Probing the Interstellar Medium towards the Double Pulsar and the Gum Nebula using Low-Frequency Pulse Broadening Measurements

Long-term timing of the double pulsar system (PSR J0737-3039A/B) has enabled some of the most stringent constraints on strong-field gravity. Yet, despite the system's high profile, there have been very few observations at frequencies below 300 MHz, where pulse broadening due to interstellar scattering becomes significant. Measurements of the pulse broadening timescale and its frequency scaling, τ s proportional toν α (where α is the scattering spectral index), provide valuable constraints on the structure of the scattering medium. Here we report on simultaneous observations of J0737-3039A and five other nearby pulsars (J0630-3824, J0742-2822, J0820-4114, J0835-4510, J0837-4135) in the frequency range 109-231 MHz using the Murchison Widefield Array (MWA). These observations have enabled the highest-quality detections ever published for these pulsars at low frequencies. For J0737-3039A, J0742-2822, and J0837-4135, we measure α to be-4.1(1), -3.8(1), and-4.1(1), respectively. These measurements provide new constraints on the density and turbulence parameters of the Gum Nebula region, and we discuss implications for models of the Galactic electron density distribution. Our current data yield a dispersion measure (DM) precision of 4 x 10-4 pc/cm-3 for J0737-3039A, which can be further improved with the MWA Phase III to reach 1 x 10-4 pc/cm-3, far exceeding the precision of previous studies. The prospects for precisely constraining temporal variations in the DM and pulse broadening timescale strengthen the importance of regular monitoring of J0737-3039A at low frequencies, with the eventual goal of contributing to long-term timing studies.