Imaging without visibilities: FAST-Effelsberg scintillometry of PSR B1508+55

Abstract: Context. The spatially coherent multipath propagation of pulsar radiation leads to a temporal and spectral interference patterns called scintillation. It is caused by density variations in the ionized interstellar medium, which often take the form of thin scattering screens filled with multiple subimages of the pulsar. PSR B1508+55 is known to be scattered by one or two such screens. Aims. We investigate appropriate methods to achieve precise astrometry for a scattering screen from simultaneous observations of only two telescopes on a very long baseline without forming visibilities. Methods. Two simultaneous observations of PSR B1508+55 were performed with the 100-m telescope at Effelsberg and the Five-hundred-meter Aperture Spherical Telescope (FAST). Using and improving existing scintillometry techniques, we leveraged the evolving, very long baseline to precisely measure the screen orientation, effective velocity, and scintillation arc curvature. We inferred the one-screen and two-screen model parameters and we imaged the closer screen. Results. Each single epoch leads to much tighter angular constraints than long-term monitoring of scintillation arcs, revealing an ongoing evolution of the orientation of the closer screen. Images of the scattered pulsar were obtained with a resolution on the order of 0.1 mas. These results confirm the highly anisotropic alignment of the scattered images, while also revealing small-scale deviations from a large-scale straight line. Conclusions. We demonstrate that simultaneous observations of scintillation can be used as a powerful substitute for very long baseline inferometry.