Dispersion and turbulence in the interstellar medium

The radio signal received from pulsars is a powerful tool for studying the interstellar medium. The frequency signal dispersion (DM) measurement, the observation of multi-propagation and scintillation phenomena, and the measurement of the Faraday rotation of the signal inform us on the electronic content along the line of sight, on the magnetic field in the Galaxy, and on their long-term variations.

The radio signal received from pulsars is a powerful tool for studying the interstellar medium. The frequency signal dispersion (DM) measurement, the observation of multi-propagation and scintillation phenomena, and the measurement of the Faraday rotation of the signal inform us on the electronic content along the line of sight, on the magnetic field in the Galaxy, and on their long-term variations. The time dispersion of the signal (“pulse broadening”), allows us to gauge the multi-propagation effect and the heterogeneity of the medium crossed. More generally, scintillation, which can be characterized by the detailed study of the dynamic spectrum, provides us with information on the turbulence scales in the interstellar medium. Finally, the Faraday rotation measurement (RM) contains information on the average magnetic field along the line of sight.

All these effects are particularly pronounced at low frequencies, making LOFAR, and especially NenuFAR, an ideal instrument for their study. Low frequency monitoring of a set of pulsars, with multiple targeting directions, therefore allows us to probe very comprehensively the overall characteristics of the interstellar medium surrounding us within a bubble of a few kpc. Pilot studies have already been carried out with the UTR2 (Ukraine) and LOFAR radio telescopes. In particular, 200 pulsars are regularly monitored with the LOFAR-FR606 station in standalone mode, highlighting variations in beam profiles and variations in dispersion measurement. This programme will continue with the new NenuFAR radio telescope.