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.
The calibration of Very Long Baseline Interferometry (VLBI) data has long been a time consuming process. The Korean VLBI Network (KVN) is a simple array consisting of three identical antennas. Because four frequencies are observed simultaneously, phase solutions can be transferred from lower frequencies to higher frequencies in order to improve phase coherence and hence sensitivity at higher frequencies. Due to the homogeneous nature of the array, the KVN is also well suited for automatic calibration. In this paper we describe the automatic calibration of single-polarisation KVN data using the…
We present a detailed description of the special procedures for calibration and quality assurance of Atacama Large Millimeter/submillimeter Array (ALMA) observations in Very Long Baseline Interferometry (VLBI) mode. These procedures are required to turn the phased ALMA array into a fully calibrated VLBI station. As an illustration of these methodologies, we present full-polarization observations carried out with ALMA as a phased array at 3mm (Band 3) and 1.3mm (Band 6) as part of Cycle-4. These are the first VLBI science observations conducted with ALMA and were obtained during a 2017 VLBI cam…
We describe a new method which achieves high precision Very Long Baseline Interferometry (VLBI) astrometry in observations at millimeter wavelengths. It combines fast frequency-switching observations, to correct for the dominant non-dispersive tropospheric fluctuations, with slow source-switching observations, for the remaining ionospheric dispersive terms. We call this method Source-Frequency Phase Referencing. Provided that the switching cycles match the properties of the propagation media, one can recover the source astrometry. We present an analytic description of the two-step calibration …
The Atacama Large mm-submm Array (ALMA) is, by far, the most sensitive mm/submm telescope in the World. The ALMA Phasing Project (APP) will allow us to phase-up all the ALMA antennas and use them as one single VLBI station. This will be a key component of the Event Horizon Telescope (EHT), a Global VLBI array at millimeter wavelengths. A problem in the APP is the calibration and conversion of the polarization channels. Most VLBI stations record their signals in a circular basis, but the ALMA receivers record in a linear basis. The strategy that will be followed in the phased-ALMA VLBI observat…
We report the development of a semi-automatic pipeline for the calibration of 86 GHz full-polarization observations performed with the Global Millimeter-VLBI array (GMVA) and describe the calibration strategy followed in the data reduction. Our calibration pipeline involves non-standard procedures, since VLBI polarimetry at frequencies above 43 GHz is not yet well established. We also present, for the first time, a full-polarization global-VLBI image at 86 GHz (source 3C 345), as an example of the final product of our calibration pipeline, and discuss the effect of instrumental limitations on …
Currently, HOPS and AIPS are the primary choices for the time-consuming process of (millimeter) Very Long Baseline Interferometry (VLBI) data calibration. However, for a full end-to-end pipeline, they either lack the ability to perform easily scriptable incremental calibration or do not provide full control over the workflow with the ability to manipulate and edit calibration solutions directly. The Common Astronomy Software Application (CASA) offers all these abilities, together with a secure development future and an intuitive Python interface, which is very attractive for young radio astron…
In this document we layout a new method to achieve "bona fide" high precision Very-Long-Baseline-Interferometry (VLBI) astrometric measurements of frequency-dependent positions of celestial sources (even) in the high (mm-wavelength) frequency range, where conventional phase referencing techniques fail. Our method, dubbed "Source/Frequency Phase Referencing" (SFPR) combines fast frequency-switching (or dual-frequency observations) with the source switching of conventional phase referencing techniques. The former is used to calibrate the dominant highly unpredictable rapid atmospheric fluctuatio…
