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Lifting the Ionospheric Veil for Precision RM Measurements

Submitter: Carlos Sotomayor, Charlotte Sobey and Jason Hessels
Description: LOFAR's low frequencies are ideal for determining precision rotation measures (RMs) of polarized sources like pulsars. By measuring high-precision pulsar RMs for a large sample of sources, we hope to deepen our understanding of the interstellar medium and the Galactic magnetic field.

One major challenge in obtaining accurate RMs, however, is adjusting for contamination from the ionosphere. The ionosphere's thickness is very dynamic, changing on timescales from minutes to years, and also varies significantly with geographic location. Using measurements of the ionosphere's total electron content (TEC), plus a model for the geomagnetic field (IGRF11), we can correct for the extra Faraday rotation that the ionosphere imparts on an astronomical signal and hence calculate the RM of pulsars due to only the passage of the signal through the interstellar medium. The Royal Observatory of Belgium provides TEC maps in a 0.5 x 0.5 degree geographic grid every 15 minutes. We're using these maps together with the IGRF11 to correct for the additional Faraday rotation imparted by the ionosphere.

On March 23rd, 2012, between 04:00 - 07:45 UT, the LOFAR Superterp stations cycled through repeated observations of three pulsars. The observed RMs of each pulsar show a clear, time-variable contribution due to the ionosphere, which is increasing as the Sun rises during these observations. After producing accurate predictions for the ionospheric Faraday rotation along the line-of-sight to each pulsar, we subtracted the constant offset between the observed pulsar RM and the ionospheric prediction. The agreement between the two curves is excellent and allows us to derive pulsar RMs with uncertainties of only ~0.1 rad/m^2 in some cases. These are among the most precise pulsar RMs ever derived, and are just the start of what will be a major campaign to derive precision RMs for hundreds of pulsars.
Copyright: CAS,CSB,JWTH
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