The team of astronomers from around the world describe their fascinating discovery and delve into its potential nature:
Sometimes astronomers encounter objects in the sky that we cannot easily explain. In our new research, published in Science, we report one such discovery, which is likely to spark debate and speculation.
Neutron stars are among the densest objects in the universe. As compact as an atomic nucleus, yet as large as a city, they push the boundaries of our understanding of extreme matter. The more massive a neutron star is, the more likely it is to eventually collapse and grow into something even denser: a black hole.
These astrophysical objects are so compact and their gravity so strong that their nuclei – whatever they may be – are permanently withdrawn from the universe by event horizons: surfaces of perfect darkness from which light cannot escape.
If we ever want to understand the physics of the tipping point between neutron stars and black holes, we need to find objects on this boundary. In particular, we need to find objects on which we can make accurate measurements over long periods of time. And that’s exactly what we found: an object that is neither clearly a neutron star nor a black hole.
Looking deep into the star cluster NGC 1851, we saw what appears to be a pair of stars that offered new insight into the extremes of matter in the universe.
The system consists of a millisecond pulsar, a type of rapidly spinning neutron star that emits radio light beams through the cosmos as it rotates, and a massive, hidden object of unknown nature.
The massive object is dark, meaning it is invisible at all light frequencies – from the radio to the optical, X-ray and gamma-ray bands. Under other circumstances this would make it impossible to study, but here the millisecond pulsar comes to our rescue.
Millisecond pulsars resemble cosmic atomic clocks. Their spins are incredibly stable and can be accurately measured by detecting the regular radio pulses they create. Although intrinsically stable, the observed spin changes when the pulsar is in motion or when its signal is affected by a strong gravitational field. By observing these changes we can measure the properties of bodies orbiting pulsars.
Our international team of astronomers used the MeerKAT radio telescope in South Africa to make such observations of the system, known as NGC 1851E.
This allowed us to accurately detail the orbits of the two objects, showing that their point of closest approach changes over time. Such changes are described by Einstein’s theory of relativity and the rate of a change tells us about the combined mass of the bodies in the system.
Our observations showed that the NGC 1851E system weighs almost four times as much as our Sun, and that the dark companion, like the pulsar, was a compact object – much denser than a normal star.
The heaviest neutron stars weigh about two solar masses, so if this were a double neutron star system (systems that are known and studied), it would have to contain two of the heaviest neutron stars ever found.
To determine the nature of the companion, we need to understand how the mass in the system is distributed among the stars. Again using Einstein’s general theory of relativity we were able to model the system in detail, finding that the mass of the companion was between 2.09 and 2.71 times the mass of the Sun.
The companion’s mass falls within the “black hole mass gap” that lies between the heaviest possible neutron stars, estimated to have about 2.2 solar masses, and the lightest black holes that can be formed by stellar collapse, about 5 solar mass. The nature and formation of objects in this aperture is an open question in astrophysics.
Possible candidates
So what exactly have we found?
One tantalizing possibility is that we have discovered a pulsar orbiting the remnants of a merger (collision) of two neutron stars. Such an unusual configuration is made possible by the dense cluster of stars in NGC 1851.
On this busy star dance floor, stars will revolve around each other and switch partners in an endless waltz. If two neutron stars are thrown too close together, their dance will end cataclysmically.
The black hole created by their collision, which can be much lighter than the holes created by collapsing stars, can then wander freely through the star cluster until it finds another pair of dancers in the waltz and, rather crudely, inserts itself into it stabs – kicking out the lighter partner. in the process. It is this mechanism of collisions and exchanges that could give rise to the system we observe today.
We are not done with this system yet. Work is already underway to definitively identify the true nature of the companion and to reveal whether we have discovered the lightest black hole or the heaviest neutron star – or perhaps neither.
At the boundary between neutron stars and black holes there is always the possibility that a new, yet unknown, astrophysical object exists.
Much speculation will certainly follow this discovery, but what is already clear is that this system holds enormous promise when it comes to understanding what is really happening to matter in the most extreme environments in the world. universe.
Ewan D. Barr, project scientist for the Transients and Pulsars with collaboration MeerKAT (TRAPUM), Max Planck Institute for Radio Astronomy; Arunima Dutta, PhD candidate at the Research Department of Fundamental Physics in Radio Astronomy, Max Planck Institute for Radio Astronomy, and Benjamin Stappers, Professor of Astrophysics, University of Manchester
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