http://www.skyandtelescope.com.au 37DC, astronomers and physicists finally
revealed their secret.Colliding neutron stars
Here’s what happened. On Thursday,
August 17, at 12:41:04 Universal Time
(UT), LIGO bagged its fifth confirmed
gravitational-wave signal, now
designated GW170817. But this signal
lasted much longer than the first four:
instead of a fraction of a second, like
the earlier detections, the ripples lasted
for a whopping 90 seconds, increasing
in frequency from a few tens of hertz
to about one kilohertz — the maximum
frequency that LIGO can observe.
This is the gravitational-wave
signal expected from closely orbiting
neutron stars, both less than two times
the mass of the Sun. Eventually they
whirled around each other hundreds
of times per second (faster than a
kitchen blender), at a fair fraction of
the speed of light. The waves emitted
by the accelerating masses kept
draining the system of orbital energy,
and before long, the two neutron stars
collided. The collision took place at a
distance of roughly 150 million light-
years from Earth.
Astronomers have known about
binary neutron stars since 1974,
when Russell Hulse and Joseph
Taylor discovered the first one, with a
separation of a few million kilometres
and an orbital period of 7.75 hours. But
their separation and period are changing
with time. In fact, the binary’s very slow
decrease in orbital period, measured
over subsequent years, perfectly matches
Einstein’s prediction for energy loss due
to the emission of gravitational waves.
Some 300 million years from now, the
two neutron stars in the Hulse-Taylor
binary will collide and merge.
That discovery of the first binary
neutron star, which earned Hulse and
Taylor the 1993 Nobel Prize in Physics,
provided a huge boost of confidence
for physicists such as Weiss and Kip
Thorne (Caltech), who were designing
the first prototypes of LIGO-like
laser interferometers and who shared
the 2017 Nobel with Barish. If onebinary neutron star would coalesce
in 300 million years, others might do
so tomorrow. The energetic burst of
gravitational waves produced by the
collision should be detectable with
extremely sensitive instruments here on
Earth. Referring to GW170817, Ralph
Wijers (University of Amsterdam, The
Netherlands) says, “We’ve been waiting
for this for 40 years.”The gamma-ray burst
Just two seconds after the gravitational-
wave event, at 12:41:06 UT, NASA’s
Fermi gamma-ray space telescope
detected a short gamma-ray burst —
a brief, powerful ‘flash’ of the most
energetic electromagnetic radiation in
nature. The European Space Agency’s
Integral gamma-ray observatory
confirmed the outburst.
Short gamma-ray bursts are thought
to be produced by colliding neutron
stars. The merger would blast two
narrow, energetic jets of particles
and radiation into space (probably
perpendicular to the neutron stars’
orbital plane). If one of the jets were
directed toward Earth, we would see
a gamma-ray burst lasting anywhere
between a fraction of a second and two
seconds or so. The natural question was,
could GRB170817A (as it was called)
possibly be related to the LIGO event
that had been observed just before?
Astronomers had doubts. Gamma-
ray bursts usually occur at distances
of billions of light-years. GRB170817A
looked about as bright to Fermi as other
GRBs, so if this burst had occurred at a
mere 150 million light-years distance,
it must have been unusually wimpy.
Moreover, it would be an uncanny
coincidence that the nearest gamma-ray
burst ever would have its jet pointed
toward Earth.Virgo to the rescue
Finding an optical counterpart to either
the gravitational ‘Einstein waves’ or to
the short gamma-ray burst would settle
the issue. Unfortunately, astronomers
couldn’t precisely pinpoint the source
of the signals on the sky. Fermi’s ‘error27, the LIGO-Virgo Collaboration
announced the detection of GW170814
— the gravitational wave signal of a
black hole merger — leading some to
assume that the earlier rumours had
been just hype.
However, because colliding
black holes don’t give off any light,
you wouldn’t expect any optical
counterpart. In a speech on October
3 after his co-reception of the physics
Nobel, Ranier Weiss (MIT) confirmed
another announcement was coming but
wouldn’t say what. On October 16, at a
UNIVERSITY OF WARWICK/MARK GARLICKlarge press conference in Washington,
An artist’s impression of the merger of
two neutron stars, responsible for the
gravitational ripples detected on August 17.