Deep Space: A mysterious object discovered within the globular cluster visible in the constellation of Dove more than 39,000 light years from Earth is at the centre of a major international scientific-astronomical discovery, which was reported in the prestigious American journal ‘Science’. The National Institute of Astrophysics and the University of Bologna confirmed the announcement in Italy. The scientist Andrea Possenti, an Inaf research director at the Cagliari Astronomical Observatory and already the author of other scientific discoveries concerning pulsars and gravitational waves, is part of the team that made it, along with other Italian and foreign colleagues.

“Thanks to the South African instrument Meerkat, to date the most powerful radio telescope that can observe the southern sky, an object of a currently mysterious nature has been discovered, leading to the following question: has the lightest black hole or the heaviest neutron star been identified?” explains the researcher. “It all started with the research of an international team of astronomers, led by researchers from the Max Planck Institute for Radio Astronomy in Bonn and including Italian scientists from Inaf and the University of Bologna, who exploited the sensitivity of the antenna of the South African radio telescope MeerKat to discover a massive object with unique characteristics: it is heavier than all known neutron stars and at the same time lighter than the lightest black holes found so far.

Unanswered questions, at least for now

The ‘special suspect’ is orbiting a rapidly rotating millisecond pulsar: pulsars are neutron stars that originate in the final collapse of very massive ordinary stars, coinciding with the associated supernova explosion. This could be the first discovery of the much-coveted radio-pulsar/black hole binary system, a stellar pair that would allow new tests of Einstein’s general theory. Speaking of pulsars, it is good to know that they are compact (some twenty kilometres in diameter) and extremely dense remnants derived from powerful supernova explosions. Besides being interesting, pulsars are exceptional laboratory tools for investigating the stars that share their orbit and the effects of the theory of general relativity on their orbit.

As part of the two international collaborations ‘Transients and Pulsars with MeerKAT’ (Trapum) and ‘MeerTime’, experts were able first to detect and then repeatedly study the faint pulses coming from one of the cluster’s stars, identifying it as a radio pulsar, a type of neutron star that spins very quickly and emits radio waves into the Universe like a cosmic beacon.

This pulsar, named NGC 1851E (i.e. the fifth pulsar in the globular cluster NGC 1851), rotates on itself more than 170 times per second, and each rotation produces a rhythmic pulse, like the clock ticking. The question then arises as to what kind of compact object forms in the mass range between 2.2 and 5 times the mass of the Sun, in what researchers call the ‘black hole mass gap’: an extremely massive neutron star, an extremely light black hole or something else? To date, there is no clear answer.