Pulsars and neutron stars/The interstellar medium
The nature of the pulsed radiation observed on Earth is affected by charged particles in the interstellar medium (ISM).
The radiation travels through the ionized gas of the ISM with group velocity (Shapiro & Teukolsky 1983):
where is the vacuum speed of light and is the plasma frequency. The time difference, between two frequencies and after travelling a distance equals:
where and are the group velocities corresponding to the two frequencies. Writing the plasma frequency in terms of fundamental constants and the charged particle density we get:
where is the electronic charge and is the electron rest mass. We define the pulsar's dispersion measure (DM) as
Hence, the time delay, between an observed pulse at observing frequency and a pulse of infinite frequency (or travelling through a vacuum) is given by:
When a pulsar is observed with a frequency channel resolution of (), the dispersion will lead to a smearing of the profile:
Dispersion measure variations
Structure function analysis
Extreme scattering events
Modelling the interstellar medium
Cordes & Lazio (2002) presented the most commonly used model for the Galactic distribution of free electrons.