Australian Sky & Telescope - 02.2019 - 03.2019

16 AUSTRALIAN SKY & TELESCOPE February | March 2019

CHIME

MeerKAT

Arecibo

Green Bank

VLA

FAST

Sardinia

Lovell Westerbork

Effelsberg

Nançay

Parkes

EPTA

NANOGrav

PPTA

New members

Now 2020

2010

2030

Vernal

equinox

Sun

2005

2040

2015

2050

2025

2000

2035

2045

more powerful waves drive a faster inspiral rate, so the

systems are visible for shorter time periods. For example, more

massive galaxies have more massive black holes, so a black

hole inspiral in a galaxy such as the gigantic elliptical M

would emit detectable gravitational waves for 4 million years.

But a humbler black hole pair in a smaller galaxy, such as the

Sombrero (M104), would offer a 160-million-year window. The

odds that a very massive binary is sending out gravitational

waves during the time that PTAs are observing is thus lower.

Unless we’re really lucky, it will probably take 10 to

15 years to build up enough data to see deep enough into

space to detect an individual binary. Theorists expect the

gravitational waveform from a single binary to be a very

simple sinusoid, and because the binary system will likely

have an orbital period of several decades, the signal will

change very little over many years.

The longer baseline could make electromagnetic follow-up

easier. If the inspiralling and merging black holes are embedded

in disks of gas — and recent work suggests that this could be

true for supermassive binaries, unlike the smaller ones that

LIGO and Virgo detect — then they would also produce light.

“If a loud individual source is detected, PTAs will give its sky

localisation, opening the possibility to identify the host galaxy

and carry out multimessenger observations of the system,

pretty much as it happened with the LIGO neutron star binary

GW170817,” Sesana says. “The difference is that with PTAs,

everything builds up slowly over the course of the years, and

you don’t hit the jackpot in a snap.”

Going global

Adding to the growing optimism is the fact that scientists

from all three teams are now combining their data sets to

form an even more powerful network: the International

Pulsar Timing Array (IPTA). The IPTA enables scientists

to analyse data from most of the observed pulsars,

including from facilities in both hemispheres. Having

more observations means shorter time gaps in data, which

increases sensitivity to shorter-wavelength gravitational

waves. All of this works to increase sensitivity to both the

stochastic background and individual binaries.

But progress has been slow for a variety of reasons, from

planning across time zones and work cultures to accounting

for each telescope’s individual quirks. There’s also the all-too-

SWORLDWIDE EFFORT Astronomers use 12 radio observatories to track roughly 75

millisecond pulsars. Of these facilities, three — CHIME in Canada, FAST in China, and MeerKAT

in South Africa — are new participants; the others have been involved for at least 10 years.

THE NEXT GRAVITATIONAL-WAVE REVOLUTION