Black hole, Neutron star? Discovered in Our Galaxy That Defies Explanation.

 

Black hole, neutron star or something new? Scientists Just Discovered an Object in Our Galaxy That Defies Explanation, The novel and extraordinary entity has the potential to facilitate the examination of Einstein's theory of general relativity.

An artist’s impression of the system assuming that the massive companion star is a black hole. The brightest background star is its orbital companion, the radio pulsar PSR J0514-4002E.Daniëlle Futselaar/Artsource.nl

In our recent study, published in the prestigious journal Science, we have made a remarkable discovery in the realm of astronomy. Occasionally, astronomers stumble upon celestial entities that defy easy explanation. This particular finding is bound to ignite extensive discussions and give rise to various speculations.

 

Neutron stars, renowned for their extraordinary density, rank among the most compact objects in the vast expanse of the universe. Comparable in size to an atomic nucleus, yet possessing the dimensions of an entire city, they challenge our comprehension of extreme matter. As a neutron star increases in mass, the likelihood of its eventual collapse into an even denser entity, namely a black hole, intensifies.

 

These cosmic entities exhibit such immense density and exert such powerful gravitational forces that their cores, whatever they may consist of, remain perpetually concealed from the universe behind event horizons. These event horizons represent surfaces of absolute darkness, impervious to the escape of light.

 

To unravel the enigmatic physics governing the transition from neutron stars to black holes, it is imperative that we identify objects situated precisely at this boundary. Specifically, we must locate objects that allow for precise measurements over extended periods of time. And that is precisely what we have accomplished – the discovery of an object that defies categorization as either a neutron star or a black hole.

Hubble Space Telescope image of the globular cluster NGC 1851. NASA, ESA, and G. Piotto (Università degli Studi di Padova)

While observing the star cluster NGC 1851, we made an intriguing discovery - a pair of stars that provide us with a unique perspective on the nature of matter in the universe. This system consists of a millisecond pulsar, a rapidly rotating neutron star that emits beams of radio light across the cosmos, and a mysterious, massive object that remains hidden from our view.

What sets this massive object apart is its darkness, rendering it invisible across all ranges of light - from radio waves to optical, x-ray, and gamma-ray frequencies. Under normal circumstances, this would pose a significant challenge for study. However, the millisecond pulsar proves to be an invaluable tool in our investigation.

Millisecond pulsars serve as cosmic timekeepers, exhibiting remarkable stability in their rotation speed, which we can accurately measure by detecting their regular radio pulses. Although inherently stable, the observed spin of the pulsar can be influenced by its motion or the presence of a strong gravitational field. By carefully observing these spin variations, we can gather valuable information about the properties of celestial bodies in orbit with pulsars.

Credit Image: The MeerKAT radio telescope in South Africa. South Africa Radio Astronomy Observatory (SARAO)

Our team of international astronomers has utilized the MeerKAT radio telescope in South Africa to carry out observations on the NGC 1851E system. Through these observations, we have been able to accurately determine the orbits of the two objects within the system, revealing that their closest point of approach changes over time. These changes align with Einstein's theory of relativity, and the rate of change provides insights into the combined mass of the bodies in the system.

Based on our findings, we have discovered that the NGC 1851E system has a mass nearly four times that of our Sun. Additionally, we have determined that the dark companion, similar to the pulsar, is a compact object with a much higher density than a typical star. The most massive neutron stars have a weight of approximately two solar masses. Therefore, if this were a double neutron star system (which are extensively studied and well-known), it would have to consist of two of the heaviest neutron stars ever observed.

To further comprehend the nature of the companion, we must ascertain how the mass is distributed between the stars within the system. By employing Einstein's general theory of relativity, we can create a detailed model of the system and calculate that the companion's mass lies between 2.09 and 2.71 times that of the Sun.

Remarkably, the mass of the companion falls within the "black hole mass gap," which exists between the heaviest possible neutron stars (estimated to be around 2.2 solar masses) and the lightest black holes that form from stellar collapse (approximately 5 solar masses). The nature and formation of objects within this mass range remain an intriguing question in the field of astrophysics.

What have we discovered, precisely?

The formation history of the system holds great potential. Initially, a low-mass X-ray binary (LMXB) gave rise to the millisecond pulsar (MSP) while leaving a white dwarf (WD) companion in its wake. Subsequently, through an exchange encounter process, the WD was substituted by the present companion star, which could either be a light black hole (BH) or a heavy neutron star (NS). This companion star is itself the outcome of a previous merger between two neutron stars (NSs). This information has been provided by Thomas Tauris from Aalborg University / MPIfR.

A fascinating prospect emerges as we unveil the presence of a pulsar encircling the remnants of a merger between two neutron stars. This extraordinary arrangement is made feasible by the dense congregation of stars within NGC 1851.

Within this bustling celestial ballroom, stars gracefully swirl around one another, engaging in a perpetual exchange of partners akin to an eternal waltz. However, if two neutron stars happen to be flung too close together, their elegant dance will abruptly culminate in a cataclysmic event.

The resulting black hole, which may possess a significantly lesser mass compared to those formed through stellar collapse, then roams freely throughout the cluster until it encounters another pair of dancers in the waltz. In a rather impolite manner, it inserts itself into the dance, displacing the lighter partner. It is through this process of collisions and exchanges that the observed system could have originated.

           Simulation of the three-body interaction that is thought to have produced the NGC 1851E system.

The investigation of this system is still in progress. Efforts are currently underway to definitively determine the true characteristics of the companion and ascertain whether it is the lightest black hole or the most massive neutron star, or potentially something entirely different.

In the realm where neutron stars and black holes converge, there remains the intriguing possibility of the existence of novel astrophysical entities that are yet to be unveiled.

Undoubtedly, this discovery will give rise to numerous speculations. However, it is already evident that this system possesses great potential in unraveling the mysteries surrounding the behavior of matter in the most extreme environments of the universe.