On October 16, 2017, LIGO Scientific Collaboration and Virgo Collaboration announced, for the first time, scientists have directly detected gravitational waves — ripples in space-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. This detection opens the window of a long-awaited multi-messenger astronomy. 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. 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.
Figure 1. Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. (Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet.)
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 LIGO-Virgo results are published in the journal Physical Review Letters; additional papers from the LIGO and Virgo collaborations and the astronomical community have been accepted for publication in various journals, e.g. Astrophysical Journal Letters, Nature, etc.
The gravitational signal, named GW170817, was first detected on Aug. 17 at 8:41 a.m. Eastern Daylight Time; the detection was made by the two identical 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.
On Aug. 17, 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 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.
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 that the objects were not as massive as binary black holes — objects that LIGO and Virgo have previously detected. Instead, the inspiraling 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. Background analysis showed an event of this strength happens less than once in 80,000 years by random coincidence.
While binary black holes produce “chirps” lasting a fraction of a second in the LIGO detector’s sensitive band, as detected by LIGO-Virgo for four times before, the Aug. 17 chirp lasted approximately 100 seconds and was seen through the entire frequency range of LIGO — about the same range as common musical instruments. Scientists could identify the chirp source as objects that were much less massive than the black holes seen to date.
Members of the LIGO Scientific Collaboration research group at Tsinghua Unviersity include Junwei Cao and Xilong Fan from Research Institute of Information Technology, and Zhihui Du and Xiangyu Guo (graduated) from Department of Computer Science and Technology, contributing to all gravitational wave detections so far since 2009. In recent two years, the group is working mainly on real-time gravitational-wave data processing and multi-messenger astronomy, including algorithm design, performance optimization, and software development. Detailed research topics include: 1. proposing an optimal statistic for a general ensemble of signals and applying it to an ensemble of known pulsars; 2. implementation of a Bayesian framework for combining astrophysical and gravitational wave information that allow us to probe short gamma-ray burst luminosities; 3. GPU acceleration and optimization of online pipelines for compact binary coalescence searching; 4. applying deep learning methods for real-time gravitational-wave data analysis, etc. These work are supported by National Natural Science Foundation of China and Tsinghua University pilot program.
In recent years, the LIGO Scientific Collaboration research group at Tsinghua University is engaged in international collaboration on gravitational-wave research, with Massachusetts Institute of Technology, California Institute of Technology, University of Western Australia, and University of Glasgow. These include the involvement in 3rd generation gravitational-wave discussions, close contacts with oversea Chinese scholars for talent introduction, and cross-disciplinary exchanging and collaboration. Domestically the group is engaged in popularization of gravitational-wave science and development of gravitational-wave research program, with great impact.
(Note: LIGO is funded by the 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 the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,200 scientists and some 100 institutions from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration and the Australian collaboration OzGrav. Additional partners are listed at http://ligo.org/partners.php. The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la RechercheScientifique (CNRS) in France; eight from the IstitutoNazionale di FisicaNucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.)