On Frequency-dependent Dispersion Measures and Extreme Scattering Events

Radio emission propagating over an Earth-pulsar line of sight provides a unique probe of the intervening ionized interstellar medium (ISM). Variations in the integrated electron column density along this line of sight, or dispersion measure (DM), have been observed since shortly after the discovery of pulsars. As early as 2006, frequency-dependent DMs have been observed and attributed to several possible causes. Ray-path averaging over different effective light-cone volumes through the turbulent ISM contributes to this effect, as will DM misestimation due to radio propagation across compact lensing structures such as those caused by "extreme scattering events (ESEs)." We present methods to assess the variations in frequency-dependent DMs due to the turbulent ISM versus these compact lensing structures along the line of sight. We analyze recent Low-Frequency Array observations of PSR J2219+4754 to test the underlying physical mechanism of the observed frequency-dependent DM. Previous analyses have indicated the presence of strong lensing due to compact overdensities halfway between the Earth and pulsar. Instead, we find the frequency dependence of the DM time series for PSR J2219+4754 is consistent with being due solely to ISM turbulence and there is no evidence for any ESE or small-scale lensing structure. The data show possible deviations from a uniform turbulent medium, suggesting that there may be an enhanced scattering screen near one of the two ends of the line of sight. We present this analysis as an example of the power of low-frequency observations to distinguish the underlying mechanisms in frequency-dependent propagation effects.