Gravitational Waves From Merging Neutron Stars

Dance of the heavyweights: two neutron stars orbit each other, spiralling ever closer together, radiating gravitational waves in the process. Astrophysicists have long been theorizing that neutron stars, the cores of burnt-out, massive suns, could create gamma rays burst. The image of the real event GW170817 comes from a numerical-relativistic simulation.

For the first time, researchers have simultaneously measured the gravitational waves and the light from two merging neutron stars.

'Multi-messenger astronomy', combining the observation of gravitational waves and electromagnetic radiation, begins with this event, registered on 17 August 2017 at 14:41:04 CEST. Together, the complementary methods will considerably increase our understanding of extreme astrophysical events. For example, this discovery confirms that two merging neutron stars are indeed the precursor to short gamma-ray bursts and that the explosion following this merger - known as a kilonova - is the source of heavy elements in the Universe. During the 17 August observations, researchers at the Max Planck Institutes for Gravitational Physics in Potsdam and Hannover, and the Max Planck Institute for Astrophysics and for Extraterrestrial Physics in Garching, played a central role.

The two LIGO detectors in Hanford (Washington state, USA) and Livingston (Louisiana) observed the signal referred to as GW170817 for around 100 seconds. The measurements made by the Virgo detector in Tuscany, near Pisa, Italy, improved localization in the heavens substantially and allowed the researchers to constrain the origin of the wave to a spot in the southern skies of only 28 square degrees - around 130 times the apparent size of the full moon.

Only 1.7 seconds later the Gamma-ray Burst Monitor (GBM) on board the Fermi satellite registered a gamma-ray burst, known as GRB 170817A, from roughly the same direction as the gravitational wave signal. The probability that this encounter was purely coincidental is one to 200 million - the odds of having six correct numbers in the lottery are considerably better!

'At first, the detection of this new gamma-ray burst by Fermi did not really seem extraordinary,' says Andreas von Kienlin, scientist at the Max Planck Institute for Extraterrestrial Physics, who also helped build the instrument. 'We observe four or five new gamma-ray bursts per week.' Only later did the researcher learn about the LIGO and Virgo observatory measurements: 'We immediately knew that this was a historical event.'

A further surprise is that the gamma-rays and the gravitational waves were not detected at exactly the same time, but with an approximately two-second time difference. 'These and the additional observations provide us with unique insights into the physics in and around this event', says von Kienlein.

Integral, another gamma-ray mission with a Max Planck instrument on board - the SPI - also observed high-energy radiation from the merger. 'The signal we observed may not have been very bright, but we can confirm the Fermi result with a completely independent gamma-ray detection,' says Roland Diehl, scientist at the Max Planck Institute for Extraterrestrial Physics and co-investigator for the SPI. 'With regard to the link between gamma-ray burst and gravitational waves, we are on safe ground here. Moreover, we can clearly connect a short gamma ray burst to a neutron star collision for the first time', says Diehl.

The very precise localization of the LIGO/Virgo observation allowed a handful of observatories around the globe to search the area of the heavens from where the signal originates only a few hours later. Optical telescopes, including the GROND instrument on the 2.2 metre Max Planck Society telescope in the La Silla observatory at the European Southern Observatory (ESO), discovered a new point of light, similar to a star, near the NGC 4993 galaxy. This lenticular galaxy system is approximately 130 million light-years from Earth.

For the astronomers, one thing is certain: the optical object, the gamma-ray burst and the gravitational waves all originate from one and the same source: the merging of a pair of neutron stars. Eventually, more than 70 ground- and space-based observatories noted the event in the x-ray, ultra-violet, visible, infra-red and radio wave spectra.

'These agreeing observations give us a detailed picture of this event from three minutes prior to the merger up to several weeks later,' says Jochen Greiner, the Max Planck Institute for Extraterrestrial Physics scientist responsible for building GROND.

This opinion is shared by the scientists at the Max Planck Institute for Gravitational Physics in Hannover and Potsdam: 'In itself, the first evidence of gravitational waves emitted by merging neutron stars is already extremely exiting. However, the combination with dozens of follow-up observations in the electromagnetic spectrum makes it truly revolutionary', say Alessandra Buonanno and her Director colleagues Bruce Allen and Karsten Danzmann.

Identification of GW170817 as a binary star system consisting of two neutron stars, and observations of electromagnetic radiation following their collision, allow conclusions on the previously mysterious origin of the short gamma-ray bursts to be reached.

