New kind of gravitational wave found!
All About Space's report on the ripples in space-time, created by the merger of neutron stars
Gravitational waves hit the headlines in February 2016 when the announcement was made of the first ever detection of these ripples in space, coming from two colliding black holes. Since then, three more detections of gravitational waves from merging black holes have been confirmed.
In August 2017, rumours began to circulate about another potential detection of gravitational waves, but one vastly different from the others in that it involved two neutron stars, rather than black holes, ploughing into each other. In October 2017 the rumour was confirmed as fact in a flurry of press conferences held around the world. The gravitational waves detected from this neutron star collision act as a smoking gun for our theories about these incredibly compact objects.
Gravitational waves were predicted by Einstein's theory of general relativity. These waves are not part of electromagnetic light, such as the visible light that we see with our eyes. Instead, they are ripples in the very fabric of space, that spread out from a source, just as a stone dropped in a pond will send ripples across the surface of the water. Gravitational waves can be detected on Earth when they interfere with an incredibly sensitive detector that has been
“The gravitational waves will finally be able to reveal what goes on inside a neutron star”
isolated from all other sources of motion. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has two sites in the USA that were used to detect the first ever gravitational-wave events. Another detector, Virgo, which is located in Italy, recently joined in the hunt after undergoing major upgrades. It was the LIGO/Virgo collaboration that detected the gravitational waves from two neutron stars that collided in a galaxy, called NGC 4993, around 130-million-light-years away.
A neutron star is formed when a giant star explodes, leaving behind its dense core as a remnant. The protons and electrons in the core get squeezed together to form neutrons, thus creating a neutron star. The neutrons are so densely packed that a neutron star with twice the mass of the Sun would fit into the area of a small city.
A binary star system where both stars explode as supernovae will produce a binary neutron star system. The neutron stars will slowly begin spiralling towards one another, and in the case of the neutron star merger discovered in NGC
4993, the shrinking of their orbits over the course of 11 billion years released energy in the form of gravitational waves. The size and pitch of these waves increased as the orbit decreased, until finally the two neutron stars smashed together. This
produces an explosive event that astronomers call a kilonova.
The gravitational waves discovered coming from this kilonova in August, an event named GW 170817 (meaning the gravitational waves were discovered on 17 August 2017), sent a shudder through the universe that was detected by LIGO and Virgo. What made this event extra special is that two seconds after the gravitational waves were first detected – at 12:41 GMT to be precise – NASA's Fermi Gamma-ray Space Telescope detected gamma rays emanating from the same region of the sky. Collisions of neutron stars were expected to release gamma rays in what is known as a short gamma ray burst (GRB), and this joint detection – the first of its kind – seemed to confirm that they do come from merging neutron stars.
In that case, there should have been an afterglow from the kilonova explosion, so all eyes turned towards a region of space around the constellation of Hydra, where the gravitational waves and short GRB had been detected. Telescopes on Earth and in space scoured the sky looking for the afterglow.
“The gravitational-wave signal from colliding neutron stars appears very different to that of a black hole collision”
It was the one-metre Swope Telescope in Chile that first spotted the faint afterglow in the lenticular galaxy NGC 4993 (a lenticular galaxy is a very dusty disk galaxy). Swiftly afterwards, pretty much every large professional telescope was pointed towards NGC 4993, observing it across the electromagnetic spectrum, including the Hubble Space Telescope, NASA's Chandra X-ray telescope, the Very Large Telescope (VLT) and the Atacama Large Millimeter/ submillimeter Array (ALMA) in Chile.
Danny Steeghs, from the University of Warwick, notes that it had been expected beforehand that a binary neutron star merger was expected to produce signals at a variety of electromagnetic wavelengths. “Because of this expectation, many groups have formed to pursue such events using an array of facilities. We have just deployed a dedicated optical instrument on La Palma, GOTO, that is specifically designed to search for prompt optical emissions coincident with gravitational wave detections. The challenge is to localise the source quickly, so that powerful narrow-field telescopes such as HST, VLT and so on can be deployed to study such signals further."