We present the results of comparative study of amplitude calibrations for East-Asia VLBI Network (EAVN) at 22 and 43 GHz using two different methods of an "a-priori" and a "template spectrum", particularly on lower declination sources. Using observational data sets of early EAVN observations, we investigated the elevation-dependence of the gain values at seven stations of the KaVA (KVN and VERA Array) and three additional telescopes in Japan (Takahagi 32m, Yamaguchi 32m and Nobeyama 45m). By comparing the independently obtained gain values based on these two methods, we found that the gain val…
Context. Very Long Baseline Interferometry (VLBI) data are extremely sensitive to the phase stability of the VLBI array. This is especially important when we reach μJy r.m.s. sensitivities. Calibration using standard phase referencing techniques is often used to improve the phase stability of VLBI data but the results are often not optimal. This is evident in blank fields that do not have in-beam calibrators. Aims. We present a calibration algorithm termed Multi-Source Self-Calibration (MSSC) which can be used after standard phase referencing on wide-field VLBI observations. This is tested on …
This paper considers the problem of accurate measurement of circular polarization in imaging spectral-line VLBI observations in the lambda=7 mm and lambda=3 mm wavelength bands. This capability is especially valuable for the full observational study of compact, polarized SiO maser components in the near-circumstellar environment of late-type, evolved stars. Circular VLBI polarimetry provides important constraints on SiO maser astrophysics, including the theory of polarized maser emission transport, and on the strength and distribution of the stellar magnetic field and its dynamical role in thi…
We present the Generalized Polarization CALibration pipeline (GPCAL), an automated pipeline for instrumental polarization calibration of very long baseline interferometry (VLBI) data. The pipeline is designed to achieve a high calibration accuracy by means of fitting the instrumental polarization model, including the second-order terms, to multiple calibrators data simultaneously. It also allows using more accurate linear polarization models of calibrators for D-term estimation compared to the conventional way that assumes similar linear polarization and total intensity structures. This assump…
(Abridged) The CASA software suite, can now reduce very long baseline interferometry (VLBI) data with the recent addition of a fringe fitter. 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) im…
Sagittarius A* is the source of near infrared, X-ray, radio, and (sub)millimeter emission associated with the supermassive black hole at the Galactic Center. In the submillimeter regime, Sgr A* exhibits time-variable linear polarization on timescales corresponding to <10 Schwarzschild radii of the presumed 4 million solar mass black hole. In previous work, we demonstrated the potential for total-intensity (sub)millimeter-wavelength VLBI to detect time-variable -- and periodic -- source structure changes in the Sgr A* black hole system using nonimaging analyses. Here we extend this work to incl…
We present the design and implementation of an automated data calibration and reduction pipeline for very-long-baseline interferometric (VLBI) observations taken at millimeter wavelengths. These short radio-wavelengths provide the best imaging resolution available from ground-based VLBI networks such as the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA), but require specialized processing due to the strong effects from atmospheric opacity and turbulence as well as the heterogeneous nature of existing global arrays. The pipeline builds upon a calibration suite (HOPS) …
Optical frequency comb integration in radio telescopes: advancing signal generation and phase calibration
Korea Advanced Institute of Science and Technology (KAIST)（韩国科学技术院）；Korea Astronomy and Space Science Institute (KASI)（韩国天文学与空间科学研究所）；University of Science and Technology (UST)（科学技术联合大学院大学）；Max-Planck Institute for Radio Astronomy（马克斯·普朗克射电天文研究所）；Korea Research Institute of Standards and Science (KRISS)（韩国标准与科学研究院）
Atmospheric propagation effects at millimeter wavelengths can significantly alter the phases of radio signals and reduce the coherence time, putting tight constraints on high frequency Very Long Baseline Interferometry (VLBI) observations. In previous works, it has been shown that non-dispersive (e.g. tropospheric) effects can be calibrated with the frequency phase transfer (FPT) technique. The coherence time can thus be significantly extended. Ionospheric effects, which can still be significant, remain however uncalibrated after FPT as well as the instrumental effects. In this work, we implem…
