by Sometimes astronomers come across things in the sky that we cannot easily explain. In our new research, published in Science, we report such a discovery, which is likely to stimulate discussion and speculation.
Neutron stars are some of the densest objects in the universe. As dense as an atomic nucleus, but as large as a city, they push the limits of our understanding of extreme matter. The heavier a neutron star is, the more likely it will eventually collapse to become even more dense: a black hole.
These astrophysical objects are so dense, and their gravity so strong, that their cores – whatever they may be – are permanently shrouded from space by event horizons: surfaces of perfect darkness that cannot light escape from them.
If we are ever to understand the physics at the tipping point between neutron stars and black holes, we need to find things at this frontier. In particular, we need to find things that we can measure precisely over long periods of time. And that’s exactly what we found – an object that is clearly not a neutron star or a black hole.
It was while looking deep into the star cluster NGC 1851 that we spotted a pair of stars, apparently offering a new view of the extreme conditions of matter in the universe. The system consists of a millisecond pulsar, a type of rapidly spinning neutron star that sweeps beams of radio light across the cosmos as it spins, and a massive, hidden object of unknown nature.
The massive object is dark, meaning it is invisible at all frequencies of light – from radio to the optical, x-ray and gamma-ray bands. In other circumstances this would make it impossible to study, but this is where the millisecond pulsar comes to our rescue.
Millisecond pulsars are like cosmic atomic clocks. Their spins are extremely stable and can be precisely measured by detecting the regular radio pulse they create. Although essentially stable, the observed spin changes when the pulsar is moving or when its signal is affected by a strong gravitational field. By observing these changes we can measure the properties of bodies in orbits with pulses.
Our international team of astronomers is using the MeerKAT radio telescope in South Africa to observe the system referred to as NGC 1851E.
These allowed us to precisely detail the orbits of the two objects, showing that their point of closest approach changes over time. Einstein’s theory of relativity describes such changes and the speed of change tells us about the combined mass of the bodies in the system.
Our observations indicated that the NGC 1851E system weighs almost four times that of our Sun, and that the dark companion, like the pulsar, was a compact object – much closer than an ordinary star. The largest neutron stars weigh about two solar masses, so if this were a double neutron star system (well known and studied systems) it would have to contain two of the heaviest neutron stars ever found to exist.
To reveal the nature of the companion, we need to understand how the mass of the system is distributed between the stars. Again using Einstein’s general relativity, we could model the system in detail, finding that the mass of the companion lies between 2.09 and 2.71 times the mass of the Sun.
The companion’s mass falls within the “black hole mass gap” between the heaviest possible neutron stars, estimated to be around 2.2 solar masses, and the lightest black holes that can form from collapsing stars, around 5 masses sun The nature and formation of objects in this gap is an outstanding question in astrophysics.
Potential candidates
So what exactly have we got there?
An intriguing 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 close packing of stars in NGC 1851.
In this crowded dance floor, the stars will spin around each other, exchanging partners in an endless waltz. If two neutron stars are thrown too close together, their dance will come to a cataclysmic end.
The black hole created by their collision, which may be much lighter than those created from collapsing stars, can travel the cluster until it finds a mate other dancers in the waltz and, in a rude way, insert himself – kicking the lighter partner. in the process. It is this mechanism of collisions and exchanges that could lead 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 reveal whether we have found the lightest black hole or the lightest neutron star – or perhaps neither.
At the boundary between neutron stars and black holes there is always the possibility that some new, as yet unknown, astrophysical object may exist.
Much speculation will undoubtedly follow this discovery, but it is already clear that this system holds enormous promise when it comes to understanding what really happens to matter in the most extreme environments in the universe.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Benjamin Stappers is funded by UKRI.
Arunima Dutta and Ewan D. Barr do not work for, consult with, own shares in, or receive funding from, or disclose any company or organization that would benefit from this article. any relevant affiliations after their academic appointment.
