Astronomers have discovered a celestial object that defies classification, perhaps revealing a new type of cosmic entity at the edge of known physics.
Sometimes astronomers encounter objects in the sky that we can't easily explain. In our new research, published in Scienceswe report such a discovery, which is likely to spark debate and speculation.
Neutron stars are some of the densest objects in the universe. Compact as the nucleus of an atom, but as large as a city, it transcends the limits of our understanding of ultimate matter. The heavier a neutron star is, the more likely it is that it will eventually collapse into something denser: a black hole.
Edge of understanding: neutron stars and black holes
These astrophysical objects are so dense, and their gravity so strong, that their cores – whatever they may be – are permanently covered from the universe by event horizons: surfaces of complete darkness from which no light can escape.
If we want to understand the physics at the tipping point between neutron stars and black holes, we must find objects at these boundaries. In particular, we must find objects for which we can make precise measurements over long periods of time. And that's exactly what we found – an object that is not clearly A Neutron star Nor a Black hole.
A cosmic dance in NGC 1851
This was when looking deep into the star cluster NGC 1851 That we have discovered what appears to be a pair of stars provides new insight into the extreme limits of matter in the universe. The system consists of one millisecond PulsarIt is a type of rapidly rotating neutron star that sweeps rays of radio light across the universe as it rotates, and it is a massive, hidden object of an unknown nature.
The massive object is dark, which means it is invisible at all frequencies of light – from radio to the bands of light, X-rays and gamma rays. In other circumstances, this would make it impossible to study, but this is where the millisecond pulsar comes to our aid.
Millisecond pulsars are like cosmic atomic clocks. Their rotations are incredibly stable and can be accurately measured by detecting the regular radio pulse they produce. Although intrinsically constant, 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 objects in the orbits of pulsars.
Unravel the mystery with MeerKAT
We have used our international team of astronomers Meerkat radio telescope In South Africa to make such observations of the system, referred to as NGC 1851E.
This allowed us to precisely detail the orbits of the two objects, showing that their point of closest approach changes with time. These changes are described by Einstein's theory of relativity The speed of change tells us the combined mass of the objects in the system.
Our observations revealed that the NGC 1851E system weighs about four times as much as our Sun, and that the dark companion was, like a pulsar, a compact object – much denser than a regular star. The most massive neutron stars weigh about twice the mass of the Sun, so if this is a double neutron star system (well-known and well-studied systems), it must contain two of the heaviest neutron stars ever discovered.
To uncover the nature of the companion, we will need to understand how mass is distributed in the interstellar system. Again using Einstein's general relativity, we can model the system in detail, finding a mass for the companion to be 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, which are thought to have a mass of about 2.2 solar masses, and the lightest black holes that can form from stellar collapse, which have a mass of about 5 solar masses. The nature and composition of objects in this gap is an outstanding question in astrophysics.
So what exactly did we find then?
An attractive possibility is that we have discovered a pulsar orbiting around the remains of a merger (collision) of two neutron stars. This unusual configuration was made possible by the dense packing of stars in NGC 1851.
On this crowded dance floor, the stars will circle around each other, swapping partners in an endless waltz. If two neutron stars were to be hurled too close to each other, their dance would end disastrously.
The black hole created by their collision, which can be much lighter than those created by collapsing stars, is free to wander through the cluster until it finds another pair of waltzing dancers, and impudently inserts itself – chasing away the lighter partner. In treatment. It is this mechanism of collisions and exchanges that could lead to the system we observe today.
We're not done with this system yet. Work is already underway to conclusively determine the true nature of the companion and reveal whether we have discovered the lightest black hole or the most massive neutron star – or perhaps neither.
At the boundary between neutron stars and black holes, there is always the possibility of new, as yet unknown, astrophysical objects.
Much speculation is sure to follow this discovery, but what is already clear is that this system holds enormous promise when it comes to understanding what really happens to matter in the most extreme environments in the universe.
- Ewan D. Barr – Project Scientist for Transiting Stars and Pulsars in collaboration with 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
- Benjamin Stubbers – Professor of Astrophysics, University of Manchester
Adapted from an article originally published in Conversation.
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