Sky and Telescope - July 2017 - 21
DA N A BERRY / SK Y WORKS DIGITA L, INC.
Measuring the pulses' shapes allows us to determine the
size of the star that emitted them. This is because the amount
that the light path bends as it leaves the surface of a neutron
star depends on how large the star is. In other words, two
neutron stars of the same mass but with different sizes, say
20 and 25 km, would create a different pattern of modulation
in the light they emit. These patterns can be calculated very
precisely and compared to the observed pulses, revealing the
sizes of these pulsars.
We are poised to conduct this experiment with an instrument called the Neutron Star Interior Composition Explorer
(NICER), which is scheduled for launch to the International
Space Station (ISS) this year. NICER is approximately a meter
across and consists of carefully designed optical elements that
focus the incoming X-rays onto 56 silicon detectors. After its
journey on a SpaceX resupply mission, it will be unpacked and
mounted onto its home on the ISS platform. A star-trackerbased pointing system will then allow the high-precision
X-ray timing instrument to point to and track pulsar targets
over nearly half of the sky.
What makes NICER unique is its unprecedented capability to record the arrival times of incoming photons with
100-nanosecond precision. This capability will enable the
highly faithful reconstruction of the pulse waveforms for a
number of pulsars. The detectors will also capture the pulsars'
spectra. Coupled with the precisely determined pulse shape,
these measurements will provide all the information necessary
for a precise size measurement within a year after its launch.
Another exciting avenue into the neutron star interior will
become possible through the detection of gravitational waves
with LIGO. Even though the ﬁrst two events detected by LIGO
were coalescing black hole binaries, LIGO is also sensitive to
signals from merging neutron stars (S&T: Dec. 2015, p. 26).
Shortly before the expected coalescence, the pair of inspiraling neutron stars start distorting and pulling each other apart
through tidal interactions, obeying the same principles as the
Moon's effect on Earth's oceans, but far more severe. How
severe it is depends on how deformable the stars are, which in
turn depends on their size, density, and interior composition.
Remarkably, the distortions caused by these tidal interactions
are then encoded into the gravitational wave signals that are
emitted throughout the inspiral, offering one more penetrating
glimpse into the neutron star interior.
If NICER and LIGO experiments conﬁrm the existing measurements of small sizes, the results would point to new physics
that emerges when matter becomes ultra-dense. Or the experiments may offer other surprises - that remains to be seen.
But no matter how small and impenetrable they may seem,
neutron stars will not be able to hold onto their innermost
secrets for much longer.
¢ FERYAL ÖZEL is a professor of astronomy and physics at
the University of Arizona and a current Guggenheim Fellow.
She studies neutron stars and black holes and is a member of
the NICER team.
STARS COLLIDE Astronomers have seen the afterglow
from two neutron stars colliding. Next, they hope to detect
gravitational waves from this kind of merger.
* See S&T: Nov. 2013, p. 12
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