Sky and Telescope - December 2015 - 30

HOW A PULSAR TIMING ARRAY WORKS Pulsars broadcast regular "beats" to us as their radiation jets spin in and out
of view. The arrival time of those beats depends on the distance
between us and the pulsar. Normally, that distance doesn't suddenly change, and so neither does the pulse's arrival time. But
when gravitational waves (concentric arcs) pass between us and
a pulsar, they will slightly stretch and squeeze the space perpendicular to their direction of motion - in the 2D representation
here, both in the direction of the double arrows and in and out
of the page. The pulsar's distance thus oscillates, making some
pulses arrive earlier and others later than expected. How much
the arrival time changes depends on how everything lines up: in
the 2D visualization here, where all objects are in the plane of the
waves' propagation, the effect will be stronger for pulsar-Earth
lines that are closer to being at right angles with the wave's direction of motion (length of arrows). By combining arrival times for
several pulsars in different parts of the sky, astronomers should
be able to figure out where the gravitational waves came from.

Of course, it's not as straightforward as it seems.
There are loads of other effects that could subtly influence pulse arrival times, explains Ryan Shannon (CSIRO,
Australia), all of which astronomers have to fully understand and compensate for. Moreover, pulsar distances
are generally not known precisely enough to accurately
apply the technique. Still, by measuring a large ensemble
of pulsars, and by carrying out a complicated statistical
analysis, the pulsar timing array (PTA) technique has a
lot of potential. Astrophysicists are excited by the prospects: while LIGO-like interferometers are sensitive to the
high-frequency gravitational waves from merging neutron stars (a few hundred hertz), PTAs would detect the
long-wavelength, very low-frequency waves from binary
black holes in distant galaxies (see graph on facing page).
In fact, says Shannon, the fact that a decade-long data
set from the Australian Parkes Pulsar Timing Array
(PPTA, published in 2013) didn't show any evidence
for those nanohertz waves has already ruled out many
existing evolutionary models that predict the frequency
30

December 2015 sky & telescope

of merging galaxies and coalescing supermassive black
holes. Meanwhile, similar programs are now in progress
in Europe (European Pulsar Timing Array, EPTA) and
the United States (North American Nanohertz Observatory for Gravitational Waves, NANOGrav); together the
three constitute the International Pulsar Timing Array
(IPTA). The hope is that a firm, statistically significant
detection will be made soon. Says Pablo Rosado (Swinburne University of Technology, Melbourne): "With so
many models already ruled out by our current nondetection, we're starting to get worried."
Frankly, you can't blame gravitational-wave scientists
for being at least a bit worried. Between 2002 and 2010,
Initial LIGO did not detect a single neutron star merger
within 50 million light-years, although that would've been
a lucky catch. But searches for long-duration "continuous
wave signals" - from known X-ray binaries like Scorpius
X-1 or from young, rapidly spinning neutron stars that are
not perfectly spherical - also have come up empty. PTAs:
nothing yet. Advanced LIGO - well, you would've seen
front-page headlines if there had been a detection by now.
Still, confidence abounds. "If there are no signals out
there, how would you explain the binary pulsar?" asks
Raab. And in Gwangju, a self-assured Deirdre Shoemaker (Georgia Institute of Technology) told her audience that "the first gravitational-wave signal that we're
going to detect has already passed Proxima Centauri." In
other words: we'll hit the jackpot before 2019.

Discovery Space
OK, so let's assume that we're indeed on the doorstep
of a revolutionary discovery - the confirmation of a
century-old prediction of general relativity, and the opening up of a new window on the high-energy universe.
Then what?
Fast forward two decades. Somewhere on the planet
- most likely in Europe - construction workers are

ical
nom
o
r
t
60°
1 as unit
Sun

NASA

S&T: LEAH TISCIONE

1OO Years of General Relativity: Gravitational Waves

eLISA ORBIT ESA's eLISA mission will comprise three spacecraft flying in a triangle about a million kilometers on a side.
Tilted 60°° to the ecliptic, the triangle formation will cartwheel
around the Sun behind Earth in our planet's orbit. Free-falling
masses inside each spacecraft will hover undisturbed by forces
other than gravitation.



Sky and Telescope - December 2015

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Sky and Telescope - December 2015 - 1
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