"NSF's LIGO, in collaboration with Virgo, has opened up the universe to future generations of scientists", stated NSF Director France Cordova. "Once again, we have witnessed the remarkable phenomenon of a neutron star merger, followed up closely by another possible merger of collapsed stars. With these new discoveries, we see the LIGO-Virgo collaborations realizing their potential of regularly producing discoveries that were once impossible. The data from these discoveries, and others sure to follow, will help the scientific community revolutionize our understanding of the invisible universe."
The National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign is engaged in this new era of discovery via the Gravity Group, a team of researchers who are contributing to various aspects of gravitational wave astrophysics, from the review of signal-processing algorithms and waveform models for gravitational wave detection, to the development of algorithms to use numerical relativity waveform to search for compact binary systems in dense stellar environments.
The Gravity Group is also spearheading the use of deep learning algorithms at scale to increase the speed and depth of gravitational wave searches. This data-driven science program has an overarching goal of building a community to accelerate the development of deep learning algorithms to tackle Big-Data science problems across disciplines.
"This is a great start to the third observing run of the advanced LIGO-Virgo detectors. The expectations for an increase in detection rate for binary black hole mergers and neutron star mergers is holding true", stated Eliu Huerta, Group Lead of the NCSA Gravity Group. The current scale and detection rate of observations indicate the timeliness to accelerate convergence between gravitational wave astrophysics and the Big Data revolution, a key endeavor of the NCSA Gravity Group."
"It is such an exciting time to be working in relativistic astrophysics. Its really invigorating for our Gravity Group at NCSA, where undergraduate and graduate students from across the University of Illinois, and even high school students, are working together on research questions in astrophysics, Big Data, machine learning, and visualization", stated Gabrielle Allen, Professor of Astronomy at Illinois. "Then, they get to see the very problems they are contributing to reported in the news."
The discoveries come just weeks after LIGO and Virgo turned back on. The twin detectors of LIGO - one in Washington and one in Louisiana - along with Virgo, located at the European Gravitational Observatory (EGO) in Italy, resumed operations April 1, after undergoing a series of upgrades to increase their sensitivities to gravitational waves - ripples in space and time. Each detector now surveys larger volumes of the universe than before, searching for extreme events such as smash-ups between black holes and neutron stars.
In addition to the two new candidates involving neutron stars, the LIGO-Virgo network has, in this latest run, spotted three 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; 13 black hole mergers; and one possible black hole-neutron star merger.
"Already at the dawn of this new era of astrophysics and cosmology we are learning so much about what the universe has to offer, having now seen evidence of black holes and neutron stars colliding in all combinations", added Edward Seidel, Vice President for Economic Development and Innovation and Professor of Physics and Astronomy at Illinois. "With many more events to come, we are off to an amazing start to this field. What comes next? I can't wait to see."
When two black holes collide, they warp the fabric of space and time, producing gravitational waves. When two neutron stars collide, they not only send out gravitational waves but also light. That means telescopes sensitive to light waves across the electromagnetic spectrum can witness these fiery impacts together with LIGO and Virgo. One such event occurred in August 2017: LIGO and Virgo 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 in space witnessed the explosive aftermath in light waves, including everything from gamma rays to optical light to radio waves.
In the case of the two recent neutron star candidates, telescopes around the world once again raced to track the sources and pick up the light expected to arise from these mergers. Hundreds of astronomers eagerly pointed telescopes at patches of sky suspected to house the signal sources. However, at this time, neither of the sources has been pinpointed.
The April 25 neutron star smash-up, dubbed S190425z, is estimated to have occurred about 500 million light-years away from Earth. Only one of the twin LIGO facilities picked up its signal along with Virgo (LIGO Livingston witnessed the event but LIGO Hanford was offline). Because only two of the three detectors registered the signal, estimates of the location in the sky from which it originated were not precise, leaving astronomers to survey nearly one-quarter of the sky for the source.
The possible April 26 neutron star-black hole collision (referred to as S190426c) is estimated to have taken place roughly 1.2 billion light-years away. It was seen by all three LIGO-Virgo facilities, which helped better narrow its location to regions covering about 1,100 square degrees, or about 3 percent of the total sky.