Characterising the response of an International LOFAR Station

Phased-array radio interferometers with fixed antennas are a highly scalable design which can achieve a large gain. However they are complex systems and challenging to calibrate. Here we examine the response of an International LOFAR Station using long-track observations of an array of flux-stable pulsars. The analyses are relevant for other arrays like the under-construction SKA-Low. In this paper, we investigate the performance of the high-band antennas of the Irish LOFAR station using long-track observations of bright pulsars. In modelling our measured responses, we account for projection effects, the frequency-dependence of the aperture efficiency and pulsar spectra, as well as sky and instrumental noise contributions. We perform full-track observations of 11 pulsars as they move across the sky. We perform RFI mitigation and determine the signal-to-noise ratio response as a function of time and evaluate the elevation and azimuth dependence. We use the DreamBeam software to model the beam response of the station to compare with what is observed. The sensitivity map so obtained was validated using observations of PSR B0329+54 on the Swedish LOFAR station. We observe the best response for pointings close to the zenith, as expected, implying an improved sensitivity at higher elevation. However, an asymmetry with respect to the zenith point is detected. Characterising the instrumental response in azimuth reveals a better performance as the targets rise, as compared to when they set. This is the case for all pulsars in the sample used for the Irish station and is also seen with the Swedish station; it also agrees with previously reported results from the Bałdy LOFAR station in Poland. The magnitude of the effect can exceed 20%. We consider variations in the beam model and noise level, both frequency- and time- dependent, to try to account for this. A possible explanation for the hysteresis-like response in elevation could be an imbalanced signal weighting of the polarisation components. This work highlights the variation in altitude and azimuth of the response of an international LOFAR station using pulsar observations, highlighting an asymmetric response. This work addresses the importance of accurate beam modelling and noise evaluation for the LOFAR 2.0 system upgrade as well as being applicable to the under-construction SKA-Low facility.