First observation of merging neutron stars by LIGO-Virgo and numerous telescopes

 

For the first time, gravitational waves emitted during the collision of two neutron stars were detected on Thursday 17th August 2017 at 14.41 (Paris time) by scientists from the LIGO-Virgo collaboration, in which teams from the Linear Accelerator Laboratory (CNRS/Paris-Sud University) are strongly involved.

The detection of a signal of a new type of gravitational waves is a world first. These waves are considerably longer than those produced from merging black holes  (one hundred seconds in comparison to a fraction of a second) and were accompanied by electromagnetic counterparts over a wide range of wavelengths (from gamma rays to radio waves) enabling 70 other observatories on Earth and in space to follow this event. This discovery was announced on Monday, 16th October 2017 at a LIGO-Virgo joint press conference, in Washington and Paris. Many scientific articles, written or co-authored by the LIGO and Virgo collaborations, or prepared by partner telescopes which have made follow-up observations of the source discovered in the galaxy NGC4993, have been made public at this time.

A neutron star

A neutron star is the remnants of a supernova, or in other words, the explosion of a giant star at the end of its life. When it has exhausted its nuclear fuel, the fusion reactions stop and nothing else counterbalances the gravitational force. The star collapses in on itself, neutrinos are emitted and the shock wave produced by the collapse expels the hot outer layers that emit light. In the centre, all that remains is a core composed almost entirely of neutrons and it is very dense: the equivalent of a teaspoon of this nuclear material weighs about a billion tonnes! Neutron stars are the smallest known stars to date; only black holes are more compact. Some neutron stars, which have a very strong magnetic field, emit a light beam and spin very quickly. The beam then sweeps through space like a lighthouse in the night and can be detected if Earth finds itself in its path. It is then called a pulsar.

 

Just like binary black hole systems, the signal expected when neutron stars merge can be accurately predicted using theoretical calculations and can be searched for in data using specific algorithms based on a model known as ‘adapted filtering’.

Several minutes after the gravitational wave was detected on Thursday 17th August 2017 at 14.41 (Paris time) and was named GW170817, the first indication of the presence of a particularly interesting “candidate” came from the LIGO-Hanford detector: the automatic analysis of data clearly indicated a merger of two neutron stars, or in any case, of stars lighter than all the black holes that are known of so far. In addition to this initial indication, an automatic alert message was received from the Fermi satellite, in orbit around the Earth. It had taken less than a minute to announce the detection of a gamma-ray burst, observed barely two seconds after the signal was recorded by LIGO-Hanford!

A gamma-ray burst

A gamma-ray burst is a burst of energetic photons (called "gamma-rays") detected for a short time (a few seconds to a few minutes) from a given direction of the sky. Approximately one gamma-ray burst is detected each day. Theoretically, it is thought that the longest ray bursts are produced by the gravitational collapse of a giant star producing a black hole or a neutron star, whilst short ray bursts are caused by merging neutron stars - a phenomenon actually observed for GW170817.

 

The estimated location of the source given by the LIGO-Virgo network was made available roughly five hours after the gravitational wave occurred - the dark-green area in the picture. It confirms the locations provided firstly, by the Fermi satellite, from the gamma-ray burst it detected (area in dark blue), and secondly, by comparing the arrival time of the gamma-rays in the Fermi and INTEEGRAL satellites (area in light blue). And it is considerably more accurate: about 30 square degrees. It is still a huge area in astronomical terms - about one hundred times the size of the full moon - but it is the most precise location known to date for a source of gravitational waves.


This image compares the different locations in the sky of the transitory event detected on 17th August 2017: from the gravitational wave signal detected by the global network of three Virgo-LIGO dectectors (areas coloured in green); by using the gamma-ray burst recorded by the Fermi and INTEGRAL satellites (area coloured in blue); with the Swope telescope which first discovered the optical counterpart. The boxes on the right show the galaxy NGC4993; in the top image, taken almost 11 hours after the gravitational waves and gamma-ray burst were detected, a new light source (shown by the reflection) is visible; however, in the bottom photo, taken about three weeks after the event, it is not there. © LIGO/Virgo/Collaborations authors of the article “Multi-Messenger Astronomy”


This map of the sky was sent to partner telescopes of the LIGO and Virgo collaborations who are going to use it to look for an optical counterpart to GW170817. Unlike mergers in binary black hole systems that should not accompanied by electromagnetic emissions, neutron stars are made of matter and a merger is expected to produce a brief gamma-ray burst (such as that recorded by Fermi and INTEGRAL). This is then followed by a phenomenon known as a kilonova: the unstable nuclei, produced in large quantities during the merger, disintegrate and emit light whose spectrum passes from blue to red as the hot matter cools and disperses. And, in this environment that is rich in neutrons, heavy chemical elements - such as lead, gold or platinum - form and are ejected into space.


Map of the world showing the position of LIGO-Virgo gravitational wave detectors (in yellow) and observatories which have helped monitor the event GW170817 (in blue). © LIGO-Virgo



11 hours after GW170817 was detected, the 1M2H collaboration which carries out observations on the American Swope telescope in Chile announced the discovery of a new light source in the galaxy NGC4993, located in the Hydra constellation, approximately 130 million light-years from Earth.
This result was very quickly confirmed by other telescopes, in particular the ESO telescopes in Chile and the Hubble Space Telescope orbiting the Earth. To date, the source has been detected in visible light, infra-red, ultraviolet, in the field of x-rays and radio waves: observations continue. The main publication that traces the chronology of this "multi-messenger" discovery is co-authored by more than three thousand scientists from sixty collaborations.

This discovery is very important and its announcement, barely two months after the signal GW170817 was detected, is the result of a major effort from the LIGO and Virgo collaborations and tens of partner collaborations. Amongst the most significant results obtained, we can cite:
•    Confirmation that at least some short gamma-ray bursts are produced by the merger of two neutron stars - something which the scientific community has suspected for a long time.
•    The first kilonova observed over a very wide spectral range.
•    Proof that some of the heavy elements in the Universe are formed during such cataclysmic phenomena.
•    A better understanding of the physics of neutron stars and the possibility of testing certain models.
•    A first estimate of the Hubble Constant (which measures the rate of expansion of the Universe), independent of the measurements made so far.
Other studies are in progress. Almost thirty years ago after the adventure began, one of the objectives set from the outset by the scientists behind the Virgo project - in particular Alain Brillet (joint winner of the CNRS gold medal 2017) in France and Adalberto Giazotto in Italy - has been achieved: to make multi-messenger astronomy including gravitational waves a reality. The best is yet to come!

The Linear Accelerator Laboratory (LAL, Paris-Sud University/CNRS) is one of the founding members of the Virgo collaboration. Previously, the LAL-Virgo group was in charge of building the large-diameter vacuum tubes which form the two 3 km arms of the interferometer, where the laser beams travel, as well as a large part of the experiment's software (longitudinal and angular control of the interferometer, vacuum command and control, general software tools, etc.). Its activities are split between two main areas: scientific exploitation of Virgo-LIGO data and activities focusing more on the detector.

To find out more:
•    Read the full article on the LAL website: https://www.lal.in2p3.fr
•    Virgo collaboration website: http://www.virgo-gw.eu

Contacts:
Patrice Hello - Leader of the VIRGO group at the Linear Accelerator Laboratory - LAL (UPSud/CNRS) - hello @ lal.in2p3.fr
Nicolas Arnaud - CNRS researcher at the Linear Accelerator Laboratory - LAL (UPSud/CNRS) - narnaud @ lal.in2p3.fr