Discovered: Compact object that’s either a very massive neutron star or an unusually low-mass black hole

Researchers who used a telescope in South Africa report discovery of an object in the Milky Way that could either be the most massive neutron star ever observed, or the least massive black hole. “Regardless of its origin … the discovery of a compact object with a mass between 2.09 and 2.71 solar masses in a globular cluster has fascinating implications,” writes Mays Fishbach in a related Perspective. It could inform understanding of the un­certain physics of extremely dense nuclear matter, of supernova explosions, or of dy­namical interactions such as neutron star mergers inside globular clusters. Astrophysical compact objects generally come in two varieties – neutron stars and black holes. However, there is a substantial gap between the masses of the heaviest measured neutron star and the lightest measured black hole. For example, the most massive neutron stars generally range between 2.2 to 2.5 solar masses, while black holes of less than 5 solar masses are rarely observed. Although a few compact objects observed in gravitational wave events with masses within this “mass gap” have been reported, the nature of these objects and the mechanisms through which they formed remain unknown. Ewan Barr, Arunima Dutta, and colleagues report the detection of one such mass gap object. Barr, Dutta et al. used observations from the MeerKAT radio telescope array in South Africa to measure the timing of a binary millisecond pulsar known as PSR J0514-4002E. Accounting for relativistic effects, the authors estimated the total mass of the binary system and then inferred the mass of the pulsar’s binary companion. The findings indicate that the binary companion is a compact object with a mass between 2.09 and 2.71 solar masses, sitting at the lower edge of the observed mass gap between neutron stars and black holes. Because of this, Barr, Dutta et al. cannot reliably classify this object as either a particularly massive neutron star or an unusually low-mass black hole. While its identity remains unknown, the authors argue that it was formed in an earlier merger between two neutron stars and possibly the result of the extreme stellar environment within the dense globular cluster in which it resides.