On Frequency-Dependent Dispersion Measures and Extreme Scattering Events

Abstract

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 dispersion measures 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". We present methods to assess the variations in frequency-dependent dispersion measures due to the turbulent ISM versus these compact lensing structures along the line of sight. We analyze recent Low-Frequency Array (LOFAR) 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 timeseries for PSR J2219+4754 is consistent with being due solely to ISM turbulence and there is no evidence for any extreme scattering event 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.

16 pages, 7 figures, accepted by ApJ

References in corpus (12)

• Assessing the Role of Spin Noise in the Precision Timing of Millisecond Pulsars
• Dispersion Measure Variations and their Effect on Precision Pulsar Timing
• Limitations in timing precision due to single-pulse shape variability in millisecond pulsars
• Wide-band Simultaneous Observations of Pulsars: Disentangling Dispersion Measure and Profile Variations
• Pulsar polarisation below 200 MHz: Average profiles and propagation effects
• The NANOGrav Nine-Year Data Set: Excess Noise in Millisecond Pulsar Arrival Times
• An improved solar wind electron-density model for pulsar timing
• Model-based asymptotically optimal dispersion measure correction for pulsar timing
• First detection of frequency-dependent, time-variable dispersion measures
• A 24-Hour Global Campaign To Assess Precision Timing of the Millisecond Pulsar J1713+0747
• Modelling and mitigating refractive propagation effects in precision pulsar timing observations
• Pulsar Timing Errors from Asynchronous Multi-Frequency Sampling of Dispersion Measure Variations

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• Detection of frequency-dependent dispersion measure toward the millisecond pulsar PSR J2241-5236 from contemporaneous wide-band observations
• A Multifrequency Census of 100 Pulsars below 100 MHz with LWA: A Systematic Study of Flux Density, Spectra, Timing, Dispersion, Polarization, and Its Variation from a Decade of Observations
• Precision Timing of PSR J0437-4715 with the IAR Observatory and Implications for Low-Frequency Gravitational Wave Source Sensitivity
• A Parallelized Bayesian Approach To Accelerated Gravitational-Wave Background Characterization