Gravitational Wave Astronomers Find Mysterious Objects in the Mass Gap

Neutron star from black hole

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

For decades, astronomers have been baffled by a gap between neutron stars and black holes, but a major new discovery has found a mysterious object in this so-called ‘mass gap’.

The group of gravitational waves of the University of PortsmouthThe Institute of Cosmology and Gravitation 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 stars that are a little less massive die, they explode into a supernova and leave dense, dead remains of stars called neutron stars.

Gravitational waves are emitted every time an asymmetric object is accelerated, with the strongest sources of detection gravitational waves being from the collision of neutron stars and black holes. Both objects are created at the end of the life of a massive star.

“The reason these findings are so exciting is because we have never before 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 dungeon it is about five solar masses.

The new study from the National Science Foundation’s Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo detector in Europe, has announced the discovery of a 2.6 solar mass object, firmly placing it in the mass gap.

LIGO consists of two gravitational wave detectors 3,000 kilometers away in the US, one in Livingston, Louisiana, and one in Hanford, Washington. The Virgo detector is in Cascina, Italy.

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

Portsmouth PhD student Connor McIsaac conducted one of the analyzes that computed the importance of this event.

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

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

The cosmic merger described in the study, an event called GW190814, resulted in a final black hole of about 25 times the mass of the sun (part of the merged mass became an explosion of energy in the form of gravitational waves). The newly formed black hole is 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 known mass ratio for a gravitational wave event. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a 3: 1 mass ratio.

“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, U.S

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

“The mysterious object may be a neutron star merging with a black hole, an exciting possibility theoretically expected but not yet confirmed by observation. However, at 2.6 times the mass of our sun, it exceeds modern predictions for the maximum mass of neutron stars, and it may be the lightest black hole ever detected. ”

When LIGO and Virgo scientists detected this fusion, they immediately sent an alert to the astronomical community. Dozens of ground and space telescopes followed in search of light waves generated at the event, but none picked up any signals. So far, such light counterparts to gravitational wave signals have been seen only once, in an event called GW170817. The event, discovered by the LIGO-Virgo network in August 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron star collisions are messy matters with matter thrown outward in all directions and are therefore expected to glow with light. In contrast, black hole mergers are believed to, in most circumstances, produce no light.

According to LIGO and Virgo scientists, 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 difficult to capture light signals. Second, if the collision involved two black holes, it probably would not have shone in any light. Third, if the object was indeed a neutron star, its 9 times more massive black hole companion could have swallowed it whole; a neutron star consumed entirely by a black hole would not emit any light.

“I think of Pac-Man eating a little 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 possibly other telescopes can detect similar events that would help reveal whether the mystery object was a neutron star or black hole, or if additional objects exist in the mass gap.

The article on detection has been accepted for publication in The Astrophysical Journal Letters.

For more information about this research:

Reference: “GW190814: Gravitational Waves from the Merger of a 23 Solar Mass Black Hole with a Compact Solar Mass 2.6 Object” by R. Abbott, et. al., June 23, 2020, Astrophysical charts.
DOI: 10.3847 / 2041-8213 / ab960f