| Pulsars maintain charged-particle flows in their magnetospheres, which are topologically constrained by TeraGauss dipolar magnetic fields. This combination gives rise to narrow beams of coherent, polarised radio emission generated above the pulsar magnetic poles. The beamed emission coupled with pulsar rotation is observed as regular pulses, each time the pulsar beam sweeps our field of view. Pulsar emission is thought to be generated at a certain height above the pulsar surface, where higher frequencies are thought to come from lower heights.
Before it reaches our telescopes, this emission has to propagate first through the pulsar magnetosphere, then through the interstellar medium, and then the Earth's ionosphere. These different media distort the intrinsic polarisation signal but the propagation effects that cause such distortion are measurable and provide invaluable information about the properties of those media.
We have used the LOFAR core stations (i.e. the inner 3km) to capture the polarisation properties of 20 pulsars at 150 MHz, with unprecedented detail for this frequency regime. By combining our LOFAR observations with archival data at higher frequencies, we were able to map the evolution of the polarisation between 1400 MHz and 150 MHz. This helped us investigate the observational effects of magnetospheric birefringence, wherein the ordinary (O) and extraordinary (X) modes of the linearly polarised emission are refracted away from each other as the radio waves propagate in the magnetosphere.
In addition, we studied the distorting effects of interstellar scattering on measurements of the amount of Faraday rotation towards pulsars. Surprisingly, these effects are more evident at higher frequencies than in LOFAR data. Finally, we have translated temporal delays between the arrival time of the total and the polarised emission at 150 MHz into emission heights, assuming that such delays are caused by relativistic aberration due to the pulsars' fast rotation. In all examined cases, we find that the low-frequency radio emission is generated within a few hundred km above the pulsar surface.
A paper describing these results has been accepted for publication in Astronomy & Astrophysics: Pulsar polarisation below 200 MHz: Average profiles and propagation effects by Noutsos et al. 2015, A&A in press