UNIVERSITY PARK, Pa. — Gravitational waves from what may be the most massive black hole merger yet observed have been detected by the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO). Produced by extreme astrophysical phenomena, these reverberations ripple forth and shake the fabric of space-time. The product of the merger is the first clear detection of an “intermediate-mass” black hole, with a mass between 100 and 1,000 times that of the sun.
The research team, which includes Penn State scientists, detected the signal, labeled GW190521, on May 21, 2019, with LIGO, a pair of identical, 4-kilometer-long interferometers in the United States; and Virgo, a 3-kilometer-long detector in Italy.
The signal seen by the detectors, which resembles about four short wiggles, is extremely brief in duration, lasting less than one-tenth of a second. From what the researchers can tell, GW190521 was generated by a source that is roughly 5 gigaparsecs, around 17 billion light years, away, when the universe was about half its age, making it one of the most distant gravitational-wave sources detected so far.
As for what produced this signal, based on a powerful suite of state-of-the-art computational and modeling tools, scientists think that GW190521 was most likely generated by a binary black hole merger with unusual properties.
"The signal that very massive black hole binaries leave in the data is very short,” said Ryan Magee, a graduate student at Penn State and a LIGO team member. “Unfortunately, this can make it difficult to distinguish between short duration noise transients, or glitches, and gravitational-wave candidates."
Every confirmed gravitational-wave signal to date has been from a binary merger, either between two black holes or two neutron stars. This newest merger appears to be the most massive yet, involving two inspiraling black holes with masses about 85 and 66 times the mass of the sun.
The LIGO-Virgo team has also measured each black hole’s spin and discovered that as the black holes were circling ever closer together, they could have been spinning about their own axes, at angles that were out of alignment with the axis of their orbit. The black holes’ misaligned spins likely caused their orbits to wobble, or “precess,” as the two goliaths spiraled toward each other.
The new signal likely represents the instant that the two black holes merged. The merger created an even more massive black hole, of about 142 solar masses, and released an enormous amount of energy, equivalent to around 8 solar masses, spread across the universe in the form of gravitational waves.
“This doesn’t look much like a chirp, which is what we typically detect,” said Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO’s first detection of gravitational waves in 2015. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”
The international team of scientists, who make up the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration, have reported their findings in two papers published today (Sept. 2). One, appearing in Physical Review Letters, details the discovery, and the other, in The Astrophysical Journal Letters, discusses the signal’s physical properties and astrophysical implications.