BOZEMAN — Last August, astronomers detected gravitational waves emitted by the collision of two massive objects 800 million light years from Earth. One of those objects was a black hole; the other is a mystery, according to Montana State University astrophysicists.

The mystery object is larger than any neutron star we’ve ever observed but smaller than our current knowledge deems possible for a black hole, and researchers at MSU say its discovery may have far-reaching implications in the study of both cosmic objects.

Neil Cornish. MSU photo by Kelly Gorham

“This mystery object is either the smallest black hole ever discovered or the most massive neutron star ever discovered,” said Montana State University Regents Professor Neil Cornish, director of the MSU eXtreme Gravity Institute.

When the most massive of stars die, they collapse under their own gravity and leave behind black holes; when stars a bit less massive die, they explode in a spectacular supernova, leaving dense, dead remnants known as neutron stars. Yet the largest neutron star ever observed is just over twice the mass of our sun, and the smallest known black hole is around five times the mass of the sun — leaving what astronomers refer to as a “mass gap” between them.

That gap presents a quandary for physicists. If the gap is real, why does it exist? If it doesn’t, why have objects not been observed within its bounds?

Cornish, a professor in the Department of Physics in MSU’s College of Letters and Science, is among the group of scientists pondering the mystery object, which totals 2.6 solar masses — well within this mass gap, in a new study from the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector in Europe published in The Astrophysical Journal Letters.

The object was found Aug. 14, 2019, as it merged with a black hole 23 times the mass of our sun, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo.

MSU researchers continue to be at the forefront of new discoveries based on gravitational waves — ripples in space-time caused by cataclysmic events. Cornish describes the waves as minute expansions and contractions of space itself. The August collision, for example, changed the distance between Earth and the sun by about the width of an atom.

Gravitational waves were first detected by LIGO in 2015, a discovery that confirmed part of Albert Einstein’s 1915 theory of relativity and led to a Nobel Prize for the collaboration’s pioneers. MSU has been part of the collaboration since 2007.

Cornish developed one of the methods used to extract the signal from the LIGO/Virgo data along with former MSU student Tyson Littenberg, a co-author on the study who is now a researcher at NASA’s Marshall Space Flight Center in Huntsville, Alabama. MSU doctoral candidate Bence Bécsy, who is also a co-author, analyzes that data as part of Cornish’s team.

“It's wonderful that these groundbreaking discoveries are being made with the participation of scientists at MSU, including by MSU graduate students,” said Dana Longcope, physics department head at MSU.

When the LIGO and Virgo scientists spotted the merger between the black hole and the mystery object among the gravitational wave data, they immediately alerted the astronomical community. Dozens of ground- and space-based telescopes followed up in search of light waves generated in the event, but none picked up any sign — astronomers believe the large black hole swallowed the mystery object whole, so no light escaped, unlike with mergers between two neutron stars, the first of which was observed by the LIGO team in 2017.

“In the future we hope to detect other mass gap objects merging with a lighter companion and getting torn apart in the process, producing the kind of fireworks show we saw with the neutron star merger,” Cornish said.

Gravitational wave events are named for the date they occurred. The merger described in the study, GW190814, left a black hole about 25 times the mass of the sun, with some of the merged mass converted to a blast of energy in the form of gravitational waves.

Before the two objects merged, the black hole was about nine times bigger than the mystery object, making this the most extreme mass ratio known for a gravitational-wave event. The new observations may change future theoretical models for merging objects within the mass gap.

“This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object,” said Vicky Kalogera, a professor at Northwestern University.

Now, with the discovery of the mystery object, scientists are wondering if this so-called mass gap even exists.

“This is going to change how scientists talk about neutron stars and black holes,” said co-author Patrick Brady, a professor at the University of Wisconsin, Milwaukee, and the LIGO Scientific Collaboration spokesperson. “The mass gap may, in fact, not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell.”

LIGO is funded by the NSF and operated by Caltech and MIT, which conceived of LIGO and lead the project. 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-OzGrav) making significant commitments and contributions. Approximately 1,300 scientists around the world participate through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo Collaboration comprises approximately 550 members from 106 institutes in 12 European countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa, Italy, and is funded by Centre National de la Recherche Scientifique in France, the Istituto Nazionale di Fisica Nucleare in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration groups can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at http://www.virgo-gw.eu.

- By Rachel Hergett, MSU News Service -