14 ASTRONOMY • APRIL 2020
QUANTUM GRAVITY
Pulsars are tiny, compact objects
called neutron stars that spin
rapidly, beaming radiation into space.
From its perch outside the International
Space Station, the Neutron star Interior
Composition Explorer (NICER) scans the
skies for X-rays from such objects. Now,
astronomers using NICER have mapped
the surface of a pulsar for the first time.
The result challenges the textbook picture
of a pulsar’s appearance.
In a series of papers published in The
Astrophysical Journal Letters in December
2019, two research teams — one led by
the University of Amsterdam and the
other led by the University of Maryland
— observed the pulsar J0030+0451, or
J0030 for short, which lies 1,100 light-
years away. Thanks to NICER’s ability
to clock the arrival of X-rays from the
pulsar with extreme precision (better
than 100 nanoseconds), they were able
to map the star’s surface and measure
its mass with unprecedented accuracy.CHARTING A STAR
The teams determined J0030 is between
1.3 and 1.4 times the mass of the Sun and
roughly 16 miles (26 kilometers) wide.By contrast, our Sun stretches just over
864,000 miles (1.4 million km) across.
NICER also allowed the teams to
map the location of hotspots on J0030’s
surface. The simple, textbook image used
to describe pulsars says they have two
hotspots: one at each magnetic pole. As
the star spins, the hotspots shoot radia-
tion out into space in thin beams, like
a lighthouse. If one or both beams pass
over Earth, astronomers observe a pulsar.
Mapping the hotspots required
supercomputer modeling to disentangle
where the X-rays NICER saw originated
on the star’s surface. J0030 is oriented
with its northern hemisphere pointed
toward Earth. So, the teams expected
a hotspot near the north pole.A NEW PICTURE
Instead, J0030 has two or three hotspots,
all in the southern hemisphere. Because
pulsars are extremely dense, their gravitybends space-time around them, offering
a glimpse of the far side of the star even
as it rotates out of view. The Amsterdam
team believes the pulsar has one small,
circular spot and one thin, crescent-
shaped spot spinning at lower latitudes.
The Maryland team found the X-rays
could alternatively be coming from two
oval spots in the southern hemisphere,
as well as one cooler spot close to the
star’s south pole.
Neither solution is the simple picture
scientists expected, indicating the pulsar’s
magnetic field, which causes the hotspots,
is more complex than originally assumed.
The result certainly leaves astronomers
wondering. But, NICER science lead and
study co-author Zaven Arzoumanian
said in a press release, “It tells us NICER
is on the right path to help us answer an
enduring question in astrophysics: What
form does matter take in the ultra-dense
cores of neutron stars?” — A.K.THE FIRST MAP OF A NEUTRON STAR
NASA’s NICER reveals these small stars are even more complex
than astronomers originally thought.The number of years that NASA’s now-completed
Operation IceBridge actively monitored the
11 glaciers, ice sheets, and polar sea ice.HOT SPOTS.
Two teams of
astronomers used
NASA’s NICER to
map the surface
of a pulsar. On the
left is one team’s
model of the star,
which shows one
circular and one
crescent-shaped
hotspot in the
star’s southern
hemisphere. At
right is the other
team’s model,
which instead
finds the star has
three circular
hotspots, one of
which is near the
south pole. NASA’S
GODDARD SPACE FLIGHT
CENTER