Neutron stars are dead stars collapsed into the densest form of matter known to humans, with a teaspoon of neutron star matter weighing a billion tons, and their collision creates a swath of galactic debris. Decades ago, stargazing scientists formed plans to detect signals from this debris. Now, in the new era of aptly named “multi-messenger astronomy,” two international projects have achieved this goal: On August 17 of this year, the Laser Interferometer Gravitational-Wave Observatory (LIGO)’s two U.S.-based interferometers and the Virgo Collaboration’s Italy-based interferometer detected for the first time gravitational waves — ripples in space-time traveling at the speed of light — from the collision and subsequent merger of two neutron stars. The detection occurred just three days after yet another “chirp” from colliding black holes.
This is LIGO’s fifth significant detection of gravitational waves from a catastrophic cosmic event, but the first detection from a kilonova. The Aug. 14 and 17 chirps were the first for Virgo. LIGO sent an alert to about a hundred observatories around the world, sparking a six-hour hunt for light and other emissions within a banana-shaped band of cosmic signals from the event. The observatories confirmed that the ripple of gravitational waves were indeed from a collision and subsequent merger of two neutron stars. A broad range of so-called cosmic messengers (gravitational waves, gamma-rays, X-rays, light, radio waves) have been recorded from the collision, marking the beginning of what astronomers refer to as “multi-messenger astronomy.”
Gravitational waves carry information on the acceleration of heavy objects, such as what occurs during the merger of two neutron stars or black holes. In this case, they told scientists that the neutron-star merger occurred about 120 million light years from Earth, closer than scientists’ previous expectations. The discovery was almost simultaneous with a sharp burst of gamma rays observed by two orbiting satellites, Fermi and INTEGRAL.
Neutron star mergers were one of the original motivations for constructing the LIGO detectors, an endeavor that began half a century ago and became the biggest project the National Science Foundation ever funded. In the Advanced LIGO project stage, the interferometers heard the first “chirp” of gravitational waves from merging black holes in 2015. The discovery, confirmed last year, heralded the beginning of gravitational-wave astronomy and earned the LIGO Scientific Collaboration much acclaim.
Scientists in the LIGO Scientific Collaboration, of which UF is a founding member, and elsewhere are using their observations of the neutron-star merger to study the universe as well as the fundamental laws of nature. Currently, gravitational-wave detectors have 232 institutions and about 1600 scientists as members. UF has made key contributions to these studies across the entire scope of the project. Physicists now better understand how extremely energetic photons are produced in outer space. LIGO enables scientists to study the expansion of the universe in unprecedented ways by comparing the observed gravitational waves to what is known about the distant galaxy in which the merger happened more than 100 million years ago. Physicists are also increasing their understanding of how matter behaves at densities greater than that of the atomic nucleus.
UF made seminal contributions to infrared and optical observations of emissions from the debris around the merged neutron stars. Led by Steve Eikenberry of UF Astronomy, UF built the FLAMINGOS2 instrument, a near-infrared imaging spectrograph installed in the Gemini-South 8-meter telescope. FLAMINGOS2 detected the infrared emission 12 hours after the chirp took place and helped confirm the signal, which was characteristic of a kilonova.
In addition, newly appointed UF faculty member Imre Bartos plays a leading role in searches for neutrinos from the merger, probing emission mechanisms at extreme energies. “Neutrinos are notoriously hard to detect, so we use IceCube, a billion-ton detector deep in the ice under the South Pole in Antarctica,” said Bartos. “No extra neutrinos were seen from this event, but even this allows us to set limits on what happened following the merger.”
UF became the third university to join LIGO after Caltech and MIT. The UF-LIGO team in the Department of Physics has spearheaded the design and construction of crucial components of the LIGO observatories and was responsible for the input optics (IO) of both the initial and the Advanced LIGO detectors. The IO is one of the most complex parts of the detector, and many key components were fabricated at UF. Florida also made significant contributions to the optical design of the main interferometer. The team working on these efforts includes Guido Mueller, David Reitze, David Tanner, Paul Fulda, and John Conklin. Hai-Ping Cheng leads the computational effort to reduce thermal noise in the detector.
UF’s Sergey Klimenko and Guenakh Mitselmakher developed the algorithm that discovered the first gravitational-wave signal in LIGO data on September 14, 2015. The UF algorithm is now being used to study the fate of the neutron stars after they merged.
The discovery was published on Oct. 16, 2017 in the Astrophysical Journal Letters.
This year’s Physics Nobel Prize was awarded to three scientists who were instrumental in the construction of LIGO and the first direct observation of gravitational waves, published last year. This award was given independently of the discovery of the neutron star collision.
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 list of UF senior LIGO members is Guenakh Mitselmakher (PI), David Tanner, David Reitze (LIGO Executive Director), Sergey Klimenko, Guido Mueller, Bernard Whiting, Steve Eikenberry, Hai-Ping Cheng, John Conklin, and Imre Bartos.
The Virgo collaboration comprises more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (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.
Contacts: Guenakh Mitselmakher, 352-871-1663; Stephen S. Eikenberry, 352-514-7632; Imre Bartos, 917-455-6264; David Tanner, 352-318-3985