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LIGO and Virgo Detect Neutron Star Smash-Ups



Ligo Virgo.jpg
Vista posterior d'un mirall suspès. El recobriment reflecteix el feix làser d'infraroig proper de Virgo, però és transparent en el rang visible.
Crèdits: EGO/Virgo Collaboration/Perciballi

Date: 03/05/2019

On April 25, 2019, the National Science Foundation 's Laser Interferometer Gravitational-Wave Observatory (LIGO) and theEuropean-based Virgo detector registered gravitational waves from what appearslikely to be a crash between two neutron stars—the dense remnants of massivestars that previously exploded. One day later, on April 26, the LIGO-Virgonetwork spotted another candidate source with a potentially interesting twist:it may in fact have resulted from the collision of a neutron star and blackhole, an event never before witnessed.

"The universe is keeping us on our toes,"says Patrick Brady, spokesperson for the LIGO Scientific Collaboration and aprofessor of physics at the University of Wisconsin-Milwaukee. "We'reespecially curious about the April 26 candidate. Unfortunately, the signal israther weak. It's like listening to somebody whisper a word in a busy café; itcan be difficult to make out the word or even to be sure that the personwhispered at all. It will take some time to reach a conclusion about thiscandidate."

"NSF's LIGO, in collaboration with Virgo,has opened up the universe to future generations of scientists," says NSFDirector France Cordova. "Once again, we have witnessed the remarkablephenomenon of a neutron star merger, followed up closely by another possiblemerger of collapsed stars. With these new discoveries, we see the LIGO-Virgocollaborations realizing their potential of regularly producing discoveriesthat were once impossible. The data from these discoveries, and others sure tofollow, will help the scientific community revolutionize our understanding ofthe invisible universe."

The discoveries come just weeks after LIGO and Virgo turned back on. The twin detectors of LIGO—one inWashington and one in Louisiana—along with Virgo, located at the European Gravitational Observatory (EGO) in Italy, resumed operations April 1, after undergoinga series of upgrades to increase their sensitivities to gravitational waves—ripplesin space and time. Each detector now surveys larger volumes of the universe thanbefore, searching for extreme events such as smash-ups between black holes andneutron stars.

"Joining human forces and instruments across the LIGOand Virgo collaborations has been once again the recipe of an incomparablescientific month, and the current observing run will comprise 11 more months,"says Giovanni Prodi, the Virgo Data Analysis Coordinator, at the University of Trento and theIstituto Nazionale di Fisica Nucleare (INFN) in Italy. "The Virgodetector works with the highest stability, covering the sky 90 percent of thetime with useful data. This is helping in pointing to the sources, both whenthe network is in full operation and at times when only one of the LIGO detectors is operating. We have a lot of groundbreaking research work ahead."

In addition to the two new candidatesinvolving neutron stars, the LIGO-Virgo network has, in this latest run, spottedthree likely black hole mergers. In total, since making history with the first-ever direct detection of gravitational waves in 2015, the network has spotted evidence for two neutron star mergers; 13black hole mergers; and one possible black hole-neutron star merger.

When two black holes collide, they warp thefabric of space and time, producing gravitational waves. When two neutron starscollide, they not only send out gravitational waves but also light. That meanstelescopes sensitive to light waves across the electromagnetic spectrum canwitness these fiery impacts together with LIGO and Virgo. One suchevent occurred in August 2017: LIGO andVirgo initially spotted a neutron star merger in gravitational waves and then,in the days and months that followed, about 70 telescopes on the ground and inspace witnessed the explosive aftermath in light waves, including everythingfrom gamma rays to optical light to radio waves.

In the case of the two recent neutron starcandidates, telescopes around the world once again raced to track the sourcesand pick up the light expected to arise from these mergers. Hundreds ofastronomers eagerly pointed telescopes at patches of sky suspected to house thesignal sources. However, [MOU1] , neither of the sources has beenpinpointed.

"The search for explosive counterparts ofthe gravitational-wave signal is challenging due to the amount of sky that mustbe covered and the rapid changes in brightness that are expected," saysBrady. "The rate of neutron star merger candidates being found with LIGOand Virgo will give more opportunities to search for the explosions over thenext year."

The April 25 neutron star smash-up, dubbed S190425z,is estimated to have occurred about 500 million light-years away from Earth. Onlyone of the twin LIGO facilities picked up its signal along with Virgo (LIGOLivingston witnessed the event but LIGO Hanford was offline.) Because only twoof the three detectors registered the signal, estimates of the location in thesky from which it originated were not precise, leaving astronomers to survey nearlyone-quarter of the sky for the source.

The possible April 26 neutron star-black holecollision (referred to asS190426c) is estimated to have taken placeroughly 1.2 billion light-years away. It was seen by all three LIGO-Virgofacilities, which helped better narrow its location to regions covering about 1,100square degrees, or about 3 percent of the total sky.

"The latest LIGO-Virgo observing run isproving to be the most exciting one so far," says David H. Reitze ofCaltech, Executive Director of LIGO. "We're already seeing hints of thefirst observation of a black hole swallowing a neutron star. If it holds up,this would be a trifecta for LIGO and Virgo—in three years, we'll have observedevery type of black hole and neutron star collision. But we've learned thatclaims of detections require a tremendous amount of painstaking work—checkingand rechecking—so we'll have to see where the data takes us."

LIGOis funded by NSF and operated by Caltech and MIT, which conceived of LIGO andled 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-Oz Grav) making significant commitments and contributions to theproject. Approximately 1,300 scientists from around the world participate inthe effort through the LIGO Scientific Collaboration, which includes the GEOCollaboration. A list of additional partners is available at .

TheVirgo Collaboration is currently composed of approximately 350 scientists,engineers, and technicians from about 70 institutes from Belgium, France,Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy,and is funded by Centre National de la Recherche Scientifique (CNRS) in France,the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in theNetherlands. A list of the Virgo Collaboration members can be found at . More information is available on the Virgo websiteat

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