The collision of two neutron stars is an incredibly important discovery. The observations of the kilonova from GW 170817 proved that some of the heaviest elements in the universe are produced by colliding neutron stars. The 'weight' of an element refers to how many protons and neutron that it has. Hydrogen is the lightest element as it only has one proton, and no neutrons. Heavy elements have
more protons and neutrons and are more difficult to create. The environment surrounding a neutron star collision is a perfect heavy element factory, as the ejected matter from the collision has such a high density that it can force neutrons together to make new elements. These elements are not stable, however, so they rapidly decay, and as they decay they produce heat that powers the kilonova afterglow for a few weeks after the collision. The detection of the afterglow in infrared light by the Visible and Infrared Survey Telescope for Astronomy (VISTA) in Chile, and the Spitzer Space Telescope, confirmed that neutron stars do actually produce these elements.
This was a 'holy grail' type of discovery, as it confirms that our models of how elements are produced are correct. In fact, you are quite possibly wearing some of these heavy elements at this moment as you read this – the elements produced in the kilonova of GW 170817 included the equivalent of Jupiter’s mass in pure gold. It’s likely that the gold in your jewellery also came from an ancient neutron star merger once upon a time.
The short GRB, and then X-rays, that were detected from GW 170817 also taught us about this dramatically violent neutron star merger.
"The coincident detection of the gravitation alwave signal that matches the pattern of a binary neutron star and the short GRB is very direct proof of this link [between short GRBs and neutron star mergers],” said Steeghs. "However, normally the GRB is only expected if the highly collimated emission points towards us, so for many mergers, we might only see the gravitational wave signal.”
The short GRB detected on 17 August was surprisingly faint, especially given that it is one of the closest GRBs ever detected. When the Chandra X-ray telescope first went looking for the expected X-rays from the kilonova on 19 August, it couldn’t see any. The X-rays weren’t spotted until 26 August, ten days after the event. Scientists suspect that the delay in the detection of the X-rays and the faintness of the GRB are connected. The gamma rays are created by a jet of material shooting out from the neutron star merger, and if this jet was ‘off-axis’, or, in other words, not pointed directly at us, it would explain the observed faintness. The X-rays are also produced by the jet, and in the offaxis theory they only became visible when the jet began to slow down and fan outwards and into clearer view.
Meanwhile, the gravitational-wave signal from colliding neutron stars appears very different to that of a black hole collision, thus allowing new information to be teased from the signal. The changing height and pitch of the wave can be modelled to deduce the exact physics of the merger. As neutron stars are less massive than black holes, the amplitude (height) of the wave is not as strong as for a black hole merger, but the signal lasted for much longer – over 60 seconds – compared to the fractions of a second in black hole mergers. If the signal was strong enough, it would be possible to determine the individual masses
of each neutron star. Astronomers have been able to deduce that the total combined mass of the two neutron stars was 2.82-times the mass of the Sun, but are more uncertain about how that mass was divided between the two neutron stars. Scientists are not even sure what the merger left behind. Did they form a new, larger neutron star or, more likely, a new black hole?
When colliding neutron stars are detected, it is hoped that the simultaneous analysis of the gravitational waves and their optical counterparts will eventually reveal what goes on inside a neutron star. This would be the first time that gravitational waves are studied alongside traditional light-based astronomy, heralding in a new era of astronomy, what scientists call ‘multi-messenger astronomy’. As more neutron star mergers are detected in the future, Einstein’s general relativity will be tested as never before, and will undoubtedly reveal surprises.
Now that Virgo is working side by side with LIGO, it is expected that more neutron star mergers will be discovered during their next observing run in 2018. By combining forces, LIGO and Virgo will be better able to pin down the location of gravitational wave sources with more precision, a precision that helped astronomers speed up the process of locating the kilonova in NGC 4993. When the two detectors of LIGO were working alone, it was impossible to tell where exactly the gravitational waves emanated, but the addition of another detector makes it easier to find the location by comparing the timing of the signals. The first confirmed detection of gravitational waves from a colliding neutron star had been eagerly awaited, and now that we know that short GRBs are produced by merging neutron stars and that such events produce heavy elements, our eyes turn to the future.
New detections should tell us more about the extreme physics of neutron stars, while signals from merging neutron stars could be used to accurately measure distances to the mergers – distances that could then be used to measure the expansion rate of the universe. They may just be the cores of dead stars colliding in a galaxy far, far away, but what they can teach us about the universe could be far greater than anyone could imagine.