For the first time, a mission designed to set its eyes on black holes
and other objects far from our solar system has turned its gaze back
closer to home, capturing images of our sun. NASA's Nuclear
Spectroscopic Telescope Array, or NuSTAR, has taken its first picture of
the sun, producing the most sensitive solar portrait ever taken in
high-energy X-rays.
"NuSTAR will give us a unique look at the sun, from the deepest to
the highest parts of its atmosphere," said David Smith, a solar
physicist and member of the NuSTAR team at University of California,
Santa Cruz.
Solar scientists first thought of using NuSTAR to study the sun about
seven years ago, after the space telescope's design and construction
was already underway (the telescope launched into space in 2012). Smith
had contacted the principal investigator, Fiona Harrison of the
California Institute of Technology in Pasadena, who mulled it over and
became excited by the idea.
"At first I thought the whole idea was crazy," says Harrison. "Why
would we have the most sensitive high energy X-ray telescope ever built,
designed to peer deep into the universe, look at something in our own
back yard?" Smith eventually convinced Harrison, explaining that faint
X-ray flashes predicted by theorists could only be seen by NuSTAR.
While the sun is too bright for other telescopes such as NASA's
Chandra X-ray Observatory, NuSTAR can safely look at it without the risk
of damaging its detectors. The sun is not as bright in the
higher-energy X-rays detected by NuSTAR, a factor that depends on the
temperature of the sun's atmosphere.
This first solar image from NuSTAR demonstrates that the telescope
can in fact gather data about sun. And it gives insight into questions
about the remarkably high temperatures that are found above sunspots --
cool, dark patches on the sun. Future images will provide even better
data as the sun winds down in its solar cycle.
"We will come into our own when the sun gets quiet," said Smith,
explaining that the sun's activity will dwindle over the next few years.
With NuSTAR's high-energy views, it has the potential to capture
hypothesized nanoflares -- smaller versions of the sun's giant flares
that erupt with charged particles and high-energy radiation. Nanoflares,
should they exist, may explain why the sun's outer atmosphere, called
the corona, is sizzling hot, a mystery called the "coronal heating
problem." The corona is, on average, 1.8 million degrees Fahrenheit (1
million degrees Celsius), while the surface of the sun is relatively
cooler at 10,800 Fahrenheit (6,000 degrees Celsius). It is like a flame
coming out of an ice cube. Nanoflares, in combination with flares, may
be sources of the intense heat.
If NuSTAR can catch nanoflares in action, it may help solve this decades-old puzzle.
"NuSTAR will be exquisitely sensitive to the faintest X-ray activity
happening in the solar atmosphere, and that includes possible
nanoflares," said Smith.
What's more, the X-ray observatory can search for hypothesized dark
matter particles called axions. Dark matter is five times more abundant
than regular matter in the universe. Everyday matter familiar to us, for
example in tables and chairs, planets and stars, is only a sliver of
what's out there. While dark matter has been indirectly detected through
its gravitational pull, its composition remains unknown.
It's a long shot, say scientists, but NuSTAR may be able spot axions,
one of the leading candidates for dark matter, should they exist. The
axions would appear as a spot of X-rays in the center of the sun.
Meanwhile, as the sun awaits future NuSTAR observations, the
telescope is continuing with its galactic pursuits, probing black holes,
supernova remnants and other extreme objects beyond our solar system.
NuSTAR is a Small Explorer mission led by Caltech and managed by
NASA's Jet Propulsion Laboratory, also in Pasadena, for NASA's Science
Mission Directorate in Washington. The spacecraft was built by Orbital
Sciences Corporation, Dulles, Virginia. Its instrument was built by a
consortium including Caltech; JPL; the University of California,
Berkeley; Columbia University, New York; NASA's Goddard Space Flight
Center, Greenbelt, Maryland; the Danish Technical University in Denmark;
Lawrence Livermore National Laboratory, Livermore, California; ATK
Aerospace Systems, Goleta, California; and with support from the Italian
Space Agency (ASI) Science Data Center.
NuSTAR's mission operations center is at UC Berkeley, with the ASI
providing its equatorial ground station located at Malindi, Kenya. The
mission's outreach program is based at Sonoma State University, Rohnert
Park, California. NASA's Explorer Program is managed by Goddard. JPL is
managed by Caltech for NASA.
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