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First Time That a Cosmic Event is Observed Optically and With Gravitational Waves

10/17/2017 05:26PM

First Time That a Cosmic Event is Observed Optically and With Gravitational Waves
For the first time, scientists have directly detected gravitational waves (ripples in space-time) together with the light from a spectacular collision of two neutron stars. This marks the first time that a cosmic event has been observed with both gravitational waves and light.

The discovery was made using the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US, the Virgo detector in Italy, and some 70 ground and space-based observatories.

Neutron stars, the smallest, densest stars known to exist, are formed when massive stars explode in supernovas. Neutron stars are so dense that a teaspoon of neutron star material has a mass of about a billion tons. As two of these neutron stars spiraled together about 130 million years ago, they emitted gravitational waves that were detected for about 100 seconds on August 17, 2017. In the days and weeks following the initial discovery, a full spectrum of light and electromagnetic radiation from the event (including X-ray, ultraviolet (UV), optical, infrared (IR) and radio waves) were detected and analyzed -- A treasure trove of material that will keep scientist busy for years to come.

When they collided, a flash of light in the form of a Gamma Ray Burst (GRB) was emitted and seen on Earth about 2 seconds after the gravitational waves. In the days and weeks following the smashup, a full spectrum of light and electromagnetic radiation (including X-ray, ultraviolet (UV), optical, infrared (IR) and radio waves) were detected.

The observations have given astronomers an unprecedented opportunity to probe a collision of two neutron stars. For example, observations made by the US Gemini Observatory, the European Very Large Telescope (VLT), and Hubble Space Telescope reveal signatures of recently synthesized material, including gold and platinum, solving a decades long mystery of where about half of all elements heavier than iron are produced.

"It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe," says France A. Cordova, director of the National Science Foundation (NSF), which funds LIGO. "This discovery realizes a long standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational wave observatories. Only through NSF's four decade investment in gravitational wave observatories, coupled with telescopes that observe from radio to gamma ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes."

A Stellar Sign

The gravitational signal, named GW170817, was first detected on August 17, 2017 at 8:41 AM EDT. The detection was made by the two identical US-based LIGO detectors located in Hanford, Washington and Livingston, Louisiana. The information provided by the third detector, Virgo, situated near Pisa, Italy, enabled an improvement in localizing the cosmic event. At the time, LIGO was nearing the end of its second observing run since being upgraded in a program called Advanced LIGO, while Virgo had begun its first run after recently completing an upgrade known as Advanced Virgo.

The NSF-funded LIGO observatories were conceived, constructed, and operated by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT). Virgo is funded by the Istituto Nazionale di Fisica Nucleare (INFN) in Italy and the Centre National de la Recherche Scientifique (CNRS) in France, and operated by the European Gravitational Observatory (EGO). Some 1500 scientists in the LIGO Scientific Collaboration and the Virgo Collaboration work together to operate the detectors and to process and understand the gravitational wave data they capture.

Each observatory consists of two long tunnels arranged in an L shape, at the joint of which a laser beam is split in two. Light is sent down the length of each tunnel, then reflected back in the direction it came from by a suspended mirror. In the absence of gravitational waves, the laser light in each tunnel should return to the location where the beams were split at precisely the same time. If a gravitational wave passes through the observatory, it will alter each laser beam's arrival time, creating an almost imperceptible change in the observatory's output signal.

On August 17th, LIGO's real-time data analysis software caught a strong signal of gravitational waves from space in one of the two LIGO detectors. At nearly the same time, the Gamma Ray Burst Monitor on NASA's Fermi Gamma Ray Space Telescope detected a burst of gamma rays. The LIGO and Virgo analysis software put the two signals together and saw it was highly unlikely to be a 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.

The LIGO data indicated that two astrophysical objects located at the relatively close distance of about 130 million light years from Earth had been spiraling in toward each other. It appeared the objects were not as massive as binary black holes -- objects that LIGO and Virgo have previously detected. Instead, the objects were estimated to be in a range from around 1.1 to 1.6 times the mass of the sun -- in the mass range of neutron stars. A neutron star is about 20 kilometers, or 12 miles, in diameter and is so dense that a teaspoon of neutron star material has a mass of about a billion tons.

While binary black holes produce "chirps" lasting a fraction of a second in the LIGO detector's sensitive band, the August 17th chirp lasted approximately 100 seconds and was seen through the entire frequency range of LIGO. Scientists could identify the chirp source as objects that were much less massive than the black holes seen to date.

"It immediately appeared to us the source was likely to be neutron stars, the other coveted source we were hoping to see... and promising the world we would see," says David Shoemaker, spokesperson for the LIGO Scientific Collaboration and senior research scientist in MIT's Kavli Institute for Astrophysics and Space Research. "From informing detailed models of the inner workings of neutron stars and the emissions they produce, to more fundamental physics such as general relativity, this event is just so rich. It is a gift that will keep on giving."

"Our background analysis showed an event of this strength happens less than once in 80,000 years by random coincidence, so we recognized this right away as a very confident detection and a remarkably nearby source," adds Laura Cadonati, professor of physics at Georgia Institute of Technology (Georgia Tech) and deputy spokesperson for the LIGO Scientific Collaboration. "This detection has genuinely opened the doors to a new way of doing astrophysics. I expect it will be remembered as one of the most studied astrophysical events in history."

Theorists have predicted that when neutron stars collide, they should give off gravitational waves and gamma rays, along with powerful jets that emit light across the electromagnetic spectrum. The gamma ray burst detected by Fermi is what's called a short gamma ray burst. The new observations confirm that at least some short gamma ray bursts are generated by the merging of neutron stars, something that was only theorized before.

