The gravitational-wave signal GW190814, detected on August 14, 2019 by NSF’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector in Europe, was generated by a compact object — a neutron star or a black hole — 2.6 times the mass of our Sun merging with a black hole of 23 times the mass of the Sun. At a ratio of 9:1, it is the largest difference in masses yet observed during a collision by gravitational wave astronomers. The less massive object is either the lightest black hole or the heaviest neutron star ever discovered in a binary system of compact objects.
An artist’s rendition of a mystery compact object in ‘mass gap’ found by LIGO and Virgo gravitational wave detectors. Image credit: LIGO / Caltech / MIT / R. Hurt, IPAC.
For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 solar masses and the lightest known black hole is about 5 solar masses.
The question remained: does anything lie in this so-called mass gap?
“We’ve been waiting decades to solve this mystery,” said Northwestern University’s Professor Vicky Kalogera.
“We don’t know if this object is the heaviest known neutron star, or the lightest known black hole, but either way it breaks a record.”
“This is going to change how scientists talk about neutron stars and black holes,” said Professor Patrick Brady, a researcher at the University of Wisconsin, Milwaukee, and spokesperson for the LIGO Scientific Collaboration.
“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.”
The GW190814 event created a new black hole 26 solar masses in size and caused a blast of energy in the form of gravitational waves, which rippled out across spacetime like when a rock is dropped into a pond. About 800 million years after the collision occurred, those ripples finally reached Earth and passed through the ultra-sensitive LIGO and Virgo detectors. Image credit: University of Glasgow.
The GW190814 merger occurred in a galaxy about 800 million light-years away from Earth.
It resulted in a final black hole about 25 times the mass of the Sun (some of the merged mass was converted to a blast of energy in the form of gravitational waves).
Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a mass ratio of about 4:1.
When the LIGO and Virgo scientists spotted GW190814, they immediately sent out an alert to 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 signals.
“This discovery is exciting — it may be the collision of a neutron star and a black hole — something we have been long searching for,” said Dr. Christopher Berry, a scientist in the Institute for Gravitational Research at the University of Glasgow.
Future observations with LIGO, Virgo and possibly other telescopes may catch similar events that would help reveal whether additional objects exist in the mass gap.
A paper on the findings was published in the Astrophysical Journal Letters.
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R. Abbott et al. 2020. GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass Black Hole with a 2.6 Solar Mass Compact Object. ApJL 896, L44; doi: 10.3847/2041-8213/ab960f
