The LIGO-Virgo collaboration has announced the first observation of gravitational waves coming from the inspiral and merger of two neutron stars.
On August 17, 2017 the LIGO-Virgo collaboration alerted more than 90 astronomy teams around the world, that they had detected a signal (GW170817) consistent with the inspiral and merger of two neutron stars. The signal duration was about 100 s, much longer than the fraction of a second signals produced by binary black hole inspirals. This simple fact alone tells us that the masses of the stars involved are typical of neutron stars.
Analyzing the signal revealed more information about the colliding objects, like the distance from Earth, which was found to be around 130 million light-years. Before the merger, the stars deformed due to the enormous gravitational forces. The imprint of these deformations on the gravitational-wave signal teaches us about their structure and about the behavior of nuclear matter at extremely high densities.
Moreover, in this case a tiny gamma ray burst had been detected by two satellites orbiting around the Earth about 2 s after the arrival of the gravitational wave signal. This confirms the hypothesis that at least some short gamma ray bursts are due to binary neutron star mergers.
The alert issued by the LIGO-Virgo teams indicated the source was located in a small patch of the sky in the southern hemisphere. Raffaele Flaminio, director of the Gravitational Wave Project Office of NAOJ and a member of the Virgo team at CNRS/LAPP explains that "Thanks to the combination of the data from the LIGO detectors in the USA and the Virgo detector in Europe, this was the best ever localized gravitational wave source."
In the following hours, days, and weeks, more than 70 observatories, both on the ground and in space, pointed in that direction. 8-m class telescopes like the Subaru Telescope in Hawai`i were part of this worldwide quest. Less than 11 hours after the alert, a 'new star' was detected near the galaxy NGC4993. This new star was subsequently observed across all electromagnetic bands: X-ray, UV, optical, infrared, and radio. These observations confirmed that a neutron star merger gives rise to an event called a kilonova, in which heavy metals like gold are produced.
"By combining the information provided by gravitational-wave detectors with the observations made with optical telescopes and other types of electro-magnetic observatories, we have been able to gain new insights into fundamental physics, astronomy, and cosmology." - says Raffaele Flaminio - "We measured the speed of gravity, obtained a completely independent measurement of the expansion of Universe and maybe discovered a new class of nearby cosmic explosions. With the start of KAGRA in Japan, the capability of this international network of gravitational wave detectors will be improved further and more surprises are to be expected".
LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,200 scientists and some 100 institutions from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration and the Australian collaboration OzGrav. Additional partners are listed at http://ligo.org/partners.php
The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.
