Giant Metrewave Radio Telescope Discovery of a Millisecond Pulsar in a Very Eccentric Binary System

We report the discovery of the binary millisecond pulsar PSR J0514-4002A, which is the first known pulsar in the globular cluster NGC 1851 and the first pulsar discovered using the Giant Metrewave Radio Telescope. The pulsar has a rotational period of 4.99 ms, an orbital period of 18.8 days, and the most eccentric pulsar orbit yet measured (e = 0.89). The companion has a minimum mass of 0.9 M☉, and its nature is currently unclear. After accreting matter from a low-mass companion star that spun it up to a (few) millisecond spin period, the pulsar eventually exchanged the low-mass star for its more massive present companion. This is exactly the same process that could form a system containing a millisecond pulsar and a black hole; the discovery of NGC 1851A demonstrates that such systems might exist in the universe, provided that stellar mass black holes exist in globular clusters.