http://www.skyandtelescope.com.au 41their combined mass was ejected into
space, but what happened to the rest?
Did the two city-sized stars merge into
a hyper-massive neutron star of a few
solar masses, or did they collapse into
a stellar-mass black hole? Astronomers
have only detected a couple of neutron
stars that weigh in above 2 solar masses
— an upper limit that might have
implications for the physics of these
stars. The merger remnant’s mass could
potentially be extremely informative.
The LIGO data can’t provide the
answer; the final stages of the merger
event weren’t observed. With the earlier
black hole collisions, LIGO could
detect hints of the collision’s ‘ring-
down phase,’ a brief period in which
the amplitude of the Einstein waves
rapidly dwindled down to zero. From
the characteristics of this ring-down,
astronomers were able to estimate the
final mass of the merged black hole.
But in the case of GW170817, the
wave frequency had become too high
for LIGO to observe it before the
two neutron stars actually collided,
and it lost the signal, says Kalogera.
So astronomers do not have any
observational data to constrain the
properties of the merged objects.
Moreover, estimates of the neutron
stars’ initial masses are not precise
enough to provide much help.
Nelemans is confident enough to
claim that the collision must have
produced a new black hole. “If there was
UWA a neutron star there right now, it would
OPTICAL OPPORTUNITY
Observatories around the world rushed to turn their telescopes toward the
sky location of the event, in the hope of picking up the faint optical glow
expected from the neutron star collision. One of those facilities was the
University of Western Australia’s (UWA) Zadko Telescope, operated by UWA’s
School of Physics and the ARC Centre of Excellence for Gravitational Wave
Discovery (OzGrav).
Normally, the only things that might spoil an observing effort are bad
weather and equipment failure. But in this case, the Zadko team was battling
a cyber attack at the time the discovery was made. Technical staff soon
had the system running again, and the telescope was able to monitor the
explosion’s aftermath for four days.
“Everything about this discovery is new. The fact that gravitational waves,
gamma rays, light and radio signals are all coming from the same source is
extraordinary,” said UWA Associate Professor, David Coward.The University of
Western Australia’s
Zadko Telescope.be extremely hot, and we would have
detected it in X-rays,” he says.
But Kalogera is not so sure. “We
really have no idea,” she says. “The
X-ray signal from the hot surface may
temporarily be absorbed by the ejecta.
I wouldn’t exclude the possibility of a
hyper-massive neutron star. Who knows,
within a few weeks or months, we may
be lucky enough to detect radiation from
its surface, or maybe even pulses [of
X-rays or radio waves], due to the object’s
extremely rapid rotation.”Making history
To sum up, the observations,
spectacular as they already are, may
turn out to be the proverbial tip of the
iceberg of future revelations on gamma-
ray bursts, binary star evolution, heavy
element synthesis, general relativity,
the behaviour of matter in extreme
environments, and the properties of
neutron stars. Physicists are particularly
interested in the material properties
of these hyper-dense stellar remnants,
which easily pack a hundred thousand
tonnes of matter into a volume of one
cubic millimetre. We can’t possibly hope
to recreate such extreme conditions in a
laboratory on Earth.
In principle, a detailed study of
gravitational-wave signals such as
GW170817 should provide more
information. As the two neutron stars
draw closer and closer, they will be
stretched and squeezed by mutual tidal
forces. The magnitude of the resultingdeformations tells physicists something
about the interior structure of the star,
the way its density changes with depth,
its material stiffness, et cetera. This
so-called equation of state has not yet
been determined on the basis of the
current GW170817 observations. In
all likelihood, it will take many more
similar events before it becomes possible
to draw the right statistical conclusions.
Still, explains Kalogera, the fact
that the neutron star coalescence
produced a massive, relativistically
expanding fireball (the kilonova) puts
some constraints on the equation of
state. “For a variety of reasons, the new
observations are more easily explained
if neutron stars are on the small side of
the postulated size range,” she says —
probably more like 20 kilometres across
than 30. Smaller sizes could indicate
extreme forms of matter deep within
the neutron stars’ cores.
So yes, Nobel laureate Barry Barish
is absolutely right: the new discovery
establishes gravitational-wave science as
a new emerging field. And it’s emerging
fast, too. Van den Heuvel can’t wait to
see the next spectacular breakthrough.
“These measurements are incredibly
hard,” he says. “Measuring spacetime
ripples that are much smaller than an
atomic nucleus is almost impossible
to imagine. But within 20 years or so,
gravitational-wave measurements may
be just as routine as X-ray observations
have become over the past 40 years. It’s
really beyond my wildest dreams.”