Copyright: John McKean / LOFAR
Part of the ongoing commissioning effort for LOFAR is determining good models for the bright A-team sources that dominate the radio sky at low-frequencies. These objects can interfere with science observations, even when they are many tens of degrees away from the target fields. One of the biggest trouble makers is Cygnus A (3C 405), the very luminous radio galaxy at redshift 0.056.
Here is a new LOFAR image of Cygnus A at 238.684 MHz that was taken with 19 core stations and 7 remote stations. The bandwidth is 0.2 MHz, which is only a fraction of the 48 MHz total bandwidth that is available. This image is significantly better than what was made only last year with a much smaller LOFAR (and an array without the benefit of updated station calibration tables). The total time on-source was 15 hours and the uv-coverage was good enough for the complex extended and compact emission to be properly imaged at this frequency for the first time. The image has an rms of 78.5 mJy / beam, which is limited to a dynamic range of 3340 because of the strong emission from Cygnus A (a total flux-density of 6737 Jy at 239 MHz). This is an excellent example of what can be achieved with an aperture array system.
The model is composed of ~6700 clean components. However, in the future the model will be defined using shaplets, since this method will allow fewer parameters to describe the source, which will greatly increase the calibration speed and will also allow information about the frequency dependent structure of the source to be included in the model.
This image will provide an initial model for further observations of Cygnus A at lower frequencies, where the effect of sources like Cygnus A is more strongly felt on science observations.
by John McKean