J 1756 − 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. Assuming this is entirely due to general relativity implies 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. Subject headings: pulsars: general — pulsars: individual PSR J1756−2251