According to the news report released by the South African Radio Astronomy Observatory (SARAO), a team of international astronomers from the Max Planck Institute for Radio Astronomy have discovered an object of an unknown nature in the globular cluster NGC 1851. The massive object, heavier than the heaviest neutron stars and challenged conventional classification, was discovered by the MeerKAT radio telescope. In addition, the object is lighter than the lightest black holes and orbits a rapidly spinning millisecond pulsar.  This could be the first discovery of the much-coveted radio pulsar–black hole binary, a stellar pairing allowing new tests of Einstein’s general relativity.

Neutron stars, the incredibly dense remnants of supernova explosions, have an upper limit to their mass. If they accumulate too much mass, potentially by absorbing another star or through collisions with their counterparts, they undergo collapse. The aftermath of this collapse remains a subject of speculation, giving rise to diverse proposals of exotic stars with unique characteristics. While various imaginative theories exist, the predominant view suggests that neutron stars, upon collapse, transform into black holes—entities so gravitationally powerful that even light cannot escape them.

This theory, backed by observation, highlights that the lightest black holes formed through star collapse weigh approximately five times more than the Sun. This exceeds the 2.2 solar masses needed for neutron star collapse, creating a phenomenon referred to as the black hole mass gap. The identity of compact objects within this mass range remains to be determined, and in-depth exploration has proven challenging. Limited glimpses of such entities have been captured solely during observations of gravitational wave merger events in the distant universe.

The discovery of an object in this mass gap in our galaxy by a team of astronomers from the international Transients and Pulsars with MeerKAT (TRAPUM) collaboration may help us finally understand these objects. Their work, published this week in Science, reports on a massive pair of compact stars in the globular cluster NGC 1851 in the southern constellation Columba (the dove). By using the sensitive MeerKAT radio telescope in South Africa, in combination with powerful instrumentation built by engineers at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, they were able to detect faint pulses from one of the stars, identifying it as a radio pulsar, a type of neutron star that spins rapidly and shines beams of radio light into the Universe like a cosmic lighthouse. 

This pulsar, designated PSR J0514-4002E, spins more than 170 times a second, with every rotation producing a rhythmic pulse, like the ticking of a clock. By observing small changes in this ticking over time, using a pulsar timing technique, they could make extremely precise measurements of its orbital motion. “Think of it like being able to drop an almost perfect stopwatch into orbit around a star almost 40,000 light years away and then being able to time those orbits with microsecond precision”, says Ewan Barr, who led the study together with MPIfR colleague and PhD candidate Arunima Dutta.

The regular timing also allowed a very precise measurement of the system’s location, showing that the object in orbit with the pulsar was no regular star – it is invisible in Hubble Space Telescope images of NGC 1851 – it is, therefore a highly dense remnant of a collapsed star. Furthermore, the observed change with time of the closest point of approach between the two stars (the periastron) showed that the companion has a mass that was simultaneously bigger than that of any known neutron star and yet smaller than that of any known black hole, placing it squarely in the black-hole mass gap.

“Whatever this object is, it is exciting news. If it is a black hole, it will be the first pulsar–black hole system known, which has been a holy Grail of pulsar astronomy for decades! If it is a neutron star, this will fundamentally affect our understanding of the unknown state of matter at these incredible densities!” commented Paulo Freire of the MPIfR

The team proposes that the formation of the massive object and its subsequent pairing with the fast-spinning radio pulsar in a tight orbit result from a rather exotic formation history that is only possible due to its particular local environment. The system is found in the globular cluster NGC 1851, a dense collection of old stars much more tightly packed than the stars in the rest of the Galaxy. Here, it is so crowded that the stars can interact with each other, disrupting orbits and, in the most extreme cases, colliding. One such collision between two neutron stars is proposed to have created the massive object that now orbits the radio pulsar. However, before the present binary was created, the radio pulsar must have first acquired material from a donor star in a so-called low-mass X-ray binary. 

Furthermore, such a “recycling” process is needed to spin up the pulsar to its current rotation rate. The team believes this donor star was replaced by the current massive object in a so-called exchange encounter. “This is the most exotic binary pulsar discovered yet,” says Thomas Tauris from Aalborg University, Denmark. “Its long and complex formation history pushes at the limits of our imagination”.

While the team cannot conclusively say whether they have discovered the most massive neutron star known, the lightest black hole known or even some new exotic star variant, what is certain is that they have uncovered a unique laboratory for probing the properties of matter under the most extreme conditions in the Universe.

“We are not done with this system yet”, says Arunima Dutta. She concludes, “Uncovering the true nature of the companion will be a turning point in our understanding of neutron stars, black holes, and whatever else might be lurking in the black hole mass gap”.

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