Speaker
Description
Very-long baseline interferometry (VLBI) across a wide field of view is notoriously difficult given the extreme data volume and compute resource requirements. Nevertheless, improvements in the calibration and imaging algorithms in the past few years made it possible to carry out wide-field imaging at sub-arcsecond resolution using the LOFAR High-Band Antenna (HBA).
The strong variations of the ionosphere across the LOFAR international baselines of up to $2000\,$km require us to carry out direction-dependent calibration of the ionospheric systematic effects. There are by now several fields across which a full direction-dependent calibration of LOFAR HBA including the international stations was achieved.
For the LOFAR Low-Band Antenna (LBA), operating at ultra-low frequencies of ~50 MHz, calibrating the international baselines is even more challenging given the increased severity of the ionospheric systematic effects and the lower signal-to-noise ratio compared to the HBA. Previously, calibration of the international baselines was only shown to be feasible in a direction-independent manner and only for a small number of exceptionally bright (${>}10\,$Jy) and compact sources. If calibration of sources with flux densities ${\leq}1\,$Jy is possible was unknown.
In this talk, I will present the first ever wide-field VLBI image obtained with the LOFAR LBA system, reaching sub-arcsecond resolution ($0.9''\times0.5''$) across 6$\,$deg$^2$. In $8\,$h of observation time of the field around the galaxy cluster Abell 2255, a noise level of $650\,\mathrm{µJy\,beam^{-1}}$ in the frequency range 42-66$\,$MHz was obtained. The ionospheric effects were calibrated using ten different direction-dependent calibrators. This breakthrough shows that, at least under mild ionospheric conditions and in presence of a 2-3 Jy compact infield-calibrator, wide-field VLBI at ultra-low frequencies is possible.
In addition to the calibration strategy, I will discuss the image quality and properties and showcase the scientific value of the data using high-resolution spectral index maps of the radio galaxies in the Abell 2255 cluster.
Lastly, I will outline important considerations for long-baseline imaging in the LBA as standard observation mode for LOFAR 2.0, also in context of NenuFAR as LOFAR Super-Station in the interferometer.
