How colliding neutron stars could shed light on universal mysteries


Date:
July 8, 2020
Source:
University of East Anglia
Summary:
Researchers have discovered an unusual pulsar - one of deep space's
magnetized spinning neutron-star 'lighthouses' that emits highly
focused radio waves from its magnetic poles. It is unusual because
the masses of its two neutron stars are quite different -- with one
far larger than the other. The breakthrough provides clues about
unsolved mysteries in astrophysics -- including the expansion rate
of the Universe (the Hubble constant).



FULL STORY ==========================================================================
An important breakthrough in how we can understand dead star collisions
and the expansion of the Universe has been made by an international team,
led by the University of East Anglia.


==========================================================================
They have discovered an unusual pulsar -- one of deep space's magnetized spinning neutron-star 'lighthouses' that emits highly focused radio
waves from its magnetic poles.

The newly discovered pulsar (known as PSR J1913+1102) is part of a binary system -- which means that it is locked in a fiercely tight orbit with
another neutron star.

Neutron stars are the dead stellar remnants of a supernova. They are
made up of the most dense matter known -- packing hundreds of thousands
of times the Earth's mass into a sphere the size of a city.

In around half a billion years the two neutron stars will collide,
releasing astonishing amounts of energy in the form of gravitational
waves and light.

But the newly discovered pulsar is unusual because the masses of its two neutron stars are quite different -- with one far larger than the other.



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This asymmetric system gives scientists confidence that double neutron
star mergers will provide vital clues about unsolved mysteries in
astrophysics - - including a more accurate determination of the expansion
rate of the Universe, known as the Hubble constant.

The discovery, published today in the journal Nature, was made using
the Arecibo radio telescope in Puerto Rico.

Lead researcher Dr Robert Ferdman, from UEA's School of Physics, said:
"Back in 2017, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected the merger of two neutron stars.

"The event caused gravitational-wave ripples through the fabric of space
time, as predicted by Albert Einstein over a century ago." Known as
GW170817, this spectacular event was also seen with traditional telescopes
at observatories around the world, which identified its location in a
distant galaxy, 130 million light years from our own Milky Way.



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Dr Ferdman said: "It confirmed that the phenomenon of short gamma-ray
bursts was due to the merger of two neutron stars. And these are now
thought to be the factories that produce most of the heaviest elements
in the Universe, such as gold." The power released during the fraction
of a second when two neutron stars merge is enormous -- estimated to be
tens of times larger than all stars in the Universe combined.

So the GW170817 event was not surprising. But the enormous amount of
matter ejected from the merger and its brightness was an unexpected
mystery.

Dr Ferdman said: "Most theories about this event assumed that neutron
stars locked in binary systems are very similar in mass.

"Our new discovery changes these assumptions. We have uncovered a binary
system containing two neutron stars with very different masses.

"These stars will collide and merge in around 470 million years, which
seems like a long time, but it is only a small fraction of the age of
the Universe.

"Because one neutron star is significantly larger, its gravitational
influence will distort the shape of its companion star -- stripping away
large amounts of matter just before they actually merge, and potentially disrupting it altogether.

"This 'tidal disruption' ejects a larger amount of hot material than
expected for equal-mass binary systems, resulting in a more powerful
emission.

"Although GW170817 can be explained by other theories, we can confirm
that a parent system of neutron stars with significantly different masses, similar to the PSR J1913+1102 system, is a very plausible explanation.

"Perhaps more importantly, the discovery highlights that there are
many more of these systems out there -- making up more than one in 10
merging double neutron star binaries." Co-author Dr Paulo Freire from
the Max Planck Institute for Radio Astronomy in Bonn, Germany, said:
"Such a disruption would allow astrophysicists to gain important new clues about the exotic matter that makes up the interiors of these extreme,
dense objects.

"This matter is still a major mystery -- it's so dense that scientists
still don't know what it is actually made of. These densities are far
beyond what we can reproduce in Earth-based laboratories." The disruption
of the lighter neutron star would also enhance the brightness of the
material ejected by the merger. This means that along with gravitational-
wave detectors such as the US-based LIGO and the Europe-based Virgo
detector, scientists will also be able to observe them with conventional telescopes.

Dr Ferdman said: "Excitingly, this may also allow for a completely
independent measurement of the Hubble constant -- the rate at which the Universe is expanding. The two main methods for doing this are currently
at odds with each other, so this is a crucial way to break the deadlock
and understand in more detail how the Universe evolved."

========================================================================== Story Source: Materials provided by University_of_East_Anglia. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ferdman, R.D., Freire, P.C.C., Perera, B.B.P. et al. Asymmetric mass
ratios for bright double neutron-star mergers. Nature, 2020 DOI:
10.1038/ s41586-020-2439-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200708121439.htm

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