AVICA: A fully automated CASA pipeline for large volume VLBI data calibration
Institute of Astrophysics, Foundation for Research and Technology – Hellas（希腊研究基金会天体物理研究所）；Department of Physics, University of Crete（克里特大学物理系）；Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University（拉德堡德大学数学、天体物理和粒子物理研究所天体物理学系）
The arrival of the Square Kilometer Array (SKA) will revitalise all aspects of Very Long Baseline Interferometry (VLBI) astronomy at the lower frequencies. In the last decade there have been huge strides towards routinely achieving high precision VLBI astrometry at frequencies dominated by the tropospheric contributions, most notably at 22GHz, using advanced phase referencing techniques. Nevertheless to increase the capability for high precision astrometric measurements at low radio frequencies (<8GHz) an effective calibration strategy of the systematic ionospheric propagation effects that is …
Heterodyne receivers register the sky signal on either a circular polarization basis (where it is split into left-hand and right-hand circular polarization) or a linear polarization basis (where it is split into horizontal and vertical linear polarization). We study the problem of interferometric observations performed with telescopes that observe on different polarization bases, hence producing visibilities that we call "mixed basis" (i.e., linear in one telescope and circular in the other). We present novel algorithms for the proper calibration and treatment of such interferometric observati…
We report on recent technical developments in the front- and back-ends for the four 20 m radio telescopes of the Japanese Very-Long-Baseline Interferometry (VLBI) project, VLBI Exploration of Radio Astrometry (VERA). We present a brief overview of a dual-circular polarization receiving and ultrawideband (16 Giga bit s$^{-1}$) recording systems that were installed on each of the four telescopes operating at 22 and 43 GHz bands. The wider-band capability improves the sensitivity of VLBI observations for continuum emission, and the dual-polarization capability enables the study of magnetic fields…
This document describes the advantages of applying "Source/Frequency Phase Referencing" (SFPR) techniques to the analysis of VLBI observations with VSOP-2, for high precision astrometric measurements and/or increased sensitivity. The SFPR calibration technique basics and a demonstration of the method applied to highest frequency VLBA observations are described in detail in VLBA Scientific Memo n. 31. Here we outline its importance in the context of space VLBI astrometry with VSOP-2, where errors in the satellite orbit determination and rapid tropospheric phase fluctuations set extreme challeng…
Context: Phase referencing is a standard calibration technique in radio interferometry, particularly suited for the detection of weak sources close to the sensitivity limits of the interferometers. However, effects from a changing atmosphere and inaccuracies in the correlator model may affect the phase-referenced images, leading to wrong estimates of source flux densities and positions. A systematic observational study of signal decoherence in phase referencing, and its effects in the image plane, has not been performed yet. Aims: We systematically studied how the signal coherence in Very-Long…
Polarization of radio emission in extragalactic jets at a sub-milliarcsecond angular resolution holds important clues for understanding the structure of the magnetic field in the inner regions of the jets and in close vicinity of the supermassive black holes in the centers of active galaxies. Space VLBI observations provide a unique tool for polarimetric imaging at a sub-milliarcsecond angular resolution and studying the properties of magnetic field in active galactic nuclei on scales of less than 10^4 gravitational radii. A space VLBI observation of high-redshift quasar TXS 0642+449 (OH 471),…
Astrometry at centimeter wavelengths using Very Long Baseline Interferometry is approaching accuracies of ~1 uas for the angle between a target and a calibrator source separated by <1 degree on the sky. The BeSSeL Survey and the Japanese VERA project are using this to map the spiral structure of the Milky Way by measuring trigonometric parallaxes of hundreds of maser sources associated with massive, young stars. This paper outlines how micro-arcsecond astrometry is done, including details regarding the scheduling of observations, calibration of data, and measuring positions.
