Australian Sky & Telescope — January 01, 2018

40 AUSTRALIAN SKY & TELESCOPE January 2018

COSMIC COLLISION

X-Shooter instrument at the Very

Large Telescope and other instruments

have indeed revealed the existence

of so-called rare earth elements and

other heavy metals like platinum, lead

and gold. The observations appear to

confirm the theory that the majority

of elements more massive than iron

are produced by the decay of nuclear

matter in the aftermath of neutron star

collisions, rather than in supernova

explosions.

Apparently, with the discovery of the

counterpart of GW170817, scientists

also literally struck gold. Edo Berger

(Harvard-Smithsonian Center for

Astrophysics) once calculated that a

run-of-the-mill neutron star merger

may produce no less than 10 times the

mass of the Moon in pure gold. Gijs

Nelemans (Radboud University, The

Netherlands) thinks it may well be

much higher, up to a few Earth masses.

According to Edward van den

Heuvel (University of Amsterdam), a

retired expert on compact binary star

evolution, 16 binary neutron stars have

so far been discovered in the Milky

Way. “From this number, I estimate

that neutron star collisions occur once

every 50,000 years or so in our Milky

Way galaxy,” he says. “Over the age

of the Milky Way, that amounts to a

few hundred thousand of these gold-

spawning events in just one galaxy.

That’s a lot of gold.”

Remaining mysteries

A few mysteries remain, though. One

is the nature of the gamma-ray signal

observed by Fermi. If GRB170817A was

a regular gamma-ray burst, one of its

jets must have been aimed at our home

planet. But in that case, astronomers

would have expected to detect a

gamma-ray brightness at least 10,000

times more powerful than what they

did, given the small distance of 130

million light-years.

However, the jets should also have

produced prompt X-ray emission, which

was not detected. Even if we happened

to observe the jet at an angle of, say, 20

degrees (which might explain the low

gamma luminosity), X-rays would have

been expected, too.

Kalogera, Eleonora Troja (NASA

Goddard and University of Maryland),

wholedtheX-rayfollow-up,and

othersstillthinkthatanoff-axisjet

isthemostlikelyexplanationforthe

weakness of the gamma-ray burst. Troja

also says that the delay in X-rays would

be natural when looking at the jet from

an angle.

Mansi Kasliwal (Caltech) suggests

adifferentscenario,inwhichthe

jetsgetstuckinathickcocoonof

material that was ejected by the

neutronstarcollision.Inhermodel,

when the cocoon becomes less dense,

itmaybrieflyemitgamma-raysitself,

atamuchweakerlevel.Wijershad

putforwardasimilarscenarioto

explain the strange behaviour of the

burstGRB980425,whichwasalso

relatively close and surprisingly weak,

andcoincidedwithasupernova-

like explosion known as SN 1998bw.

Wijersalsonotesthatthemodel

neatly accounts for the transition of

theopticalcounterpartofGRB170817

from blue to red wavelengths within

48 hours.

Adetailedanalysisofkilonova

observationsmayeventuallysolvethe

issue. And future observations of the

siteofthecosmiccatastrophecouldalso

shed light on another as-yet-unsolved

mystery:whatwasthefateofthetwo

neutronstars?Sure,asmallfractionof

EYES ON THE SKY

Two telescope systems in Western Australia were ideally placed to investigate the

discovery. Curtin University’s Desert Fireball Network, which scans the sky for bright

meteors, was the only facility of its kind to be looking in the right direction at the time

of the neutron stars collision. “It was tremendously exciting to be part of something

like this, and that our facility designed for planetary science was able to contribute

to such an important event,” said the project’s leader, Professor Phil Bland.

And the Murchison Widefield Array (MWA) radio telescope’s wide field of view en-

abled its astronomers to search a large area of the sky immediately after the gravita-

tional wave detection, in an effort to narrow down the event’s exact location in space.

“The data that were collected and captured is incredibly exciting in terms of our

understanding of how the universe ticks, and for Curtin researchers to be able to

play a part in this reflects the high value of our planetary science and radio astrono-

my work,” said MWA Director and Curtin Associate Professor, Randall Wayth.

Antennae of the Murchison Widefield Array.

ICRAR/MWA