A pulsar in a binary with a compact object in the mass gap between neutron stars and black holes

Understand

Among the compact objects observed in gravitational wave merger events a few have masses in the gap between the most massive neutron stars (NSs) and least massive black holes (BHs) known.

• Their nature and the formation of their merging binaries are not well understood.
• We report on pulsar timing observations using the Karoo Array Telescope (MeerKAT) of PSR J0514-4002E, an eccentric binary millisecond pulsar in the globular cluster NGC 1851 with a total binary mass of $3.887 \pm 0.004$ solar masses.
• The companion to the pulsar is a compact object and its mass (between $2.09$ and $2.71$ solar masses, 95% confidence interval) is in the mass gap, so it either is a very massive NS or a low-mass BH.