Gravitational waves will let us see inside stars as supernovae happen

Artistic representation of the material around the
supernova 1987A. Credit: ESO/L. Calçada

On February 11th, 2016, scientists at the Laser
Interferometer Gravitational-wave Observatory (LIGO)
announced the first detection of gravitational waves. This
development, which confirmed a prediction made by Einstein’s
Theory of General Relativity a century ago, has opened up new
avenues of research for cosmologists and astrophysicists.
Since that time, more detections have been made, all of which
were said to be the result of black holes merging.

However, according to a team of astronomers from Glasgow and
Arizona, astronomers need not limit themselves to detecting
waves caused by massive gravitational mergers. According to a
study they recently produced, the Advanced LIGO, GEO 600, and
Virgo gravitational-wave detector network could also detect the
gravitational waves created by supernova. In so doing,
astronomers will able to see inside the hearts of collapsing
for the first time.

The study, titled “Inferring the Core-Collapse Supernova
Explosion Mechanism with Three-Dimensional Gravitational-Wave
Simulations,” recently appeared online. Led by Jade Powell, who
recently finished her Ph.D. at the Institute for Gravitational
Research at the University of Glasgow, the team argue that
current gravitational wave experiments should be able to detect
the waves created by core collapse supernovae (CSNe).

Otherwise known as Type II supernovae, CCSNe are what happens
when a massive star reaches the end of its lifespan and
experiences rapid collapse. This triggers a massive explosion
that blows off the outer layers of the star, leaving behind a
remnant neutron star that may eventually become a black hole.
In order for a star to undergo such collapse, it must be at
least 8 times (but no more than 40 to 50 times) the mass of the
Sun.

When these types of supernovae take place, it is believed that
neutrinos produced in the core transfer gravitational energy
released by core collapse to the cooler outer regions of the
star. Dr. Powell and her colleagues believe that this
gravitational energy could be detected using current and future
instruments. As they explain in their study:

“Although no CCSNe have currently been detected by
gravitational-wave detectors, previous studies indicate that an
advanced detector network may be sensitive to these sources out
to the Large Magellanic Cloud (LMC). A CCSN would be an ideal
multi-messenger source for aLIGO and AdV, as neutrino and
electromagnetic counterparts to the signal would be expected.
The gravitational waves are emitted from deep inside the core
of CCSNe, which may allow astrophysical parameters, such as the
equation of state (EOS), to be measured from the reconstruction
of the gravitational-wave signal.”

Dr. Powell and her also outline a procedure in their study that
could be implemented using the Supernova model Evidence
Extractor (SMEE). The team then conducted simulations using the
latest three-dimensional models of gravitational-wave core
collapse supernovae to determine if background noise could be
eliminated and proper detection of CCSNe signals made.

As Dr. Powell explained to Universe Today via email:

“The Supernova Model Evidence Extractor (SMEE) is an algorithm
that we use to determine how supernovae get the huge amount of
energy they need to explode. It uses Bayesian statistics to
distinguish between different possible explosion models. The
first model we consider in the paper is that the explosion
energy comes from the neutrinos emitted by the star. In the
second model the explosion energy comes from rapid rotation and
extremely strong magnetic fields.”

From this, the team concluded that in a three-detector network
researchers could correctly determine the explosion mechanics
for rapidly-rotating supernovae, depending on their distance.
At a distance of 10 kiloparsecs (32,615 light-years) they would
be able to detect signals of CCSNe with 100% accuracy, and
signals at 2 kiloparsecs (6,523 light-years) with 95% accuracy.

In other words, if and when a supernova takes place in the
local galaxy, the global network formed by the Advanced LIGO,
Virgo and GEO 600 gravitational wave detectors would have an
excellent chance of picking up on it. The detection of these
signals would also allow for some groundbreaking science,
enabling scientists to “see” inside of exploding stars for the
first time. As Dr. Powell explained:

“The are emitted from
deep inside the core of the star where no electromagnetic
radiation can escape. This allows a gravitational wave
detection to tell us information about the explosion mechanism
that can not be determined with other methods. We may also be
able to determine other parameters such as how rapidly the star
is rotating.”

Gravitational waves will let us see inside stars as supernovae happen

Illustration showing the merger of two black holes and the
gravitational waves that ripple outward as the black holes spiral
toward each other. Credit: LIGO/T. Pyle

Dr. Powell, having recently completed work on her PhD will also
be taking up a postdoc position with the RC Centre of
Excellence for Gravitational Wave Discovery (OzGrav), the
gravitational wave program hosted by the University of
Swinburne in Australia. In the meantime, she and her colleagues
will be conducting targeted searchers for supernovae that
occurred during the first and seconds advanced detector
observing runs.

While there are no guarantees at this point that they will find
the sought-after signals that would demonstrate that supernovae
are detectable, the team has high hopes. And given the
possibilities that this research holds for astrophysics and
astronomy, they are hardly alone!

Explore further:

Collapsing star gives birth to a black hole

More information: Inferring the core-collapse supernova
explosion mechanism with three-dimensional gravitational-wave
simulations. arxiv.org/pdf/1709.00955.pdf

Provided by: Universe Today