Abstract
Context. High-precision pulsar-timing experiments are affected by temporal variations of the dispersion measure (DM), which are related to spatial variations in the interstellar electron content and the varying line of sight to the source. Correcting for DM variations relies on the cold-plasma dispersion law which states that the dispersive delay varies with the squared inverse of the observing frequency. This may, however, give incorrect measurements if the probed electron content (and therefore the DM) varies with observing frequency, as is predicted theoretically due to the different refraction angles at different frequencies. Aims. We study small-scale density variations in the ionised interstellar medium. These structures may lead to frequency-dependent DMs in pulsar signals. Such an effect could inhibit the use of lower-frequency pulsar observations as tools to correct time-variable interstellar dispersion in higher-frequency pulsar-timing data. Methods. We used high-cadence, low-frequency observations with three stations from the German LOng-Wavelength (GLOW) consortium, which are part of the LOw-Frequency ARray (LOFAR). Specifically, 3.5 yr of weekly observations of PSR J2219+4754 are presented. Results. We present the first detection of frequency-dependent DMs towards any interstellar object and a precise multi-year time-series of the time- and frequency-dependence of the measured DMs. The observed DM variability is significant and may be caused by extreme scattering events. Potential causes for frequency-dependent DMs are quantified and evaluated. Conclusions. We conclude that frequency dependence of DMs has been reliably detected and is indeed caused by small-scale (up to tens of AUs) but steep density variations in the interstellar electron content. We find that long-term trends in DM variability equally affect DMs measured at both ends of our frequency band and hence the negative impact on long-term high-precision timing projects is expected to be limited.
Highlights
Pulsars are highly magnetised, rapidly rotating neutron stars, the remnants of massive stars that ended their life in a supernova
We have presented strong and rapid dispersion measure (DM) variations along the line of sight towards PSR J2219+4754, which have a similar amplitude as the variations commonly seen in millisecond pulsars
The ESEs in this paper would be some of the longest and most persistent observed to date. This would be the first time an ESE is observed in electron density and scattering at the same time, the scattering may well be caused by different ionised interstellar medium (IISM) structures that have only a minimal impact on the DM
Summary
Pulsars (first discovered by Hewish et al 1968) are highly magnetised, rapidly rotating neutron stars, the remnants of massive stars that ended their life in a supernova. Structure function analysis Given the strong and significant DM variations we detected – as well as the frequency dependence of these measurements – it is worthwhile to evaluate the overall structure of the IISM towards this pulsar in order to verify whether the DM variability could be explained by standard IISM turbulence or not. 10−3m−6.67 fits a the idea that the observed variability is part of the general IISM turbulence rather than a stand-alone ESE
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