All around us there are different types of light, some visible and some that elude the eye. Of the latter kind, X-rays are a common form of electromagnetic radiation. Despite its abundance, the source of X-rays remains one of the most contentious topics in science. A new study has attempted to shed some light on the subject, while also uncovering an astounding mystery: a hitherto-unidentified family of X-rays that doesn’t seem to be coming from any known source.
Recently published in The Astrophysical Journal, the latest research is based on information collected during NASA’s Diffuse X-ray emission from the Local Galaxy (DXL) mission. According to the scientists, the findings confirm our general ideas about the origin of X-rays: solar winds and the Local Hot Bubble. Solar wind refers to the mass of charged particles that emanate from the sun’s outer atmosphere and spread throughout the solar system. The Local Bubble, on the other hand, is believed to searing cavity surrounding our solar system. The team explained:
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. So the rest of the X-rays must come from the Local Hot Bubble, proving that it exists. At higher energies, these sources contribute less than a quarter of the X-ray emission. So there’s an unknown source of X-rays in this energy range.
As pointed out by the researchers, the DXL was launched in 2012 using a sounding rocket, which is a type of research rocket that flies for only 15 minutes or so. During its flight, the rocket observes space beyond Earth, taking important measurements that would be impossible to gather from the ground. This is even more important in case of X-rays, since most of these emissions are blocked by our atmosphere.
Apart from ROSAT, DXL is the only spacecraft to study these rays. The DXL probe took place when our planet was going through what is called helium-focusing cone, which is a localized region in space occupied primarily by neutral helium several times denser than the remainder of our solar system. Youaraj Uprety, the study’s lead author, said:
The solar system is moving through interstellar space at about 15 miles per second. This space is filled with hydrogen and helium. The helium is a little heavier, so it carves around the sun to form a tail.
According to the team, the measurements taken in the helium focus cone have in turn enabled them to estimate the percentage of X-rays coming from solar winds as well as from the Local Hot Bubble. The former accounts for nearly 40-percent of the commonly-observed emissions, with the remaining coming from the Local Hot Bubble. At higher levels of energy, solar winds seem to produce less than 25-percent of the ambient X-rays. The amount generated by the Local Hot Bubble is negligible. The researchers went on to say:
The temperature of the Local Hot Bubble is not high enough to produce X-rays in this energy range.So we’re left with an open question on the source of these X-rays… Identifying the X-ray contribution of the Local Hot Bubble is important for understanding the structure surrounding our solar system. It helps us build better models of the interstellar material in our solar neighborhood.
While the study has confirmed the sources of X-rays at lower energy levels in our solar system, it also pointed to an unknown source of X-rays that is yet to be uncovered.