Is it a black hole? A neutron star? Scientists find mystery object inside puzzling area known as ‘mass gap’

Scientists have so far believed that dying stars do not leave remnants, whatever their nature, with a mass between 2.5 and 5 times the mass of the Sun

                            Is it a black hole? A neutron star? Scientists find mystery object inside puzzling area known as ‘mass gap’
(Getty Images)

Astronomers have discovered a mystery object inside the puzzling area known as the “mass gap” — the range that lies between the heaviest known neutron star and the lightest known black hole. The new observation is important because it challenges astrophysicists' understanding both of how stars die and how they pair up into binary systems, that is, two astronomical objects orbiting around each other.

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 supernovas and leave behind dense, dead remnants of stars called neutron stars. 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 times the mass of our Sun or 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?

In the new study from the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European Virgo observatory, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The research team confirmed that it is a record-breaker: It is more massive than any neutron star and lighter than any black hole yet observed. The cosmic merger resulted in a final black hole about 25 times the mass of the Sun. The newly-formed black hole lies about 800 million light-years away from Earth.

“We've been waiting decades to solve this mystery. Mergers of a mixed nature — black holes and neutron stars — have been predicted for decades, but this compact object in the mass gap is a complete surprise. Even though we can't classify the object with conviction, we have seen either the heaviest known neutron star or the lightest known black hole. Either way, it breaks a record,” says co-author Vicky Kalogera, a professor at Northwestern University, in the analysis published in the Astrophysical Journal Letters. A leading astrophysicist in the LIGO Scientific Collaboration (LSC), Kalogera is an expert in the astrophysics of compact object binaries and analysis of gravitational-wave data.

“This merger event is one of the most unusual ones observed in gravitational waves to date. It pushes our understanding of the nature of the lighter companion and how it is formed to the limits. This will keep astrophysicists occupied for a while,” says Dr Patricia Schmidt, Lecturer at the Institute for Gravitational Wave Astronomy and member of the LIGO team.

Scientists do not know if the mystery object was a neutron star or black hole, but either way, it set a record as being either the heaviest known neutron star or the lightest known black hole (LIGO/Caltech/MIT/R. Hurt/IPAC)

The intriguing object was found on August 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo observatories. The highly unusual gravitational wave signal was named GW190814. The study describes that the signal was generated by a “compact object 2.6 times the mass of our sun, merging with a black hole of 23 solar masses.”  

“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. This discovery implies these events occur much more often than we predicted, making this an intriguing low-mass object,” explains Kalogera. “The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally. However, at 2.6 times the mass of our sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected.”

Before the two objects merged, their masses differed by a factor of nine, 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, says the study.

“Whereas we are not sure about the nature of the low-mass compact object, we have obtained a very robust measure of its mass, which falls right into the so-called mass gap. This exciting and unprecedented finding, combined with the unique mass ratio of the merger event, challenges all the astrophysical models that try to shed light on the origins of this event. However, we are quite sure that the universe is telling us, for the umpteenth time, that our ideas on how compact objects form, evolve and merge are still very fuzzy,” says co-author Mario Spera, who studies the formation of merging binaries. He is a Virgo collaboration member and a European Union Marie Curie postdoctoral fellow at CIERA and the University of Padova.

The highly unusual gravitational wave signal, detected by the LIGO and Virgo observatories, was named GW190814 (Getty Images)

When the LIGO and Virgo scientists spotted this merger, 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. 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 collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron star collisions are complex 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.

The scientists say there could be a few possible reasons why the August 2019 event was not seen by light-based telescopes. “This event was six times farther away than the merger observed in 2017, making it harder to pick up any light signals. Secondly, if the collision involved two black holes, it likely would have not shone with any light. Thirdly, 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,” the findings state.

How will researchers determine if the mystery object was a neutron star or black hole? 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, say experts. According to Charlie Hoy, a member of the LIGO Scientific Collaboration and a graduate student at Cardiff University, this is just the start. “As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe,” says Hoy.

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