray emission from the Local galaxy, that scientists had concrete answers about the X-rays' sources.
DXL's data confirms some of our ideas about where these X-rays come from, in turn strengthening our understanding of our solar neighborhood's early history. But it also reveals a new mystery -- an entire group of X-rays that don't 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 theorized area of hot interstellar material that surrounds our solar system.
"We show that the X-ray contribution from the solar wind charge exchange is about forty percent in the galactic plane, and even less elsewhere," said Massimiliano Galeazzi, an astrophysicist at the University of Miami and an author on the study. "So the rest of the X-rays must come from the Local Hot Bubble, proving that it exists."
However, DXL also measured some high-energy X-rays that couldn't 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 Youaraj Uprety, lead author on the study and an astrophysicist at University of Miami at the time the research was conducted. "So there's an unknown source of X-rays in this energy range."
In the decades since we first discovered the X-ray emission that permeates space, three main theories have been bandied about to explain its origins. First, and quickly ruled out, was the idea that these X-rays are a kind of background noise, coming from the distant reaches of the universe.
Our galaxy has lots of neutral gas that would absorb X-rays coming from distant sources - meaning that these X-rays must originate somewhere near our solar system.
So what could produce this kind of X-ray so close to our solar system? Scientists theorized that there was a huge bubble of hot ionized 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 ionized 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."
The Local Hot Bubble was the prevailing theory for many years. Then, in the late 1990s, scientists discovered another source of X-rays - a process called solar wind charge exchange.
Our sun is constantly releasing solar material in all directions, a flow of charged particles called the solar wind. Like the sun, the solar wind is made up of ionized gas, where electrons and ions have separated. This means that the solar wind can carry electric and magnetic fields.
When the charged solar wind interacts with pockets of neutral gas, where the electrons and ions are still tightly bound together, it can pick up electrons from these neutral particles, exciting them.
As these electrons settle back into a stable state, they lose energy in the form of X-rays - the same type of X-rays that had been thought to come from the Local Hot Bubble.
The discovery of this solar wind X-ray source posed a problem for the Local Hot Bubble theory, since the only indication that it existed were these X-ray observations. But if the hot bubble did exist, it could tell us a lot about how our corner of the galaxy formed. ■