Double-lens scintillometry: the variable scintillation of pulsar B1508 + 55

Abstract

We report on observations of PSR B1508+55’s scintillation at the Effelsberg 100-m telescope spanning from early 2020 to early 2022. In the autumn of 2020, close to the time the pulsar was predicted to cross echoes in its pulse profile, a sudden transition in the scintillation arcs from peculiar stripe-like features to parabolic arclets was observed. To infer a geometric model of the scattering we measure the effects of the annual velocity curve of Earth, of the relative movement of the line of sight, and of the projection of points on a second scattering screen and develop novel methods to do so. The latter phenomenon was discovered by this study and strongly indicates a two-screen scattering geometry. We derive an analytical two-screen model and demonstrate in a Markov Chain Monte Carlo analysis as well as simulations that it can be successfully applied to explain the observations by interpreting the transition as a change of relative amplitudes of images as well as a shift in the orientation of anisotropy. The collection of methods we demonstrate here is transferable to other pulsars with the potential to strongly improve constraints on scattering models.

A Simultaneous Dual-Frequency Scintillation Arc Survey of Six Bright Canonical Pulsars Using the Upgraded Giant Metrewave Radio Telescope

“…Interestingly, the arc curvatures measured in the last two epochs (MJDs 59115 and 59497) are a factor of two to three times smaller than the first two epochs (MJDs 59064 and 58987), possibly indicating a detection of multiple scattering screens along the LOS to this pulsar. This result augments the results of Sprenger et al (2022), who also found significant variability along the LOS to this pulsar during the same period of time and propose a double screen model as a possible explanation. An example observation from the earlier two epochs is shown in Figure 8, while an example from the later two epochs is shown in Figure 9.…”

A Scintillation Arc Survey of 22 Pulsars with Low to Moderate Dispersion Measures

“…Low curvature arclets could be due to localized scattering near the pulsar (i.e., small value of s) interfering with a core in brightness at small angles of deflection. The scattering geometry is complex for this pulsar, as documented by Bansal et al (2020), who observed remarkable echoes of the pulse arriving 30 ms after the main pulse at 50 and 80 MHz that persisted over about 3 yr. Also see Sprenger et al (2022) who develop a two-screen explanation for the scattering from this pulsar. Arc credibility indices η cred at 340, 825, 1400 MHz are (2, 2, 2).…”

“…Note the wide valley near the τ axis, indicative of anisotropic scattering in a thin screen. This high-velocity pulsar exhibits several intriguing scintillation phenomena that are under current investigation (Sprenger et al 2022).…”

Pulsar Double Lensing Sheds Light on the Origin of Extreme Scattering Events

“…We use the screens package (Van Kerkwijk & van Lieshout 2022) for this purpose, which also uses the inverted matrix to calculate time derivatives V  and a  given velocities of the observer, screens, and pulsar (and thus r  in the equations), as well as the implied delay τ and its time derivative t  for each ray. We note that the same scattering geometry is obtained by considering the geometric limit of the two-screen scattering model in Sprenger et al (2022). It is different, however, from the geometry considered by Simard et al (2019), as those authors assumed that the scattering points were fixed on their respective screens, while we assume that light can only be bent perpendicular to the linear structures, which implies that the scattering images move along those structures as the relative positions of the observer, screens, and pulsar change.…”