The NANOGrav Nine-Year Data Set: Measurement and Interpretation of Variations in Dispersion Measures

TL;DR

This study analyzes nine years of pulsar data to measure and interpret variations in dispersion measures, revealing diverse behaviors and potential ISM structures, with implications for understanding the interstellar medium and pulsar timing.

Contribution

It provides a comprehensive analysis of DM variations in 37 pulsars, identifying periodic, monotonic, and complex trends, and constrains the size of dispersing regions, advancing understanding of ISM turbulence.

Findings

Significant DM variations in nearly all pulsars.

Detection of periodic and monotonic trends in DM.

Evidence for Kolmogorov-like turbulence in some pulsars.

Abstract

We analyze dispersion measure (DM) variations of 37 millisecond pulsars in the 9-year NANOGrav data release and constrain the sources of these variations. Variations are significant for nearly all pulsars, with characteristic timescales comparable to or even shorter than the average spacing between observations. Five pulsars have periodic annual variations, 14 pulsars have monotonically increasing or decreasing trends, and 13 pulsars show both effects. Several pulsars show correlations between DM excesses and lines of sight that pass close to the Sun. Mapping of the DM variations as a function of the pulsar trajectory can identify localized ISM features and, in one case, an upper limit to the size of the dispersing region of 13.2 AU. Finally, five pulsars show very nearly quadratic structure functions, which could be indicative of an underlying Kolmogorov medium. Four pulsars show roughly Kolmogorov structure functions and another four show structure functions less steep than Kolmogorov. One pulsar has too large an uncertainty to allow comparisons. We discuss explanations for apparent departures from a Kolmogorov-like spectrum, and show that the presence of other trends in the data is the most likely cause.