PSR B1802 – 07: a globular cluster pulsar in an eccentric binary system

We report the discovery of the first binary radio pulsar to possess both a low-mass companion and a substantial orbital eccentricity. PSR B 1802–07 is a 23-ms pulsar, located within the core of the globular cluster NGC 6539 and a member of a binary system with an orbital period of 2.6 d and ccentricity of 0.21. Timing observations reveal an advance in the longitude of periastron of $$0.^\circ 06\pm 0.^\circ 01$$ yr^–1. If the periastron advance is due solely to relativistic effects, the total mass of the system is 1.65 ± 0.4 M_⊙ , consistent with a neutron star of mass 1.4 M_⊙ and a white dwarf companion of mass 0.3 M_⊙ . Most pulsar binary systems with low-mass companions have orbits which are extremely circular, due to tidal forces which they experience during the giant phase of their companion’s evolution. The eccentricity of the system may have been induced by interactions with other stars of the globular cluster, or may result from incomplete circularization of an initially more eccentric orbit.