10 ASTRONOMY • MARCH 2020
The Neutron star Interior Composition
Explorer (NICER) sits outside the
International Space Station, scan-
ning the skies for blasts of X-rays from
neutron stars — the dense remnants of
massive stars — and black holes. On
August 20, the instrument saw a sudden
spike of X-rays that released as much
energy in 20 seconds as the Sun puts
out in about 10 days.
This burst was the brightest NICER
had ever recorded. And it showed a
combination of features that astrono-
mers hadn’t seen together in a single
event. Astronomers studying the burst
reported their findings October 23 in
The Astrophysical Journal Letters.
X-ray bursts often come from neu-
tron stars in binary systems containing
a neutron star and another object. This
flash came from SAX J1808.4–3658,
or J1808 for short, a neutron star part-
nered with a brown dwarf, which is an
object too massive to be a planet but
not massive enough to be a star.
As J1808 pulls hydrogen off its
companion, the gas accumulates on
the neutron star. As more gas falls on
the star, the intense heat and pres-
sure turns some of this hydrogen into
helium. When enough hydrogen settles
onto the surface, the added weightmakes the helium suddenly fuse into
carbon — and explode.
That’s what caused the August
burst. Astronomers saw evidence that
the explosion blew material off of the
neutron star in two layers. A layer of
hydrogen was ejected first, followed
by a layer of helium. The helium layer
outraced the hydrogen; then the helium
slowed, stopped, and settled back
down onto the neutron star’s surface.
After that, J1808 grew brighter by about
20 percent before finally fading.
NICER has previously seen X-ray
bursts corresponding to the ejection
of a hydrogen or helium layer, but not
both in a single burst like this, Peter
Bult at NASA’s Goddard Space Flight
Center, one of the paper’s authors, tells
Astronomy. Additionally, astronomers
cannot explain the brief brightening
episode. Finally, NICER spotted X-ray
“flickers” from X-rays reflecting off the
swirling disk of hydrogen surrounding
J1808, which is made of material pulled
off the brown dwarf.
“I’m most excited by the fact that we
see these various phenomena in the
same burst,” Bult says. Taken together,
these observations could tell scientists
much about the physics of X-ray bursts
and neutron stars. — ERIK A K. CARLSON, A.K.NICER sees brightest
X-ray burst yet
THE DANCE
OF NEPTUNE’S
MOONS
Neptune’s moons Naiad and
Thalassa are a matched pair. Each
only about 60 miles (100 kilome-
ters) wide, they orbit the ice giant
every 7 and 7.5 hours, respectively.
And yet, they don’t collide, thanks
to strange cosmic choreography
called an orbital resonance.
Resonances are common in the
solar system — for example, Pluto
circles the Sun twice for every
three orbits Neptune completes.
Objects in an orbital resonance are
more stable, which is why these
patterns pop up so frequently.
But the resonance between
Naiad and Thalassa is unique in
the solar system so far. Naiad’s
orbit is tilted almost 5° relative
to Thalassa’s. For an observer
on Thalassa, Naiad appears to
weave up and down in a wavelike
motion. The motion keeps Naiad
about 2,200 miles (3,540 km) away
from Thalassa every time it laps
the slower-moving moon.
The newly discovered resonance
was published March 1, 2019, in
Icarus. According to co-author
Marina Brozović of NASA’s Jet
Propulsion Laboratory, the current
arrangement implies that an
orbital resonance with some other
moon must have bumped Naiad
into this tilted orbit. After that,
Naiad and Thalassa were able to
begin their dance. — E.K.C., A.K.QUANTUM GRAVITY
IN A FLASH. X-ray bursts from a neutron star, shown in this artist’s concept, eject hydrogen and
helium from the surface of the star. Additionally, X-rays from the explosion can bounce around the
swirling accretion disk of gas around the neutron star (blue and orange), causing the object to flicker.NASA/JPL-CAL
TECHNASA
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