THE BINARY COMPANION OF YOUNG, RELATIVISTIC PULSAR J1906+0746

© JvL

PSR J1906+0746 is a young pulsar in the relativistic binary with the second-shortest known orbital period, of 3.98 hours. Our timing study based on five years of observations, conducted with the 5 largest radio telescopes in the world, including Westerbork, aimed at determining the companion nature. The paper was recently accepted for ApJ and is now in arxiv (van Leeuwen et al. 2014).

The complete set of 28,000 pulsar times of arrival (TOAs) is one the largest, most detailed datasets ever fit. From these we derived parameters describing the state and evolution of both the individual pulsar and the binary system. These include three post-Keplerian orbital parameters -- changes in the orbit that only occur in the General Relativistic regime. These are the rate of periastron advance, the gravitational redshift, and the orbital period derivative. These all depend, in different ways, on the two masses of the system. Thus by plotting these three relations on a plane spanned by the masses of the two stars, the highest-confidence value is determined. That plot (Fig. 7 in the paper) is shown above. The colors range from yellow (low probability) to red (highest probability). Above, the figure is further stretched by an impression of the time-space distortions produced by the two components of the binary system.

Through the measurement of these three post-Keplerian orbital parameters we find the pulsar mass to be 1.29 solar mass, and the companion mass 1.32 solar mass respectively. These masses fit well in the observed collection of double neutron stars, but are also compatible with other white dwarfs around pulsars that are young like PSR J1906+0746. We conclude that young pulsar J1906+0746 is likely part of a double neutron star -- one of only ten known!