An article published in the journal “Science” reports the discovery of a compact object within the globular cluster cataloged as NGC 1851 whose nature is uncertain because mass estimates place it on the border between a neutron star and a black hole. A team of researchers used observations conducted with the MeerKAT radio telescope to identify traces of a binary system composed of a so-called millisecond pulsar and the mysterious compact object. The estimated mass for this object is between 2.09 and 2.71 times the Sun’s, so it could be a massive neutron star or a small black hole.
Astronomical knowledge obtained from observations and theoretical models indicates that the remnant of a massive star constitutes a neutron star of up to 2.2 times the Sun’s mass and at the same time, that smaller black holes have a mass that is at least 5 times the Sun’s. Between these two values, there’s what is called the black hole mass gap and astronomers can’t tell the nature of an object with that type of mass. Studying a newly discovered compact object could help find an answer.
The Transients and Pulsars with MeerKAT (TRAPUM) collaboration uses the MeerKAT radio telescope to search for new pulsars and transient objects to expand our knowledge of radio wave sources. The MeerTime project complements it by studying over 1,000 radio pulsars to test various theoretical models.
The TRAPUM and MeerTime collaborations made it possible to identify a pulsar within the globular cluster NGC 1851, full of stars much older than the Sun very close to each other. This is the fifth pulsar discovered in that cluster and was consequently cataloged as NGC 1851E, although it was also cataloged as PSR J0514-4002E.
The discovered pulsar spins over 170 times per second and is an interesting object precisely because of this rotation speed: in the past, it probably stripped materials from another nearby star to increase the rotation speed. The study of this pulsar’s orbital motion made it possible to identify the presence of a companion and estimate its mass. The problem is that the estimate between 2.09 and 2.71 times the Sun’s mass places the companion in the black hole mass gap, so at least for the moment, it wasn’t possible to ascertain its nature.
According to the researchers, the mysterious object could be the result of the merger between two neutron stars. They also believe that only after this event did the new object form a pair with the pulsar. That’s possible precisely because the globular cluster NGC 1851 is very compact, allowing frequent interactions between stars. Those consequences are even more likely when events such as supernovae and mergers significantly affect the gravitational balance within it.
This discovery was possible thanks to the sensitivity of the MeerKAT radio telescope, one of the precursors of SKA, the next generation radio telescope which is scheduled to enter service in the next few years. This means that studies of objects such as the strange binary system subject of this article will be able to continue obtaining even more precise information.
Some hypothesize that in the black hole mass gap, there may be compact objects that are neither neutron stars nor black holes, and instruments such as MeerKAT and in the future SKA will help to better understand the boundaries between the different types of objects and if other “exotic” objects really exist within that gap. These studies intersect with those on mergers between these compact objects, at the center of gravitational wave detections, and the ones testing relativistic effects near those extreme objects.
