NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) was built to observe black holes and other objects beyond the solar system, but the X-ray telescope has lately turned back its gaze to an object closer to home.
The U.S. space agency shared on Monday the first image taken by NuSTAR of the sun producing the most sensitive measurement to date of high energy solar X-rays ever taken. The image, which is marked by green and blue portions that show the sun's highly energetic X-ray emission, was overlaid in an image taken by another NASA space telescope, the Solar Dynamics Observatory (SDO), which showed mostly red parts that represent ultraviolet light.
The bluish-green X-ray patches occurred near the areas where gas temperatures reach over 3 million degrees Fahrenheit while the ultraviolet image captured by the SDO offers a glimpse of lower-energy materials with lower temperature. Collated, the two images suggest that the sun's active regions, which can be seen as loops and swirls, may not be altogether responsible for producing the highly energetic X-rays that cluster in areas above some active regions.
"Although the structures can look similar in the extreme ultraviolet images, they in fact contain material at different temperatures," said Iain Hannah, an astrophysicist from the University of Glasgow. "We don't know the exact mechanism(s) that do the heating."
The idea to use NuSTAR to study the sun was first considered about seven years ago before the space telescope was launched in 2012. NuSTAR principal investigator Fiona Harrison said that the idea of using the most sensitive high energy space X-ray telescope intended to study objects in deep space for the sun seemed absurd at first.
Solar physicist and NuSTAR team member David Smith, however, eventually persuaded the citing that the theoretical faint- x-ray flashes can only be seen by the telescope. Although other telescopes cannot look at the sun because of its brightness, NuStar can safely do so without posing risks to its detector as the solar system's main star is not as bright in the higher-energy X-ray NuStar detects.
"NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares," said Smith.
Should the hypothesized nanoflares exist, they could explain why the corona, the sun's outer atmosphere, is hotter than the surface of the sun, a mystery known as the "coronal heating problem."
NuSTAR's first image of the sun proves that the telescope can also do a good job of imaging and gathering data about it. Future images are anticipated to gather better information about the sun as its activity is expected to dwindle over the next few years.