'While we did have strong hints that merging neutron stars are indeed the progenitors for short gamma ray bursts, we did not have final proof. The simultaneous observation of this two-second burst by Integral and Fermi, and by the gravitational wave detectors, is the first conclusive evidence that at least some of these gamma ray bursts are indeed powered by neutron star mergers,' says Rashid Sunyaev, Director at the Max Planck Institute for Astrophysics.

Analyses of the LIGO data returned a relatively small distance of the merging neutron stars from the earth of around 85 to 160 million light years, in agreement with the 130 million light years to the assumed parent galaxy NGC 4993. In contrast to previous gravitational wave observations, the scientists calculated the masses of the merging objects as 1.1 to 1.6 times that of our Sun, comparable to that of known neutron stars and dissimilar to that of black holes.

Neutron stars are the extremely dense, exhausted remains of massive stars, with diameters of only around 20 kilometres. Astrophysicists have studied the merger of two such star corpses in theory for a long time - and have enjoyed confirmation in the observation of the GW170817 signal: 'The physical parameters of the transient event match the theoretical predictions for a so-called kilonova from a neutron star merger remarkably well', says Anders Jerkstrand from the Max Planck Institute for Astrophysics.

'In particular, the rate at which the light from the source dimmed over the ten days following the merger exactly corresponds to the forecast that the ejecta are dominated by radioactive elements much heavier than iron' says Jerkstrand.

Two neutron stars merging in a gamma ray burst liberates enormous quantities of energy. At the same time, dense matter is expelled at high velocities. Because the ejected matter displays a high concentration of free neutrons, the heaviest elements in the universe can form from them. The process involved is referred to as rapid neutron capture or, more briefly, the r-process.

'The origin of the heaviest chemical elements in the universe was a mystery to us scientists for a long time', says Hans-Thomas Janka, lead scientist at the Max Planck Institute for Astrophysics. 'Now we have the first observational proof that neutron star collisions may be the sources of these elements. They may even be the main source of the r-process elements.'

The observation and characterization of minutes-long signals from merging neutron stars, concealed in detector noise, demands extremely precise waveform models. Members of the 'Astrophysical and Cosmological Relativity' department at the Max Planck Institute for Gravitational Physics developed models which were deployed as templates in the optimal filter searches that discovered GW170817.

In addition, Max Planck researchers played a central role in the development and implementation of the search algorithms used to observe GW170817. As members of the team that investigated the signal immediately after its discovery, they immediately removed a temporary spurious signal from data delivered by the LIGO Livingston instrument.

This allowed the position in the heavens to be so precisely pinpointed that astronomers were able to observe a quickly fading visible light glow within twelve hours of GW170817's discovery. They had thus identified the parent galaxy and redshift, leading to measurement of the Hubble constant and therefore to the distance.

The Max Planck Institute for Gravitational Physics also helped to develop and apply analysis algorithms and waveform models, allowing them to identify the source of GW170817 as a binary neutron star system. In an additional insight, the results contradict theories that assume highly repulsive nuclear forces, because they predict relatively large and therefore highly deformable neutron stars.

In Potsdam, both analytical methods and numerical simulations were adopted to construct modern waveform models which predict weaker repulsive nuclear forces in GW170817's neutron stars. Moreover, Max Planck scientists have investigated electromagnetic signals created by the ejection of matter as the stars merged. These signals contain information on the creation of heavy elements in the universe, as discussed above.

'In a domino effect, GW170817 has set off a spectacular sequence of astrophysical observations, simultaneously solving long-standing mysteries and presenting us with new ones', says Alessandra Buonanno, Director at the Max Planck Institute in Potsdam. 'Remarkably, GW170817 has also delivered insights into the nature of ultradense matter in the interior of the most fascinating and extreme objects in the universe - neutron stars.'

Scientists in the 'Observation-based Relativity and Cosmology' department at the institute in Hannover played a central role during the first hours of analyses, as well as in the characterization of the gravitational wave signal and understanding the source. 'Even in my wildest dreams I would not have dared to hope that we would simultaneously demonstrate the first discovery of a binary neutron star by gravitational waves, the corresponding gamma-ray burst and the electromagnetic signals. I imagined we would only see such a thing after 20 or more observations of two merging neutron stars. This is fantastic!', as Director Bruce Allen stated.

'This is the beginning of multi-messenger astronomy and a greater understanding of our universe. We are very proud to play a central role in measuring gravitational waves, because our lasers, developed and tested in the GEO600 project, are at the heart of all gravitational wave observatories', summarizes Max Planck Director Karsten Danzmann, Allen's colleague at the Hannover institute.

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