"For decades we've suspected short gamma ray bursts were powered by neutron star mergers," says Fermi Project Scientist Julie McEnery of NASA's Goddard Space Flight Center. "Now, with the incredible data from LIGO and Virgo for this event, we have the answer. The gravitational waves tell us that the merging objects had masses consistent with neutron stars, and the flash of gamma rays tells us that the objects are unlikely to be black holes, since a collision of black holes is not expected to give off light."

But while one mystery appears to be solved, new mysteries have emerged. The observed short gamma ray burst was one of the closest to Earth seen so far, yet it was surprisingly weak for its distance. Scientists are beginning to propose models for why this might be, McEnery says, adding that new insights are likely to arise for years to come.

A Patch in the Sky

Though the LIGO detectors first picked up the gravitational wave in the United States, Virgo, in Italy, also played a key role in the story. Due to its orientation with respect to the source at the time of detection, Virgo recovered a small signal. Combined with the signal sizes and timing in the LIGO detectors, this allowed scientists to precisely triangulate the position in the sky. After performing a thorough vetting to make sure the signals were not an artifact of instrumentation, scientists concluded that a gravitational wave came from a relatively small patch in the southern sky.

"This event has the most precise sky localization of all detected gravitational waves so far," says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration. "This record precision enabled astronomers to perform follow-up observations that led to a plethora of breathtaking results."

"This result is a great example of the effectiveness of teamwork, of the importance of coordinating, and of the value of scientific collaboration," adds EGO Director Federico Ferrini. "We are delighted to have played our relevant part in this extraordinary scientific challenge. Without Virgo, it would have been very difficult to locate the source of the gravitational waves."

Fermi was able to provide a localization that was later confirmed and greatly refined with the coordinates provided by the combined LIGO-Virgo detection. With these coordinates, a handful of observatories around the world were able, hours later, to start searching the region of the sky where the signal was thought to originate. A new point of light, resembling a new star, was first found by optical telescopes. Ultimately, about 70 observatories on the ground and in space observed the event at their representative wavelengths.

"This detection opens the window of a long awaited "multi-messenger" astronomy," says Caltech's David H. Reitze, Executive Director of the LIGO Laboratory. "It's the first time that we've observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves -- our cosmic messengers. Gravitational wave astronomy offers new opportunities to understand the properties of neutron stars in ways that just can't be achieved with electromagnetic astronomy alone."

A Fireball and an Afterglow

Each electromagnetic observatory will be releasing its own detailed observations of the astrophysical event. In the meantime, a general picture is emerging among all observatories involved that further confirms that the initial gravitational wave signal indeed came from a pair of in-spiraling neutron stars.

Approximately 130 million years ago, the two neutron stars were in their final moments of orbiting each other, separated only by about 300 kilometers, or 200 miles, and gathering speed while closing the distance between them. As the stars spiraled faster and closer together, they stretched and distorted the surrounding space-time, giving off energy in the form of powerful gravitational waves, before smashing into each other.

At the moment of collision, the bulk of the two neutron stars merged into one ultra-dense object, emitting a fireball of gamma rays. The initial gamma ray measurements, combined with the gravitational wave detection, also provide confirmation for Einstein's general theory of relativity, which predicts that gravitational waves should travel at the speed of light.

Theorists have predicted that what follows the initial fireball is a "Kilonova," a phenomenon by which the material that is left over from the neutron star collision, which glows with light, is blown out of the immediate region and far out into space. The new light-based observations show that heavy elements, such as lead and gold, are created in these collisions and subsequently distributed throughout the Universe.

In the weeks and months ahead, telescopes around the world will continue to observe the afterglow of the neutron star merger and gather further evidence about various stages of the merger, its interaction with its surroundings, and the processes that produce the heaviest elements in the Universe.

"When we were first planning LIGO back in the late 1980s, we knew that we would ultimately need an international network of gravitational wave observatories, including Europe, to help localize the gravitational wave sources so that light-based telescopes can follow up and study the glow of events like this neutron star merger," says Caltech's Fred Raab, LIGO Associate Director for Observatory Operations. "Today we can say that our gravitational wave network is working together brilliantly with the light-based observatories to usher in a new era in astronomy, and will improve with the planned addition of observatories in Japan and India."

Astronomers using the NASA-ESA Hubble Space Telescope have observed the visible counterpart to gravitational waves for the first time -- the merging of two neutron stars. This merger created a kilonova — an object first predicted by theory more than 30 years ago. This event also provides the strongest evidence yet that short duration gamma ray bursts are caused by mergers of neutron stars. These observations may help solve another long standing question in astronomy -- the origin of heavy chemical elements, like gold and platinum. In the merger of two neutron stars, the conditions appear just right for their production. (Video Credit: NASA, ESA/Hubble, ESO, LIGO-Virgo, Directed by Mathias Jager, Visual design and editing by Martin Kornmesser, Written by Izumi Hansen, Rosa Jesse, Richard Hook and Mathias Jager, Narration by Sara Mendes da Costa, Music by STAN DART ( and Johan B. Monell (, Web and technical support by Mathias Andre and Raquel Yumi Shida, Executive producer Lars Lindberg Christensen)

LIGO is funded by the National Science Foundation (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 NSF with significant commitments and contributions to the project by Germany (Max Planck Society), the UK (Science and Technology Facilities Council), and Australia (Australian Research Council). More than 1200 scientists and some 100 institutions from around the world participate in the effort through the LIGO Scientific Collaboration.

The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups, including the Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, Nikhef in the Netherlands, MTA Wigner RCP in Hungary, the POLGRAW group in Poland, the University of Valencia in Spain, and the European Gravitational Observatory (EGO) that hosts the Virgo detector near Pisa in Italy, which is funded by CNRS, INFN, and Nikhef.

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