For the first time, scientists have directly detected gravitational waves — ripples in space and time — in addition to light from the spectacular collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light.
The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 70 ground- and space-based observatories.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. As these neutron stars spiraled together, they emitted gravitational waves that were detectable for about 100 seconds; when they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the smashup, other forms of light, or electromagnetic radiation — including X-ray, ultraviolet, optical, infrared, and radio waves — were detected.
The initial detection of the gravitational signal, named GW170817, was first made on Aug. 17 at 8:41 a.m. Eastern Daylight Time; by the two identical LIGO detectors, located in Hanford, Washington, and Livingston, Louisiana. Virgo, situated near Pisa, Italy, also recovered a signal that allowed scientists to precisely triangulate the position of the source in a relatively small patch in the southern sky.
At nearly the same time that this detection was made the Gamma-ray Burst Monitor on NASA’s Fermi space telescope had detected a burst of gamma rays. LIGO-Virgo analysis software put the two signals together and saw it was highly unlikely to be a chance coincidence, and another automated LIGO analysis indicated that there was a coincident gravitational wave signal in the other LIGO detector.
Rapid gravitational-wave detection by the LIGO-Virgo team, coupled with Fermi’s gamma-ray detection, enabled the launch of follow-up by telescopes around the world. In this follow up an optical transient was identified in the galaxy known as NGC 4993 – which is located over 130 million lights years away from Earth, and in the same region of the southern sky that the gravitational wave detection was made. While at the other end of the electromagnetic spectrum, the first detection of a faint, transient radio emission was reported from the same galaxy by the Jansky Very Large Array (VLA) on Sep. 2, 16 days after the initial detection of the gravitational signal.
“This is the first time that we have witnessed gravitational waves from merging neutron stars, and, in fact, this is the first time that we could identify an astronomical source of gravitational radiation” said Zsolt Paragi, Head of User Support at the Joint Institute for VLBI ERIC. “Our European collaboration has been involved in the observing campaign. Telescope arrays distributed within the UK (e-Merlin) and globally (the European VLBI Network – EVN) are looking for radio emission from the colliding neutron stars witnessed by LIGO and Virgo. These observations are part of ongoing efforts to localise the source of transient radio emission in the sky as it becomes brighter, with the highest precision to date.”
The current findings have only been possible due to the collaboration of multiple institutions and observing stations across the globe, and further work is now required to fully understand their significance.
“We expect this discovery is only the first of many gravitational wave sources which will have detected electromagnetic counterparts, and which can then be studied across the spectrum” said John Conway, Chairman of the EVN Board of Directors. “Radio observations using the EVN are unique in that they provide the highest accuracy location and imaging of any technique in astronomy. The EVN is continuously increasing its sensitivity with higher observing bandwidth and new dishes planned in the coming years. The radio detection of these gravitational wave sources at the very limit of present detectability increases the importance of these sensitivity increases; which we expect will open up a whole new area of study for the EVN.”
The LIGO-Virgo results are published today in the journal Physical Review Letters; additional papers from the LIGO and Virgo collaborations and the astronomical community have been either submitted or accepted for publication in various journals.
Adapted from an original press release written by Jennifer Chu, MIT News Office
European VLBI Network telescopes
The telescopes involved in the observation of the neutron stars were part of the EVN and included: Effelsberg Radio Telescope (Max-Planck Institute for Radio Astronomy, Germany), Hartebeesthoek Radio Astronomy Observatory (National Research Foundation, South Africa), Jodrell Bank Observatory (University of Manchester, UK), Medicina Radio Observatory (National Institute for Astrophysics, Italy), Onsala Space Observatory (Chalmers University of Technology, Sweden), Noto Radioastronomical Station (National Institute for Astrophysics, Italy), Toruń Centre for Astronomy (Nicolaus Copernicus University, Poland), Ventspils International Radio Astronomy Centre (Latvian Academy of Sciences, Latvia), Westerbork Synthesis Radio Telescope (ASTRON, the Netherlands), and Yebes Observatory (National Geographic Institute, Spain).
More about VLBI, the European VLBI Network and JIVE
VLBI is an astronomical method by which multiple radio telescopes distributed across great distances observe the same region of sky simultaneously. Data from each telescope is sent to a central “correlator” to produce images with higher resolution than the most powerful optical telescopes.
The European VLBI Network (EVN; www.evlbi.org) is an interferometric array of radio telescopes spread throughout Europe, Asia, South Africa and the Americas that conducts unique, high-resolution, radio astronomical observations of cosmic radio sources. Established in 1980, the EVN has grown into the most sensitive VLBI array in the world, including over 20 individual telescopes, among them some of the world’s largest and most sensitive radio telescopes. The EVN is administered by the European Consortium for VLBI, which includes a total of 15 institutes, including the Joint Institute for VLBI ERIC (JIVE).
The Joint Institute for VLBI ERIC (JIVE; www.jive.eu) has as its primary mission to operate and develop the EVN data processor, a powerful supercomputer that combines the signals from radio telescopes located across the planet. Founded in 1993, JIVE is since 2015 a European Research Infrastructure Consortium (ERIC) with six member countries: France, Latvia, the Netherlands, United Kingdom, Spain and Sweden; additional support is received from partner institutes in China, Germany, Italy and South Africa.
Paper: “GW170817: Observation of gravitational waves from a binary neutron star merger.”
** Paper will be available to read online at 10:00 AM EDT on Oct. 16, 2017.
More information about EVN: http://www.evlbi.org/