Astronomers have found either the lightest ever black hole or the heaviest neutron star. Either way, it’s an exciting breakthrough.
The mysterious object sits in what is considered the “mass gap” between the two and its discovery “is going to change how scientists talk about neutron stars and black holes”, suggests Patrick Brady from the University of Wisconsin, US.
“The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities,” he says. “Time and more observations will tell.”
The object, which is 2.6 times the mass of the Sun, was found last August when it merged with (perhaps was swallowed by) a black hole of 23 solar masses about 800 million light-years from Earth.
This surprising event, now known as GW190814, resulted in a final black hole of about 25 solar masses. Some of the merged mass was converted to a blast of energy in the form of gravitational waves, which were picked up by the LIGO detector in the US and Virgo detector in Italy.
The findings are reported by a large international team in a paper in The Astrophysical Journal Letters.
When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in a supernova and leave behind dense, dead remnants of stars called neutron stars.
The heaviest known neutron star is no more than 2.5 solar masses and the lightest black hole is about five solar masses. The question remains: does anything lie in this so-called mass gap?
Before the two objects merged in August, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event, the authors say. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a mass ratio of about 4:1.
“It’s a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap,” says co-author Vicky Kalogera, from Northwestern University, US.
“This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object.”
After the initial detection in August, dozens of ground- and space-based telescopes around the world went searching for light waves from the event, but without success.
So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817 in August 2017, the authors say. That involved a fiery collision between two neutron stars that was witnessed by the LIGO-Virgo network and by dozens of telescopes on Earth and in space.
Neutron star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light. Conversely, black hole mergers, in most circumstances, are thought not to produce light.
According to the LIGO and Virgo scientists, the 2019 event was not seen by light-based telescopes for three possible reasons.
First, it was six times farther away than the merger observed in 2017, making it harder to pick up any light signals. Second, if the collision involved two black holes, it likely would have not shone with any light.
Third, if the object was in fact a neutron star, its 9-fold more massive black-hole partner might have swallowed it whole; a neutron star consumed whole by a black hole would not give off any light.
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