Gravitational Waves provide first light on a Neutron Star Merger
Scientists of the LIGO Collaboration and Virgo Collaboration, including the Optomechanics and Quantum Measurement team of Laboratoire Kastler Brossel, have observed for the first time the gravitational wave emission from a merger of two Neutron Stars (not two Black Holes that were the previous detections). In another first, this source of gravitational waves also produced electromagnetic signals, observed in the hours, days and weeks following by a collaboration of 60 Earth and Space-based observatories. This ensemble of observation, marking the start of a new type of « Multi-messenger » astronomy, has given a multitude of results, from the resolution of the enigma of short gamma-ray bursts to the origin of the heaviest chemical elements (such as lead, gold or platinum), to the study of the properties of neutron stars, to an independent measurement of the speed of expansion of the Universe. Around 10 scientific articles have been released on the 16 October 2017 detailing these many results, and summarised in the Science Summaries linked at the end of this page.
First observation of a Binary Neutron Star Merger
In an article published in the journal Physical Review Letters, the scientists of the LIGO and Virgo collaborations present the first observation of gravitational waves emitted from a binary neutron star merger.
This observation, seen on the 17 August 2017 with all three gravitational-wave detectors of LIGO-Virgo, is very clear with a combined signal-to-noise ratio of 32, the strongest signal of any detection seen to date.
This signal is much longer in time than that of a binary black hole merger (100s of seconds versus a fraction of 1 second), meaning that the two merging objects are much lighter than black holes. The signal’s frequency reveals these objects orbitted very quickly around each other, so they are very compact objects. With the observation of electromagnetic signals, it excludes that they are light black: these are therefore two neutron stars with masses between 1.17 and 1.60 times that of the Sun, based on a more detailed analysis of the data.
The gravitational waves signals are very visible in the LIGO-Livingston instrument, a little less strong in the LIGO-Hanford instrument. It is not visible in the Virgo instrument because the sky position of the source was located in a zone that Virgo is not sensitive to – a blind spot. Nevertheless, Virgo was essential to greatly localise the sky position, allowing the exclusion of certain zones of the sky and focussing the search zone sent to astronomers from 190 square degrees to 31 square degrees.
A new « Multi-messenger » Astronomy
The publication appearing in the journal Astrophysical Journal Letters consists of an even larger number of authors : co-signed by scientists of LIGO and Virgo, as well as those of 70 other groups who have made observations with instruments situated on Earth or on satellites.
It is the first time that an astrophysical object has been observed using two different cosmic messenger signals: gravitational waves and electromagnetic signals. The LIGO and Virgo collaboration has information-sharing agreements with nearly 90 different groups to observe the sky with a range of different instruments.
The figure below summarises the results for source localisation in the sky. The results from the LIGO and Virgo gravitational-wave detectors are shown in green (pale green regions for LIGO only, dark green when we add information from Virgo). The results from the gamma ray detectors are in blue (dark blue for Fermi-GBM and light blue for Fermi and INTEGRAL). The upper inset image shows the host galaxy NGC 4993, and is the first electromagnetic image of a gravitational-wave source, taken by the Swope telescope 10.9 hours after the binary neutron star merger (indicated by the cross-mark to the upper left of NGC 4993, and is not present in the lower image, taken 20.5 days before the merger).
Link to the articles : http://public.virgo-gw.eu/gw170817_papers
Link to GW170817 Science Summary webpage : https:/ligo.org/Publication-GW170817BNS/index.php
Link to Multi-messenger Astronomy Science Summary webpage : http://ligo.org/science/Publication-GW170817MMA/index.php
Link to LIGO/Virgo Press Release : PDF
“Binary Neutron Stars” Image Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
Contact: Pierre-Francois Cohadon