Giant Neutron Star or Small Black Hole? A New Star Intrigues Scientists

A surprising duo

An antenna of the MeerKAT telescope. Photo: CC Wikimedia Commons/Morganoshell Observing the globular cluster NGC 1851 with the telescope MeerKATscientists were lucky. This radio telescope, located in South Africa and in operation since 2018, has 64 antennas. It first detected a millisecond pulsar, that is, a type of neutron star that rotates on itself more than 170 times per second, emitting an extremely regular beam of radio waves. But in this case, something was jamming the signal. By analyzing these disturbances, astronomers concluded that this “something” was an unknown massive object orbiting the pulsar: its gravitational field was deflecting the radio signals. The team was able to estimate its mass. Conclusion: the object, whatever it is, is likely located in the “empty” zone… Which may not be so empty after all. “This void is partly explained by a lack of observations,” explains Maya Fishbach, from the University of Toronto, who wrote a comment article in Science devoted to this study. Either these objects are very rare, and we don’t know why, or we have difficulty detecting them.”

The list goes on

For some time now, however, the number of observations has been growing, thanks to another approach: detection of gravitational wavesthese ripples in space-time generated by extremely powerful cosmic events. “With data from the LIGO and Virgo detectors, we sometimes observe objects present in this mass gap,” says Marie-Anne Bizouard, an astrophysicist at the Côte d’Azur Observatory in France and a member of the Virgo team. There are few of them, but we manage to find some.” Indeed, analyses of the gravitational waves generated by cosmic collisions make it possible to determine the masses of the two objects before the shock, then the mass of the new body produced by the fusion of the two objects. In about ten years, observations have made it possible to slowly populate the empty zone. “There is an abundance of objects with certain masses, and others are rarer, but there is not really a hole,” notes Marie-Anne Bizouard. Last April, precisely, the LIGO-Virgo-KAGRA collaboration (which brings together three gravitational wave detectors) announced that he had recorded waves from the collision between an object of about 1 to 2 solar masses, probably a neutron star, and another whose mass is right in the improbable range: between 2.5 and 4.5 solar masses. The idea of ​​an empty zone, “which has existed for a quarter of a century, was born from electromagnetic observations [au télescope] “, explained one of the authors, Michael Zevin, in a press release. ” [Cette] The discovery is exciting because it suggests that this “mass gap” is less empty than astronomers previously thought.

Blurred identity

So, are these mid-range stars black holes or neutron stars? It’s hard to decide, because these mysterious objects are particularly tricky to observe. Black holes don’t emit light, and neutron stars are extremely small, with a diameter no larger than that of a city. There are still some hypotheses. The object described in Science could be a neutron star that rotates extremely quickly: we know that rapid rotation prevents collapse. The star could therefore support a mass slightly higher than the “classical” mass. Another scenario: it could be a black hole from a massive neutron star, whose rotation has slowed down and which has collapsed on itself. “If we had been able to see jets of matter projected by the object, we could have learned more,” says Annabelle Richard-Laferrière, a doctoral student in astrophysics at the University of Montreal and a specialist in black holes. Since black holes do not emit them, we could have determined that it was a neutron star. But here, the only information we have is the mass.” Third hypothesis to explain the existence of this object: fusion. The globular cluster in which it is located is “a very dense and quite unique environment,” emphasizes Maya Fishbach. The models show a veritable cosmic ballet in which stars orbit each other before changing partners, which sometimes causes cataclysmic mergers. “All combinations of stars are possible,” the astronomer continues. We can therefore imagine that the merger of two neutron stars of “normal” mass gave birth to a more massive star.

Close monitoring

“It’s extremely interesting,” Marie-Anne Bizouard enthusiastically emphasizes. “Most similar objects detected by gravitational waves are located in galaxies other than ours. This one is in ours, and we’re going to be able to follow it.” Indeed, the PSR J0514-4002E duo will be able to be examined from every angle, unlike the phenomena detected by LIGO and Virgo, which only “see” the objects once, when the violent collision folds the cosmos. It’s impossible to come back to them later! “A star like this is rare,” adds Marie-Anne Bizouard. “It will be the subject of years of observation by increasingly powerful radio telescopes.” In particular, the SKA (Square Kilometre Array), currently being built on two sites in South Africa and Australia, is expected to point its antennas at this curiosity. Developed by an international consortium including Canada, this ultra-sensitive radio telescope is to take over from MeerKAT from 2027, at the earliest. One thing is certain, scientists have every interest in closing this “mass gap” by finding more objects located at the tipping point between neutron stars and black holes. They could challenge current models that describe what happens to stars at the end of their lives. “These are environments where there are physical phenomena that are impossible to recreate on Earth,” adds Annabelle Richard-Laferrière. “Extraordinary pressure conditions, with a high temperature, an intense gravitational field…” By their very existence, these incongruous objects are windows onto the physics of extremes.