PSR J1756-2251: a new relativistic double neutron star system

We report the discovery during the Parkes Multibeam Pulsar Survey of PSR J1756 − 2251, a 28.5 ms pulsar in a relativistic binary system. Subsequent timing observations showed the pulsar to have an orbital period of 7.67 hrs and an eccentricity of 0.18. They also revealed a significant advance of periastron, 2 . 585 ± 0 . 002 deg. yr − 1 . Assuming this is entirely due to general relativity im-plies a total system mass (pulsar plus companion) of 2.574 ± 0.003 M ⊙ . This mass and the significant orbital eccentricity suggest that this is a double neutron star system. Measurement of the gravitational redshift, γ , and an evaluation of the Shapiro delay shape, s , indicate a low companion mass of < 1.25 M ⊙ . The expected coalescence time due to emission of gravitational waves is only ∼ 1.7 Gyr substantially less than a Hubble time. We note an apparent correlation between spin period and eccentricity for normally evolving double neutron star systems.