The First Gravity Wave and Telescopic Neutron Star Merger

The First Gravity Wave and Telescopic Neutron Star Merger

The key thing about gravitational wave detection, is that LIGO and Virgo are measuring the changes in gravitational field strength in stretching and compressing space, which only falls off like 1/r, whereas the energy or radiation strength in electromagnetic radiation falls off like field strength squared, as 1/r^2, that is, one over distance squared. So although the Gravity disturbance is extremely weak, a stretching of one part in 10^22, it can be measured from large distances.

One of the main results is that the chemical elements, starting with Strontium (Sr), of atomic number 38, can be made in neutron star mergers. This is by the rapid or r-process, where neutrons are rapidly packed into nuclei before they can beta decay into a proton plus electron plus anti-neutrino. We have data on element production from Science magazine, M. R. Drout et al, Science 10.1126/Science.aaq0049 (2017) As much gold was made to equal 40-100 times the mass of the earth. The r-process made as much heavy elements as 0.05 plus or minus 0.02 times the mass of the sun. The lanthanides, starting at atomic number 57, were 0.1 to 0.05 of that. The Milky Way r-process production rate of 3×10^-7 solar masses per year can be accounted for by neutron star mergers every 20,000 to 30,000 years.

We have data on the neutron star merger in what is called a “kilonova” from Dennis Overbye of the New York Times on Oct. 16, 2017. There was a 100 second long gravity wave detected in Advanced LIGO, at a null spot for Advanced Virgo, that triangulated the source. There were gamma rays detected 1.7 seconds after coalescence by the Fermi Gamma-Ray Space Telescope, and the signal lasted for two seconds. Because the source was close to the sun, telescope detection was eleven hours later.

The source is 130 million light years away. At that time the gas was at 8,000 degrees F, and the size of Neptune’s orbit. It was radiating 200 million times as much energy as the sun. X-rays were detected by the Chandra X-Ray telescope 9 days later. At 16 days, the Very Large Array detected radio emission. The initially blue optical source had turned to red.

The neutron stars were formed in supernova about 11 billion years ago. They are in the range of 1.17  to  1.60 solar masses, with the total mass of 2.74 solar masses.  This is from B. P. Abbott et all., Physical Review Letters, 20 October 2017, PRL 119, 161101 (2017).  Neutron stars with masses up to 2.0 solar masses have been detected elsewhere.

Jets were emitted at the magnetic poles that led to gamma rays. As the material hit gas, it made X-rays and radio waves.

LIGO is being turned off for a year to increase its sensitivity. At its design sensitivity, it can detect such sources out to 650 million light years at a rate of 3-12 per year.

The astronomical detections were described in an Astrophysical Journal Letters article with 3,500 authors.

Careful background explanations are presented by Prof. Matt Strassler at

There are many new states of matter that can occur in neutron stars, which are described in the Wikipedia article “neutron stars”. Layers near the surface are called “nuclear pasta” in another Wikipedia article.

A lifetime of new physics can be revealed in continued observation of many neutron star mergers.

About Dennis SILVERMAN

I am a retired Professor of Physics and Astronomy at U C Irvine. For a decade I have been active in learning about energy and the environment, and in lecturing and attending classes at the Osher Lifelong Learning Institute (OLLI) at UC Irvine.
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