The daytime sky has been recently demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K Inouye Solar Telescope (DKIST) and other large telescopes under construction can benefit from precise polarimetric calibration of large mirrors. Several atmospheric phenomena and instrumental errors potentially limit the techniques accuracy. At the 3.67m AEOS telescope on Haleakala, we have performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting cons…
This paper presents a polarization calibration method applied to a microwave polarimeter demonstrator based on a near-infrared (NIR) frequency up-conversion stage that allows both optical correlation and signal detection at a wavelength of 1550 nm. The instrument was designed to measure the polarization of cosmic microwave background (CMB) radiation from the sky, obtaining the Stokes parameters of the incoming signal simultaneously, in a frequency range from 10 to 20 GHz. A linearly polarized input signal with a variable polarization angle is used as excitation in the polarimeter calibration s…
Bayesian polarization calibration and imaging in very long baseline interferometry
Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；Max Planck Computing and Data Facility（马克斯·普朗克计算与数据中心）；Max-Planck-Institut für Astrophysik（马克斯·普朗克天体物理研究所）；Technische Universität München (TUM)（慕尼黑工业大学）；School of Space Research, Kyung Hee University（庆熙大学宇宙研究学院）；Ludwig-Maximilians-Universität（路德维希-马克西米利安大学）；Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University（拉布德大学数学、天体物理学和粒子物理研究所天体物理学系）；Institut für Experimentalphysik（实验物理研究所）
Verification of the Polarimetric Capability of the East Asia VLBI Network
School of Space Research, Kyung Hee University（庆熙大学宇宙研究学院）；G-LAMP NEXUS Institute, Kyung Hee University（庆熙大学G-LAMP NEXUS研究所）；Institute of Astronomy and Astrophysics, Academia Sinica（中央研究院天文及天文物理研究所）；Korea Astronomy and Space Science Institute（韩国天文学与空间科学研究所）；University of Science and Technology（科学技术联合大学院大学）；Graduate School of Science, Nagoya City University（名古屋市立大学理学研究科）；Mizusawa VLBI Observatory, National Astronomical Observatory of Japan（日本国立天文台水泽VLBI观测站）
We propose a probabilistic framework for performing simultaneous estimation of source structure and fringe-fitting parameters in Very Long Baseline Interferometry (VLBI) observations. As a first step, we demonstrate this technique through the analysis of synthetic short-duration Event Horizon Telescope (EHT) observations of various geometric source models at 230 GHz, in the presence of baseline-dependent thermal noise. We perform Bayesian parameter estimation and model selection between the different source models to obtain reliable uncertainty estimates and correlations between various source…
Very Long Baseline Interferometry (VLBI) astrometry is a well established technique for achieving $\pm10~μ$as parallax accuracies at frequencies well above 10~GHz. At lower frequencies, uncompensated interferometer delays associated with the ionosphere play the dominant role in limiting the astrometric accuracy. Multiview is a novel VLBI calibration method, which uses observations of multiple quasars to accurately model and remove time-variable, directional-dependent changes to the interferometer delay. Here we extend the Multiview technique by phase referencing data to the target source ("inv…
We report astrometric results of VLBI phase-referencing observations of 22 GHz \hho\ masers emission toward the red hypergiant \vxsgr, one of most massive and luminous red hypergiant stars in our Galaxy, using the Very Long Baseline Array. A background source, \Jtwoze, projected 4\d4 from the target \vxsgr, was used as the phase reference. For the low declinations of these sources, such a large separation normally would seriously degrade the relative astrometry. We use a two-step method of tropospheric delay calibration, which combines the VLBI geodetic-block (or GPS) calibration with an image…
Aims: To propose a method for the polarimetric calibration of large astronomical mirrors that does not require use of special optical devices nor knowledge of the exact polarization properties of the calibration target. Methods: We study the symmetries of the Mueller matrix of mirrors to exploit them for polarimetric calibration under the assumptions that only the orientation of the linear polarization plane of the calibration target is known with certainty. Results: A method is proposed to calibrate the polarization effects of single astronomical mirrors by the observation of calibration targ…
New generation of radio interferometers are envisaged to produce high quality, high dynamic range Stokes images of the observed sky from the corresponding under-sampled Fourier domain measurements. In practice, these measurements are contaminated by the instrumental and atmospheric effects that are well represented by Jones matrices, and are most often varying with observation direction and time. These effects, usually unknown, act as a limiting factor in achieving the required imaging performance and thus, their calibration is crucial. To address this issue, we develop a global algorithm, nam…
