Polarization calibration techniques for the new-generation VLBI

The calibration and analysis of polarization observations in Very Long Baseline Interferometry (VLBI) requires the use of specific algorithms that suffer from several limitations, closely related to assumptions in the data properties that may not hold in observations taken with new-generation VLBI equipment. Nowadays, the instantaneous bandwidth achievable with VLBI backends can be as high as several GHz, covering several radio bands simultaneously. In addition, the sensitivity of VLBI observations with state-of-the-art equipment may reach dynamic ranges of tens of thousands, both in total intensity and in polarization. In this paper, we discuss the impact of the limitations of common VLBI polarimetry algorithms on narrow-field observations taken with modern VLBI arrays (from the VLBI Global Observing System, VGOS, to the Event Horizon Telescope, EHT) and present new software that overcomes these limitations. In particular, our software is able to perform a simultaneous fit of multiple calibrator sources, include non-linear terms in the model of the instrumental polarization and use a self-calibration approach for the estimate of the polarization leakage in the antenna receivers.

[1]  Daniel C. M. Palumbo,et al.  Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution , 2020 .

[2]  P. Ho,et al.  Resolved magnetic-field structure and variability near the event horizon of Sagittarius A* , 2015, Science.

[3]  A. Readhead,et al.  Image Formation by Self-Calibration in Radio Astronomy , 1984 .

[4]  R. Sault,et al.  Understanding radio polarimetry. I. Mathematical foundations , 1996 .

[5]  W. Cotton.  Calibration and imaging of polarization sensitive Very Long Baseline Interferometer observations , 1993 .

[6]  Daniel C. M. Palumbo,et al.  THEMIS: A Parameter Estimation Framework for the Event Horizon Telescope , 2020, The Astrophysical Journal.

[7]  Uwe Bach,et al.  PROBING THE INNERMOST REGIONS OF AGN JETS AND THEIR MAGNETIC FIELDS WITH RADIOASTRON. I. IMAGING BL LACERTAE AT 21 μas RESOLUTION , 2015, 1512.04690.

[8]  K. Hada,et al.  Linear polarization in the nucleus of M87 at 7 mm and 1.3 cm , 2020, Astronomy & Astrophysics.

[9]  J. Zensus,et al.  Linear Polarization Imaging with Very Long Baseline Interferometry at High Frequencies , 1995 .

[10]  Daniel C. M. Palumbo,et al.  First M87 Event Horizon Telescope Results. III. Data Processing and Calibration , 2019, The Astrophysical Journal.

[11]  S. T. Timmer,et al.  First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole , 2019, 1906.11238.

[12]  T. Cornwell,et al.  A multi-scale multi-frequency deconvolution algorithm for synthesis imaging in radio interferometry , 2011, 1106.2745.

[13]  W. Alef,et al.  The ALMA Phasing System: A Beamforming Capability for Ultra-high-resolution Science at (Sub)Millimeter Wavelengths , 2017, 1711.06770.

[14]  L. Gurvits,et al.  Multi-frequency polarization properties of ten quasars on deca-parsec scales at z > 3 , 2011, 1202.2193.

[15]  O. Smirnov.  Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism , 2011, 1101.1764.

[16]  T. Krichbaum,et al.  Evidence of internal rotation and a helical magnetic field in the jet of the quasar NRAO150 , 2014, 1404.5961.

[17]  W. Alef,et al.  On the calibration of full-polarization 86 GHz global VLBI observations , 2012, 1203.1424.

[18]  S. Muller,et al.  UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data , 2014, 1401.4984.

[19]  E. F. Arias,et al.  THE SECOND REALIZATION OF THE INTERNATIONAL CELESTIAL REFERENCE FRAME BY VERY LONG BASELINE INTERFEROMETRY , 2015 .

[20]  Lindy Blackburn,et al.  rPICARD: A CASA-based calibration pipeline for VLBI data , 2019, Astronomy & Astrophysics.

[21]  G. Swenson,et al.  Interferometry and Synthesis in Radio Astronomy , 1986 .

[22]  L. Blackburn,et al.  Calibration of ALMA as a Phased Array. ALMA Observations During the 2017 VLBI Campaign , 2019, Publications of the Astronomical Society of the Pacific.

[23]  Des Small,et al.  rPICARD: A CASA-based Calibration Pipeline for VLBI Data , 2019 .

[24]  Alan Roy,et al.  Calibration of mixed-polarization interferometric observations Tools for the reduction of interferometric data from elements with linear and circular polarization receivers , 2016, 1601.04266.

[25]  Kevin A. Dudevoir,et al.  First M87 Event Horizon Telescope Results. II. Array and Instrumentation , 2019, 1906.11239.