Sky and Telescope - January 2016 - 18

News Notes

SUN I Resonant Waves
Might Heat Corona
Astronomers have detected waves
working together in the solar atmosphere,
creating turbulence that superheats the
corona's ionized gas.
The Sun's corona is more than a hundred times hotter than the photosphere
below it. One explanation for this temperature surge is Alfvénic waves, motions
in the solar plasma governed by the magnetic ﬁeld. The corona is essentially a sea
of these waves. Solar physicists think that
the waves carry enough energy to heat
the corona - but pinning down how the
magnetic motions do it has been tough.
New spacecraft observations, coupled
with computer simulations, might solve
the mystery. Takenori Okamoto (JAXA),
Patrick Antolin (National Astronomical
Observatory of Japan), and their team
watched threads of superheated plasma
wiggle in 3D in a coronal ﬁlament. These
prominences are cooler and denser than
the surrounding corona, making them
easier to observe.
Using these data, the researchers realized it's how the waves add together that
matters. Seen from the side, the threads
vibrate with transverse waves, as a plucked
guitar string does. But the threads also
twist toward and away from us, just as
your hand twists one way, then the other,
on your wrist. When these two types of
vibrations travel at the same speed, they
resonate with the same beat.
Guided by simulations, the team
suggests in two papers in the August
10th Astrophysical Journal that the waves'
resonance causes the guitar-string waves
to transfer their energy to the twist
waves, pumping up their motion. This in
turn exacerbates the shear between the
threads' boundaries and the thinner, hotter gas in the atmosphere around them,
creating eddies. The eddies combine to
create larger-scale turbulence. And the
turbulence, due to the friction and electrical currents it creates, steals the wave
energy from the threads and dumps it
into the coronal plasma, heating it.
■ CAMILLE M. CARLISLE

Januar
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STELLAR I White Dwarfs with Hiccups
Observations of two cool white dwarfs
show irregular outbursts in the stars' otherwise steady rhythm of pulsations. Pulsating white dwarfs, or ZZ Ceti stars, are
a type of white dwarf that glows and fades
in a steady rhythm. White dwarfs start
to pulse when they cool from an initial
temperature of around 100,000K to about
12,600K. The atmosphere becomes a mix
of ionized and neutral hydrogen atoms,
and this mix stores and releases energy in
regular intervals of a few minutes, driving
pulsations. These make the white dwarf
brighten and fade by a few percent in regular beats, like a person's rhythmic breathing. The star continues to cool slowly over
time, but once it falls below about 10,800K,
it ceases pulsating altogether.
Using Kepler data, which has far fewer
gaps than ground-based observations, J. J.
Hermes (University of Warwick, UK) and
colleagues tracked two pulsating white

dwarfs, KIC 4552982 and PG 1149+057.
The temperatures of the two stars are
both about 11,000K, cooler than most
ZZ Cetis previously studied. The team
discovered that hiccups interrupted
the dwarfs' otherwise even pulsations.
Shooting high above the expected crests
and falls in brightness every few minutes appeared sporadic outbursts lasting
several hours. The regular pulsations
change the stars' brightness by 1% to 3%,
but KIC 4552982's outbursts were 2% to
17% above the norm, and PG 1149+057's
hiccups spiked up to 45% above the star's
mean brightness. As the researchers
report in the August 10th Astrophysical
Journal and September 1st Astrophysical
Journal Letters, they don't know why these
outbursts happen, but the surges might
be the dwarfs' last gasps before pulsations shut down.
■ NATALIA GUERRERO

MISSIONS I India Launches First Astronomy Satellite
Indian astrophysics got a boost on
September 28th when the Indian Space
Research Organization (ISRO) launched
the Astrosat spacecraft into a low-altitude
orbit. The multipurpose observatory will
study the universe across the X-ray spectrum, accompanied by simultaneous visible- and ultraviolet-light observations.
India has successfully sent spacecraft
to the Moon and Mars and, more recently,
has lofted X-ray detectors aboard highaltitude balloons and suborbital sounding
rockets. But this is ISRO's ﬁrst astronomical satellite. It's capable of studying everything from nearby white dwarfs, pulsars,
and supernova remnants to faraway galaxy
clusters. Its data will be available to the
Indian astronomy community via proposals for observations.
Astrosat's ﬁve instruments each
observes a diﬀerent wavelength range.
They comprise low-resolution X-ray spectral instruments, a binocular Ritchey-Chrétien telescope, and a detector with a huge

10°°-by-90° ﬁeld of view that will scan the
sky for transient X-ray sources.
The project is essentially a combination
of NASA's Swift and now-retired Rossi
X-ray Timing Explorer (RXTE) satellites.
Like Swift, Astrosat will hunt for X-ray transients and observe these sources across
the visible-to-X-ray spectrum. And like
RXTE, Astrosat will measure the arrival
time of each photon. Its Large Area Xenon
Proportional Counters (LAXPC) have the
largest collecting area of any X-ray instrument ever built, and it's currently the only
one capable of studying X-ray ﬂuctuations
over millisecond time scales.
Astrosat joins several other X-ray observatories already in orbit: Chandra makes
out ﬁne details in low-energy X-ray images,
NuSTAR brings high-energy X-rays into
sharp focus, Swift monitors the sky for distant explosions bright in X-rays and gamma
rays, and ESA's XMM-Newton is a light
bucket for low-energy X-rays. ✦
■ DAVID DICKINSON

Sky and Telescope - January 2016

Table of Contents for the Digital Edition of Sky and Telescope - January 2016