Sky and Telescope - February 2016 - 10

News Notes
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NASA / JPL-CALTECH

BLACK HOLES I How Flares Happen

This series of artist's illustrations shows what might happen when a supermassive black hole
ﬂares. The purplish glow around the black hole represents the corona, a haze of energetic particles
that generate X-ray light. During a ﬂare, the corona contracts (left), then launches (middle and
right), beaming X-rays at observers in the direction of its motion.

Astronomers regularly watch black
holes ﬂare, but they're not actually sure
why ﬂares happen. New X-ray observations now suggest that the brilliant surges
occur when a black hole's atmosphere
contracts and launches away from it.
This atmosphere, called the corona, is
a haze of high-energy electrons that hovers over the black hole and its accretion

disk. Photons from the disk collide with
the electrons and receive a mighty energy
boost, becoming the X-rays we observe.
Dan Wilkins (Saint Mary's University, Canada) and colleagues used the
Swift and NuSTAR space telescopes to
watch how X-ray emission changed during a month-long ﬂare from the active
supermassive black hole in the galaxy

Markarian 335, which lies roughly 300
million light-years away in the constellation Pegasus. They noticed that, even
though the corona was bright and compact during the ﬂare, the disk reﬂected
fewer than half of the X-rays expected.
The corona was still producing X-rays, yet
most of them weren't hitting the disk.
The team thinks that, before a ﬂare,
the corona is spread out across the disk's
surface. Then it gathers itself together
into a vertical, jetlike structure, like a cat
preparing to spring. This compact haze
then launches oﬀ the disk at relativistic
speeds. When an object moves that fast, it
beams more of its radiation in the direction it's moving - which, in this case, is
away from the disk. This eﬀect is called
relativistic beaming. Also, with the corona
now farther from the black hole, more of
the photons can escape the hole's gravity
and reach us. These eﬀects produce the
apparent ﬂare.
The ﬂare ends when the jetlike corona
collapses back to the disk. However,
astronomers don't know why the corona
launches in the ﬁrst place. The study
appears in the December 21st Monthly
Notices of the Royal Astronomical Society.
■ CAMILLE M. CARLISLE

STELLAR I Magnetic Fields Weaken Stars' Inner Sloshing
Astronomers have found an indirect
way to discern the strength of magnetic
ﬁelds within red giant stars.
Stars are roiling balls of plasma, and
as they roil, their ﬂickers and pulsations
reveal clues to what's going on inside.
Jim Fuller (Caltech) and colleagues used
these changes to study a few dozen red
giant stars that had been monitored for
years by the Kepler spacecraft.
Red giants have ﬁnished fusing their
core hydrogen into helium. They often
exhibit half-and-half behavior: ﬁrst one
hemisphere brightens, then it dims as
the other lights up. This half-on, halfoﬀ pattern can be chalked up to sound
waves sloshing through the star's interior.
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Februar y 2016 sky & telescope

But the ﬂuctuations seen in these few
dozen red giants are weaker than those in
hundreds of others that Kepler observed
during the same time period.
To explain this weird behavior, Fuller
and colleagues propose that a superstrong inner magnetic ﬁeld is weakening the brightness variations. In a red
giant star, a large convective envelope of
plasma surrounds a radiative core where
fusion happens. Sound waves churn the
outer envelope, interacting with ocean
wave-like motions called gravity waves
within the star's hot, dense plasma core.
Sound waves can "talk" to the gravity
waves, donating energy to the core. But in
the presence of a strong magnetic ﬁeld,

the gravity waves can't "reply" - they're
trapped in the core. So the sound waves'
energy bleeds into the gravity waves,
which themselves gradually dissipate.
The lost energy has little eﬀect on the
giant star other than the slight weakening
of the hemisphere brightness variations.
To explain the observations, a ﬁeld
of at least 100,000 gauss (10 teslas) must
be lurking in these stars' cores. That's
100,000 times stronger than the Sun's
polar magnetic ﬁeld and 30 times greater
even than sunspots, the strongest magnetic concentrations found on the Sun.
The team published the results in the
October 23rd Science.
■ MONICA YOUNG

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Sky and Telescope - February 2016

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