[Paper Review] A Scintillation Arc Survey of 22 Pulsars with Low to Moderate Dispersion Measures

Daniel R. Stinebring, B. J. Rickett+7 more|arXiv (Cornell University)|Jul 18, 2022

Advanced Frequency and Time Standards4 citations

TL;DR

This study conducts a systematic survey of scintillation arcs in 22 low-to-moderate dispersion measure pulsars using multi-frequency observations, revealing that scintillation arcs are prevalent and more prominent at higher frequencies and lower DM. The key finding is that well-defined arcs do not imply scattering anisotropy; only reverse arclets with deep delay-axis valleys—present in ~20% of pulsars—indicate significant anisotropy, pointing to patchy, localized scattering structures in the ionized ISM on au-scale transverse to the line of sight.

ABSTRACT

Context: By providing information about the location of scattering material along the line of sight (LoS) to pulsars, scintillation arcs are a powerful tool for exploring the distribution of ionized material in the interstellar medium. Here, we present observations that probe the ionized ISM on scales of $\sim$~0.001 -- 30~au. Aims: We have surveyed pulsars for scintillation arcs in a relatively unbiased sample with DM < 100 pc cm-3. We present multi-frequency observations of 22 low to moderate DM pulsars. Many of the 54 observations were also observed at another frequency within a few days. Methods: For all observations we present dynamic spectra, autocorrelation functions, and secondary spectra. We analyze these data products to obtain scintillation bandwidths, pulse broadening times, and arc curvatures. Results: We detect definite or probable scintillation arcs in 19 of the 22 pulsars and 34 of the 54 observations, showing that scintillation arcs are a prevalent phenomenon. The arcs are better defined in low DM pulsars. We show that well-defined arcs do not directly imply anisotropy of scattering. Only the presence of reverse arclets and a deep valley along the delay axis, which occurs in about 20\% of the pulsars in the sample, indicates substantial anisotropy of scattering. Conclusions: The survey demonstrates substantial patchiness of the ionized ISM on both au size scales transverse to the line of sight and on $\sim$~100~pc scales along it. We see little evidence for distributed scattering along most lines of sight in the survey.

Motivation & Objective

• To investigate the prevalence and characteristics of scintillation arcs in a statistically unbiased sample of pulsars with DM < 100 pc cm⁻³.
• To determine whether scintillation arcs are indicative of anisotropic scattering or localized, patchy structures in the interstellar medium.
• To assess the role of frequency and dispersion measure in shaping arc morphology and detectability.
• To quantify the distribution of scattering material along the line of sight using dynamic spectra, autocorrelation functions, and secondary spectra.
• To evaluate the implications of arc morphology for the spatial structure of ionized interstellar plasma on scales of ~0.001–30 au.

Proposed method

• Multi-frequency observations were conducted using the Green Bank Telescope and Arecibo Observatory to capture dynamic spectra across multiple frequency bands.
• Dynamic spectra were analyzed to identify parabolic modulations indicative of scintillation arcs, using power spectrum techniques to detect arc-like features.
• Autocorrelation functions were computed to estimate scintillation bandwidths and pulse broadening timescales.
• Secondary spectra were generated to extract arc curvatures and assess the presence of reverse arclets and deep valleys along the delay axis.
• Scattering anisotropy was inferred only when both reverse arclets and a deep valley in the delay direction were observed, following established criteria from prior studies.
• Statistical analysis of arc prominence across the sample was performed as a function of dispersion measure and observing frequency.

Experimental results

Research questions

• RQ1 How prevalent are scintillation arcs in a representative sample of low-to-moderate DM pulsars?
• RQ2 Does the presence of well-defined scintillation arcs imply anisotropic scattering in the interstellar medium?
• RQ3 What is the relationship between arc morphology, dispersion measure, and observing frequency?
• RQ4 What fraction of pulsars exhibit the diagnostic features of anisotropic scattering—specifically reverse arclets and deep delay-axis valleys?
• RQ5 What does the distribution of arc features suggest about the spatial structure of scattering material on au-scale transverse to the line of sight?

Key findings

• Scintillation arcs were definitively or probably detected in 19 out of 22 pulsars and in 34 out of 54 observations, confirming their prevalence in low-to-moderate DM pulsars.
• Arcs were more prominent and better defined in low DM pulsars, with higher-frequency observations showing sharper and more distinct arc features.
• Well-defined arcs alone do not indicate scattering anisotropy; only about 20% of the pulsars exhibited the combination of reverse arclets and a deep valley along the delay axis, which is the diagnostic signature of anisotropic scattering.
• Power asymmetries in the secondary spectra suggest that the scattering medium is patchy on scales of ~0.001–30 au transverse to the line of sight.
• The absence of widespread distributed scattering along most lines of sight implies that scattering is dominated by discrete, localized structures rather than a smooth, extended medium.
• The results support the hypothesis that interstellar scattering is primarily caused by scattering at the boundaries of dense plasma structures—such as those in the Local Bubble or stellar bow-shocks—rather than by homogeneous turbulence.