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 variationsEdit

Structure function analysisEdit



Extreme scattering eventsEdit

Modelling the interstellar mediumEdit

Cordes & Lazio (2002) presented the most commonly used model for the Galactic distribution of free electrons.