rPICARD: A CASA-based calibration pipeline for VLBI data. Calibration and imaging of 7 mm VLBA observations of the AGN jet in M 87

Abstract

Context. The Common Astronomy Software Application (CASA) software suite, which is a state-of-the-art package for radio astronomy, can now reduce very long baseline interferometry (VLBI) data with the recent addition of a fringe fitter. <BR /> Aims: Here, we present the Radboud PIpeline for the Calibration of high Angular Resolution Data (rPICARD), which is an open-source VLBI calibration and imaging pipeline built on top of the CASA framework. The pipeline is capable of reducing data from different VLBI arrays. It can be run non-interactively after only a few non-default input parameters are set and delivers high-quality calibrated data. CPU scalability based on a message-passing interface (MPI) implementation ensures that large bandwidth data from future arrays can be processed within reasonable computing times. <BR /> Methods: Phase calibration is done with a Schwab-Cotton fringe fit algorithm. For the calibration of residual atmospheric effects, optimal solution intervals are determined based on the signal-to-noise ratio (S/N) of the data for each scan. Different solution intervals can be set for different antennas in the same scan to increase the number of detections in the low S/N regime. These novel techniques allow rPICARD to calibrate data from different arrays, including high-frequency and low-sensitivity arrays. The amplitude calibration is based on standard telescope metadata, and a robust algorithm can solve for atmospheric opacity attenuation in the high-frequency regime. Standard CASA tasks are used for CLEAN imaging and self-calibration. <BR /> Results: In this work we demonstrate the capabilities of rPICARD by calibrating and imaging 7 mm Very Long Baseline Array (VLBA) data of the central radio source in the M 87 galaxy. The reconstructed jet image reveals a complex collimation profile and edge-brightened structure, in accordance with previous results. A potential counter-jet is detected that has 10% of the brightness of the approaching jet. This constrains jet speeds close to the radio core to about half the speed of light for small inclination angles.