PALFA Discovery of a Highly Relativistic Double Neutron Star Binary

We report the discovery and initial follow-up of a double neutron star (DNS) system, PSR J1946+2052, with the Arecibo L-Band Feed Array pulsar (PALFA) survey. PSR J1946+2052 is a 17 ms pulsar in a 1.88 hr, eccentric (e = 0.06) orbit with a greater than or similar to 1.2 M-circle dot companion. We have used the Jansky Very Large Array to localize PSR J1946+2052 to a precision of 0 ''.09 using a new phase binning mode. We have searched multiwavelength catalogs for coincident sources but did not find any counterparts. The improved position enabled a measurement of the spin period derivative of the pulsar ((P)over dot = 9 +/- 2 x 10(-19)); the small inferred magnetic field strength at the surface (B-S = 4 x 10(9) G) indicates that this pulsar has been recycled. This and the orbital eccentricity lead to the conclusion that PSR J1946+2052 is in a DNS system. Among all known radio pulsars in DNS systems, PSR J1946+2052 has the shortest orbital period and the shortest estimated merger timescale, 46 Myr; at that time it will display the largest spin effects on gravitational-wave waveforms of any such system discovered to date. We have measured the advance of periastron passage for this system, (omega)over dot = 25.6 +/- 0.3 deg yr(-1), implying a total system mass of only 2.50 +/- 0.04 M-circle dot, so it is among the lowest-mass DNS systems. This total mass measurement combined with the minimum companion mass constrains the pulsar mass to less than or similar to 1.3 Me-circle dot.