CONSTRAINTS ON NATAL KICKS IN GALACTIC DOUBLE NEUTRON STAR SYSTEMS

Since the discovery of the first double neutron star (DNS) system in 1975 by Hulse and Taylor, there are currently eight confirmed DNS in our galaxy. For every system, the masses of both neutron stars, the orbital semimajor axis, and eccentricity are measured, and proper motion is known for half of the systems. Using the orbital parameters and kinematic information, if available, as constraints for all systems, we investigate the immediate progenitor mass of the second-born neutron star (NS2) and the magnitude of the supernova kick it received at birth, with the primary goal to understand the core-collapse mechanism leading to neutron star formation. Compared to earlier studies, we use a novel method to address the uncertainty related to the unknown radial velocity of the observed systems. For PSR B1534+12 and PSR B1913+16, the kick magnitudes are 150-270 km s(-1) and 190-450 km s(-1) (with 95% confidence), respectively, and the progenitor masses of the NS2 are 1.3-3.4 M-circle dot and 1.4-5.0 M-circle dot (95%), respectively. These suggest that the NS2 was formed by an iron core-collapse supernova in both systems. For PSR J0737-3039, on the other hand, the kick magnitude is only 5-120 km s-1 (95%), and the progenitor mass of the NS2 is 1.3-1.9 M-circle dot (95%). Because of the relatively low progenitor mass and kick magnitude, the formation of the NS2 in PSR J0737-3039 is potentially connected to an electron capture supernova of a massive O-Ne-Mg white dwarf. For the remaining five Galactic DNS, the kick magnitude ranges from several tens to several hundreds of km s(-1), and the progenitor mass of the NS2 can be as low as similar to 1.5 M-circle dot or as high as similar to 8 M-circle dot. Therefore, in these systems it is not clear which type of supernova is more likely to form the NS2.