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

TL;DR

This study investigates the scintillation of PSR B1508+55 using a 10,000-km baseline between GMRT and ARO, revealing highly anisotropic 2D scattering and proposing multiple scattering screens at different distances.

Contribution

It provides the first measurement of scintillation cross-correlation on a 10,000-km baseline and suggests the presence of at least two distinct scattering screens affecting pulsar signals.

Findings

Low cross-correlation coefficient indicates anisotropic 2D scattering.

Detection of scintillation confined to low delays suggests separate scattering regions.

Proposes multiple scattering screens at different distances, including a 125 pc screen.

Abstract

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 isotropic 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 parabolic scintillation arc detected at the 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 125~pc to the screen causing these echoes. These previous measurements, alongside our observations, lead us to propose that there are at least two scattering screens: the closer 125 pc screen causing the scintillation arc detected at GMRT, and a screen further beyond causing the scintillation detected on the GMRT-ARO baseline. We advance the hypothesis that the 125-pc screen partially resolves the speckle images on the screen beyond leading to loss of coherence in the scintillation dynamic spectrum, to explain the low cross-correlation coefficient.