Sometimes astronomers encounter objects in the sky that we cannot easily explain. In our new research published in the journal Science, we report one such discovery that is likely to spark debate and speculation.
Neutron stars are some of the densest objects in the universe. As compact as an atomic nucleus but as large as a city, these substances push the limits of our understanding of extreme matter. The heavier a neutron star is, the more likely it is that it will eventually collapse into something even denser: a black hole.
These astrophysical objects are so dense and their gravitational pull so strong that their cores (whatever they are) are permanently hidden from the universe by their event horizons (perfectly dark surfaces from which light cannot escape).
If we are to understand the physics at the tipping point between neutron stars and black holes, we must find objects at this boundary. We need to find objects where we can make precise measurements, especially over long periods of time. And that’s exactly what we found: an object that is clearly neither a neutron star nor a black hole.
Peering deep into the NGC 1851 star cluster, we saw what appears to be a double star that offers a new perspective on the far reaches of matter in the universe. The system consists of a millisecond pulsar, a type of rapidly rotating neutron star that scans beams of radio light across the universe as it rotates, and a massive, hidden object of unknown nature.
The massive object is dark, meaning it is invisible in all frequencies of light, from radio to optical, x-ray and gamma ray bands. In other cases this makes it impossible to study, but this is where the millisecond pulsar comes to our aid.
Millisecond pulsars are similar to cosmic atomic clocks. Their spins are incredibly stable and can be measured precisely by detecting the regular radio pulse they produce. Although inherently 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 objects in the orbits where pulsars are located.
Our international team of astronomers is using the MeerKAT radio telescope in South Africa to perform such observations of the system called NGC 1851E.
These allowed us to fully detail the orbits of the two objects, showing that their point of closest approach changed over time. Such changes are described by Einstein’s theory of relativity, and the rate of change gives us information about the total mass of objects in the system.
Our observations revealed that the NGC 1851E system is almost four times more massive than our Sun, and that this dark companion, like the pulsar, is a compact object much denser than a normal star. The largest neutron stars weigh about two solar masses; Therefore, if this were a double neutron star system (well-known and studied systems), it would contain two of the heaviest neutron stars ever found.
To reveal 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 relativity, we could model the system in detail and find 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” between the heaviest possible neutron stars, thought to be about 2.2 solar masses, and the lightest black holes that can form from the collapse of stars (about 5 solar masses). The nature and formation of objects in this space is an important question in astrophysics.
possible candidates
So what exactly did we find then?
One tantalizing possibility is that we have uncovered a pulsar orbiting around the remnants of the merger (collision) of two neutron stars. Such an unusual configuration is made possible by the dense packing of stars in NGC 1851.
On this crowded star dance floor, the stars will spin around each other and change partners in an endless waltz. If two neutron stars are launched too close to each other, their dance will result in disaster.
The black hole formed by their collisions, which may be much lighter than those formed by collapsing stars, then wanders freely through the cluster until it finds another pair of dancers in the waltz, and rather rudely thrusts itself in and ejects the lighter partner. in the process. It is this mechanism of collision and change that could give rise to the system we observe today.
We’re not done with this system yet. Studies are currently underway to pinpoint the companion’s true nature and reveal whether we have discovered the lightest black hole, the most massive neutron star, or both.
There is always the possibility that a new, as yet unknown, astrophysical object may exist at the boundary between neutron stars and black holes.
Much speculation is sure to follow this discovery, but what is already clear is that this system holds great promise for understanding what actually happens to matter in the most extreme environments in the universe.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Benjamin Stappers receives funding from UKRI.
Arunima Dutta and Ewan D. Barr do not work for, consult for, own shares in, or receive funding from any company or organization that would benefit from this article, and they have disclosed no relevant affiliations other than their academic appointments.