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Gravitational wave astronomers find the mysterious object in the mass gap

Black Hole Neutron Star

Artist’s representation of a black hole about to swallow a neutron star. Credit: Carl Knox, ARC of Excellence Center of OzGrav

For decades, astronomers have been perplexed by a gap between neutron stars and black holes, but an important new discovery has found a mysterious object in this so-called “mass gap”.

The group of gravitational waves since University of PortsmouthThe Institute of Cosmology and Gravitation has played a key role in the study, which will change the way scientists look at neutron stars and black holes.

When the most massive stars die, they collapse under their own gravity and leave black holes. When slightly less massive stars die, they explode in a supernova and leave dense and dead remains of stars called neutron stars.

Gravitational waves are emitted every time an asymmetric object accelerates, with the most powerful detection sources gravitational waves being from the collision of neutron stars and black holes. Both of these objects are created at the end of a massive star’s life.

“The reason these findings are so interesting is because we never detected an object with a mass that is firmly within the theoretical mass gap between neutron stars and black holes.” – Dr. Laura Nuttall, astrophysics, University of Portsmouth

The heaviest known neutron star it is not more than two and a half times the mass of our sun, or 2.5 solar masses, and the lightest known black hole it is about five solar masses.

The new study by the National Science Foundation’s Laser Interferometer Observatory on Gravitational Waves (LIGO) and the detector of the Virgin in Europe, announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap.

LIGO is made up of two gravitational wave detectors 3000 km away in the United States: one in Livingston, Louisiana, and one in Hanford, Washington. The detector of the Virgin is in Cascina, in Italy.

Dr. Laura Nuttall, gravitational wave expert from the University’s Institute of Cosmology and Gravitation, said: “The reason why these results are so exciting is because we have never detected an object with a mass firmly inside of the theoretical mass gap between neutron stars and black holes before. Is it the lightest black hole or the heaviest neutron star we’ve ever seen? “

Portsmouth doctoral student Connor McIsaac conducted one of the analyzes that calculated the significance of this event.

Dr Nuttall added: “Connor’s analysis assures us that it is a true astrophysical phenomenon and not a strange instrumental behavior.”

The object was found on August 14, 2019, while it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected on Earth by LIGO and Virgo.

The cosmic fusion described in the study, an event nicknamed GW190814, resulted in a final black hole about 25 times the mass of the sun (part of the combined mass was converted into an explosion of energy in the form of gravitational waves). The newly formed black hole is located about 800 million light years from Earth.

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-Virgin event, called GW190412, occurred between two black holes with a mass ratio of 3: 1.

“I think of Pac-Man eating a small spot, when the masses are highly asymmetric, the smallest neutron star can be eaten in one bite.” – Vicky Kalogera, Northwestern University, United States

Vicky Kalogera, a professor at Northwestern University in the United States, said: “It is a challenge for current theoretical models to form pairs of compact objects with such a large mass ratio where the low mass partner resides in the mass gap. This discovery implies that these events occur much more often than we expected, making it a truly intriguing low-mass object.

“The mysterious object could be a neutron star that merges with a black hole, an exciting possibility theoretically foreseen but not yet confirmed on an observational level. However, at 2.6 times the mass of our sun, it exceeds modern forecasts for the maximum mass of neutron stars and may instead be the lightest black hole ever detected. “

When the scientists from LIGO and Virgo noticed this merger, they immediately sent a notice to the astronomical community. Dozens of terrestrial and space telescopes followed in search of the light waves generated in the event, but nobody collected any signals. So far, similar light counterparts to gravitational wave signals have only been seen once, in an event called GW170817. The event, discovered by the LIGO-Virgo network in August 2017, resulted in a fiery collision between two neutron stars which was subsequently witnessed by dozens of telescopes on Earth and in space. Collisions of neutron stars are messy affairs with matter projected outward in all directions and are therefore expected to shine with light. In contrast, black hole mergers are believed to, in most cases, not produce light.

According to scientists from LIGO and Virgo, the August 2019 event was not seen by light-based telescopes for some possible reasons. First, this event was six times farther than the merger observed in 2017, making it harder to pick up light signals. Second, if the collision had affected two black holes, it probably wouldn’t have shone brightly. Third, if the object were actually a neutron star, its black hole partner 9 times more massive could have swallowed it whole; a neutron star consumed whole from a black hole would not emit light.

“I think of Pac-Man eating a small point,” said Kalogera. “When the masses are highly asymmetric, the smallest neutron star can be eaten in one bite.”
Future observations with LIGO, Virgo and perhaps other telescopes could capture similar events that could help reveal if the mysterious object was a neutron star or a black hole or if additional objects exist in the mass gap.

The disclosure article was accepted for publication in The Astrophysical Journal Letters.

For more information on this research:

Reference: “GW190814: Gravitational waves from the coalescence of a black hole of 23 solar masses with a compact object of solar mass 2.6” by R. Abbott, et. al., June 23, 2020, Astrophysical letters from the diary.
DOI: 10.3847 / 2041-8213 / ab960f

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