Orbital Modulation of the Apparent Dispersion Measure towards Psr J0737−3039a

We report the discovery of systematic orbital variations in the apparent Dispersion Measure (DM) towards PSR J0737−3039A in the double-pulsar binary using the Green Bank Telescope. The peak-to-peak amplitude of the modulation is ∼0.09 pc cm−3, with the peaks occurring near maximum elongation (i.e. when the pulsars are separated by the greatest angle on the sky). The apparent DM variation is fit equally well by models linearly proportional to the radial velocity of the pulsar (δDMV ) or its distance from the system’s center-of-mass along the line of nodes in the plane of the sky (δDMX ). The best-fit values for the constants of proportionality were determined over a 50 MHz bandpass centered at 427 MHz and are δDMV = −0.0001488(19) pc cm−3 (km s−1)−1 and δDMX = −0.03222(20) pc cm−3 (lt-sec)−1, respectively. The reference DM of the pulsar is ∼48.93(1) pc cm−3. We argue that the model considering the DM variation to be proportional to the radial velocity is the most physically plausible. It implies that a modification to the standard cold plasma dispersion relation is required to account for propagation effects within the wind of the rapidly orbiting pulsar. Subject headings: ISM: general — pulsars: general — pulsars: individual (PSR J0737−3039A)