Scintillation of PSR B1508+55 -- the view from a 10,000-km baseline

We report on the simultaneous Giant Metrewave Radio Telescope (GMRT) and Algonquin Radio Observatory (ARO) observations at 550-750 MHz of the scintillation of PSR B1508+55, resulting in a $\sim$10,000-km baseline. This regime of measurement lies between the shorter few 100-1000 km baselines of earlier multi-station observations and the much longer earth-space baselines. We measure a scintillation cross-correlation coefficient of $0.22$, offset from zero time lag due to a $\sim 45$ s traversal time of the scintillation pattern. The scintillation time of 135 s is $3\times$ longer, ruling out anisotropic as well as strictly 1D scattering. Hence, the low cross-correlation coefficient is indicative of highly anisotropic but 2D scattering. The common scintillation detected on the baseline is confined to low delays of $\lesssim 1 \mu$s, suggesting that this correlation may not be associated with the prominent parabola detected at GMRT. Detection of pulsed echoes and their direct imaging with the Low Frequency Array (LOFAR) by a different group enable them to measure a distance of 120 pc to the screen causing these echoes. These previous measurements, alongside our observations, lead us to propose that there are two scattering screens: the closer 120 pc screen causing the prominent parabola detected at GMRT, and a screen further beyond causing the scintillation detected on the GMRT-ARO baseline. We advance the hypothesis that the 120-pc screen partially resolves the speckle images on the screen beyond and that their scattering axes are mutually misaligned, resulting in a two-dimensional scattered disc, to explain the low cross-correlation coefficient.