A high-Galactic-altitude scattering screen: scintillation from Globular Cluster pulsar M53A

Abstract: While known interstellar scattering screens are largely confined to the Galactic disk, the scattering properties of the circumgalactic medium (CGM) in the deep Galactic halo have remained unexplored. Using observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we report the first detection of scintillation arcs in the secondary spectra of the high-latitude pulsar J1312+1810A (M53A). Under the 1D anisotropic and 2D isotropic thin-screen models considered here, the inferred scintillation screen distances are $12.8_{-3.5}<sup>{+2.6}$ and $11.7_{-3.5}<sup>{+2.8}$\,kpc, respectively. These model-dependent estimates correspond to Galactic heights of $12.6_{-3.5}<sup>{+2.6}$ and $11.5_{-3.5}<sup>{+2.7}$\,kpc, placing the candidate screen among the highest-altitude scattering structures inferred to date. The strong, time-variable scintillation asymmetry suggests substantial electron-density inhomogeneity within the screen. We speculate that this screen is associated with small-scale structures near the shock boundary of the northern eROSITA bubble. These results support the presence of discrete, inhomogeneous scattering structures deep within the Galactic halo, far beyond the regions where localized stellar feedback is typically concentrated, and highlight pulsar scintillation as a promising probe of small-scale ionized structures in the CGM.