Gravitational waves from merging Neutron stars

About 130 million years ago, in a galaxy far, far away, emitting gravitational waves, two neutron stars collided. LIGO employees have been waiting for such an event since the launch of an improved version of the interferometer in 2015.

A hundred years later, after Einstein suggested the existence of gravitational waves, we saw them, traced them to the source and found an explosion with new physics that we could only dream of before.

Andy Howell, Astronomer at Las Cumbres Observatory

What is a Neutron star?

When a star with a mass of 8-30 solar masses runs out of fuel capable of supporting the process of thermonuclear fusion, its core collapses under the pressure of the outer layers. Gravity compresses matter so much that protons and electrons combine to form neutrons. At this moment, a monstrous explosion occurs – a supernova is born. The former red supergiant’s neutron core becomes a neutron star.

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Only about 20 kilometers in diameter, about the size of a large city, but with a mass 1.4-2 times that of the sun. These are the smallest and densest stars known. Just a teaspoon of neutron star matter weighs a billion tons, and surface gravity is on average 2 billion times stronger than that of the Earth’s surface. In addition, the force of the supernova explosion spins the star, forcing it to rotate several times per second (over time, the speed decreases). At the moment, the pulsar PSR J1311-3430 is considered the record holder, which makes 43,000 rpm.

Some neutron stars have jets (directed streams of particles) that eject matter above the magnetic poles at a near-light speed. Usually the axis of rotation and the magnetic axis are displaced relative to each other, and on the Earth flashes of radiation are seen at equal short intervals. Such neutron stars are called “pulsars”. As of 2010, approximately 1,800 pulsars were discovered from radio emission, and another 70 were found from gamma rays.

Several classes of neutron stars are distinguished, such as radio pulsars, magnetars, X-ray pulsars, and radio quiet neutron stars. Sometimes neutron stars enter binary systems with a wide variety of companions, from white dwarfs to red giants. And sometimes the system consists of two neutron stars.

Chronology of events

The GW170817 gravitational wave burst was first detected on August 17, 2017 at 8:41 am ET by one of the LIGO interferometers. At almost the same time, the Fermi (NASA) and INTEGRAL (ESO) space telescopes recorded a short GRB, about 2 seconds long. The chance of this being a coincidence was assessed as extremely small. Additional data analysis confirmed the presence of a signal on another detector of the LIGO team, initially mistaken for noise. The approximate position of the signal source was established and an alert was sent to astronomers around the world.

Although the absence of a signal on the European Virgo detector (it was located in such a way that it was not sensitive to the wave) made it possible to significantly refine the search area, it was still very significant. The first one to cope with the task of searching for visual confirmation (after only 11 hours) was an old 1-meter telescope in Chile, which was put into operation in 1971. The new light source was discovered in the galaxy NGC 4993, located about 130 million light-years from Earth.

Later visible and infrared photographs from the Hubble Telescope showed that the source was brighter than a nova, but dimmer than a supernova. Additional confirmation of the observation of the object from about 70 telescopes around the world did not keep itself waiting long.

Analysis of gravitational waves made it possible to approximately establish the parameters of the stars participating in the merger – from 1.1 to 1.6 solar masses and about 20 kilometers in diameter. The previous four signals from black hole merger observations were fractions of a second, while on August 17 it was about 100 seconds long. The proximity of the source provided an exceptionally clear signal that, according to LIGO spokeswoman Laura Cadonati, could occur “less than once every 80,000 years by coincidence.” Two records were broken at once: the nearest source of gravitational waves and the nearest gamma-ray burst were registered.

From 0.03 to 0.05 solar masses of matter (approximately 13,000 Earth masses) were thrown into space. The initial ejection velocity according to ESA data was about one-fifth the speed of light. A significant part of the ejected matter is elements heavier than iron, including many rare ones, primarily platinum and gold. For a long time, it was believed that the r-process, during which heavy elements are formed, can also occur during the collapse of a supernova core.

At the moment, scientists are inclined to believe that the neutron density inside a supernova is too low to launch it. Also, by analyzing the spectrum, the emission of cesium and tellurium was confirmed, but the presence in the ejected substance of many elements predicted in the theoretical model could not be unambiguously established.

Gravitational waves and the speed of light

In 1961, Einstein, in his general theory of relativity, predicted the possible existence of gravitational waves. According to this theory, moving masses create “ripples” in the spacetime metric that diverge from the source, as if you threw a pebble into a pond. These waves can give us information about where, when and how they were generated. Propagation of gravitational waves with the speed of light was predicted, now the theory has been confirmed.

Before the general theory about Einstein appeared, it was believed that gravity spreads instantly. According to Newton’s theory, as the Earth moves along its orbit, its gravitational field instantly shifts and these changes can be immediately detected from anywhere in the Universe. We know that if you throw a pebble into a pond, it will take some time before the waves reach the edge of the reservoir; now, after the discovery of both gravitational and light waves generated by one event, we can confidently say that this behaviour is also characteristic of gravity.

Considering the distance to the merging point of neutron stars and the difference between the registration of light and gravitational signals in 1.7 seconds, we can assume that the speed of propagation of gravity is, with great accuracy, equal to the speed of light. Even in the lower estimate, assuming that the light was emitted 10 seconds after the gravitational waves, the accuracy is 3 * 10 -15 . The slight delay between the signals can be explained by the fact that the reactions that caused the gamma radiation took some time to reach the surface.

Significance for astrophysics and emerging questions

– For the first time, the theoretically predicted collision of neutron stars has been registered. The registration of gravitational waves was also visually confirmed for the first time. The absorption of a neutron star by a black hole is another event that the astronomical community hopes to record in the future.

– Received the first confirmation that gravitational waves and light travel at the same speed, in full accordance with the predictions of Einstein.

– This event confirmed the theory of the origin of chemical elements heavier than iron, such as gold and platinum. However, a new question arises: can collisions of neutron stars explain the entire volume of such elements in our Universe?

– The possibility of specifying the expansion rate of the Universe (Hubble constant) by analyzing gravitational waves from such mergers is being actively discussed. Recording such events in the future will help to make independent and more accurate distance estimates.

– The theory of the origin of short GRBs has been confirmed. However, the recorded burst was rather weak, despite the fact that it was received from a record source close to Earth. Perhaps this is due to the fact that the burst ray was directed at a large angle with respect to the observer.

– By analyzing the spectrum, it is impossible to establish with final clarity what type of object was the result of the merger. Supposed options: a supermassive neutron star or, conversely, the lightest black hole known. Increasing the sensitivity of interferometers and additional measurements of electromagnetic radiation are likely to provide a more definite answer in the future.

Recommended book:

Neutron Stars, Black Holes and Gravitational Waves

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