A NASA-funded study has answered a longstanding mystery over the cause of X-rays that fill space in our Solar System, but in doing so, it's also detected a complete group of high-energy X-rays that cannot be explained. The research comes from a new study of data recorded by NASA's DXL rocket mission, which was launched in 2012 to resolve the question of what generates these low-energy X-ray discharges, called the diffuse soft X-ray background, in our corner of the galaxy.
Astrophysicist Massimiliano Galeazzi from the University of Miami, says, "We demonstrated that the X-ray contribution from the solar wind charge interchange is about 40% in the galactic level, and even less elsewhere. So the rest of the X-rays must come from the Local Hot Bubble, showing that it exists."
But while the discoveries of the new DXL, Diffuse X-ray emission from the Local galaxy, the mission may have answered an old debate on the source of these space X-rays, which were only themselves identified in the 1960s, the data have now unlocked another galactic mystery.
When DXL (Diffuse X-ray emission from the Local galaxy) took off from White Sands Missile Range in New Mexico back in 2012, it was only a small flight. Carried onboard a NASA Black Brant IX 'sounding rocket', a term used for suborbital research missions, in short, the DXL payload took its readings of X-ray emissions in just 5 minutes above Earth's atmosphere, before free-falling back to the surface. But that small window was long enough for DXL's devices to pick up something the researchers were not supposing: evidence of high-energy X-rays that the scientists say could not perhaps originate from the solar wind or the Local Hot Bubble.
Although nobody knows where these high-energy X-rays are coming from, the scientists are certain they cannot be connected to solar wind or the Local Hot Bubble. After researchers became aware of space X-rays in the 1960s, they knew the emissions had to be created locally, as the levels of neutral gas in our Solar System intended X-rays would be absorbed if they originated from a distant source somewhere else in the galaxy. The first main supposition was that the Local Hot Bubble, a huge peanut-shaped bubble of hot ionized gas about 300 light years long, inside which our Solar System exists, was responsible.
Galeazzi says, "We consider that around 10 million years ago, a supernova exploded and ionized the gas of the Local Hot Bubble. But one supernova would not be enough to generate such a huge cavity and reach these temperatures, so it was perhaps two or three supernova over time, one inside the other."
More recently, researchers have detected that diffuse X-rays could also be created inside the Solar System, which gave rise to the hypothesis that solar wind, precisely something called solar wind charge exchange (SWCX), was making them.When solar wind acts together with pockets of neutral gas in space, it can pick up electrons from the neutral particles. But when the electrons settle down into a stable state, they drop energy, in the form of X-ray emissions.
Thanks to the DXL mission, we now recognize that both of these wonders are responsible for creating the diffuse soft X-ray background in our Solar System. That is a win in itself, particularly since it tells us more about the natural surroundings of the Local Hot Bubble, which researchers still have a slight understanding of.
Uprety says, "Finding the X-ray involvement of the Local Hot Bubble is important for understanding the arrangement surrounding our Solar System. It helps us construct better simulations of the interstellar material in our solar neighborhood."
But as for those unidentified high-energy X-rays ? No one knows for sure where they come from. The scientists say solar wind produces less than a quarter of X-ray emissions at extreme energy levels, and it does not appear like the Local Hot Bubble is responsible either.
Uprety says, "The temperature of the Local Hot Bubble is not high enough to create X-rays in these energy regions. So we are left with an open mystery on the cause of these X-rays."
The good news is that DXL's successor, DXL-2 was launched in December last year, and when Galeazzi's crew has a chance to examine the new data, we might just be capable of solving the most recent mystery too.
The discoveries are reported in The Astrophysical Journal.