Gravitational-wave observatories LIGO and Virgo have observed the biggest black hole merger yet. In May 2019, the three detectors (LIGO has two) reported the detection of a collision that ended up forming a black hole weighing 142 times the mass of our Sun, the heaviest ever seen by the observatories. This final black hole also fits into the still-mysterious class of intermediate-mass black holes, which if confirmed would be the first direct observation of one. The heavier of the two colliding black holes also hints at a knowledge gap in stellar evolution.

It's not only the most massive final black hole detected, it's also the furthest observed yet. The signal traveled for half the lifetime of the universe before reaching us, around 7 billion years. Analysis of the event, GW190521, is reported in Physical Review Letters and The Astrophysical Journal Letters. This event was first in the news in June 2020. Before the merger was confirmed, its location was noted as being peculiar. It happened in the disk surrounding a supermassive black hole.

"This doesn't look much like a chirp, which is what we typically detect," Nelson Christensen, a researcher at the French National Centre for Scientific Research, said in a statement, comparing this signal to past detections. "This is more like something that goes 'bang,' and it's the most massive signal LIGO and Virgo have seen."

Intermediate-mass black holes have so far only been indirectly observed, so this could be the first exciting direct detection of one. These objects have masses between 100 to 100,000 times that of our Sun. They are much lighter than the supermassive ones we find at the core of galaxies, which weigh millions, if not billions, that of regular stars but they are heavier than what we expect to be produced by stars, which are around five to a few tens the mass of our Sun. The lack of in-between objects has long perplexed astronomers,

Another surprising discovery in this detection is that the two colliding black holes that caused GW190521 were around 66 and 85 solar masses respectively, with the heaviest one falling into the so-called “pair-instability” gap. 

“From our understanding of how stars age and evolve we expect to find black holes with either less than 65 solar masses or more than 135 solar masses, but none in between,” Frank Ohme, leader of an Independent Max Planck Research Group at AEI Hannover, said in a statement. “The 85 solar-mass black hole in the GW190521 origin system falls right in that gap where it shouldn’t be. This can mean two things: our understanding of star evolution is incomplete or something different has happened here.”

The team is leaning towards the latter. They believe that the heaviest black hole might be the product of previous mergers; either two smaller black holes or a collision between two larger stars. 

To gain more insights it will be necessary to see it in context with other the discoveries made by LIGO and Virgo's third run to understand if it’s a limited case or one of many unexpected objects.

“We don’t know yet whether GW190521, this surprising discovery and first observation of an intermediate-mass black hole, is an entirely new class of binary black holes or just the high-mass end of the source spectrum we’ve seen so far,” said Karsten Danzmann, director at the AEI Hannover, and director of the Institute for Gravitational Physics at Leibniz University Hannover. “Soon, when we have analyzed all binary black hole mergers seen by LIGO and Virgo in their third observing run we might know more.”

The LIGO and Virgo observatories have detected 56 possible gravitational-wave events in their third run of operation, which lasted between April 1, 2019 and March 27, 2020. Four of these events have been confirmed and made public with the remaining 52 still being analyzed.