There is growing evidence of relativistic jets in radio-loud narrow-line Seyfert 1 (RL-NLS1) galaxies. We constrain the observational properties of the radio emission in the first RL-NLS1 galaxy ever detected in gamma-rays, PMN J0948+0022, i.e., its flux density and structure in total intensity and in polarization, its compactness, and variability. We performed three real-time e-VLBI observations of PMN J0948+0022 at 22 GHz, using a global array including telescopes in Europe, East Asia, and Australia. These are the first e-VLBI science observations ever carried out with a global array, reachi…
We present MeqSilhouette v2.0 (MeqSv2), a fully polarimetric, time-and frequency-resolved synthetic data generation software for simulating millimetre (mm) wavelength very long baseline interferometry (VLBI) observations with heterogeneous arrays. Synthetic data are a critical component in understanding real observations, testing calibration and imaging algorithms, and predicting performance metrics of existing or proposed sites. MeqSv2 applies physics-based instrumental and atmospheric signal corruptions constrained by empirically-derived site and station parameters to the data. The new versi…
We present a novel system to calibrate millimeter-wave polarimeters for CMB polarization measurements. This technique is an extension of the conventional metal mirror rotation approach, however it employs cryogenically-cooled blackbody absorbers. The primary advantage of this system is that it can generate a slightly polarized signal ($\sim100$ mK) in the laboratory; this is at a similar level to that measured by ground-based CMB polarization experiments observing a $\sim$ 10 K sky. It is important to reproduce the observing condition in the laboratry for reliable characterization of polarimet…
Active Galactic Nuclei (AGN) play a decisive role in galaxy evolution, particularly so when operating in a radiatively inefficient mode, where they launch powerful jets that reshape their surroundings. However, identifying them is difficult, since radio observations commonly have resolutions of between 1 arcsec and 10 arcsec, which is equally sensitive to radio emission from star-forming activity and from AGN. Very Long Baseline Interferometry (VLBI) observations allow one to filter out all but the most compact non-thermal emission from radio survey data. The observational and computational de…
The Simons Observatory: Detector Polarization Angle Calibration using Sparse Wire Grid with Initial Data Sets of the Small Aperture Telescope
Kyoto University（京都大学）；Okayama University（冈山大学）；Princeton University（普林斯顿大学）；University of California San Diego（加州大学圣迭戈分校）；KEK（高能加速器研究机构）；The University of Tokyo（东京大学）；University of Milano-Bicocca（米兰比可卡大学）
We analyse Very Long Baseline Interferometry (VLBI) observations of the blazar CGRaBS J0809+5341 using Bayesian inference methods. The observation was carried out at 5 GHz using 8 telescopes that form part of the European VLBI Network. Imaging and deconvolution using traditional methods imply that the blazar is unresolved. To search for source structure beyond the diffraction limit, we perform Bayesian model selection between three source models (point, elliptical Gaussian, and circular Gaussian). Our modelling jointly accounts for antenna-dependent gains and system equivalent flux densities. …
The DKIST is designed to deliver accurate spectropolarimetric calibrations across a wide wavelength range and large field of view for solar observations. Instruments deliver spectral resolving powers up to 300,000 in multiple cameras of several instrument. We require detailed knowledge of optical coatings on all optics to ensure we can predict and calibrate the polarization behavior of the system. Optical coatings can be metals protected by many dielectric layers or several-micron thick dichroics. Strong spectral gradients up to 60deg retardance per nanometer wavelength and several percent dia…
In this paper we describe a new approach for mm-VLBI calibration that provides bona-fide astrometric alignment of the mm-wavelength images from a single source, for the measurement of frequency dependent effects, such as `core-shifts' near the black hole of AGN jets. We achieve our astrometric alignment by solving firstly for the ionospheric (dispersive) contributions using wide-band cm-wavelength observations. Secondly we solve for the tropospheric (non-dispersive) contributions by using fast frequency-switching at the target mm-wavelengths. These solutions can be scaled and transferred from …
Modern observatories and instruments require optics fabricated at larger sizes with more stringent performance requirements. The Daniel K. Inouye Solar Telescope will be the world's largest solar telescope at 4.0 m aperture delivering a 300 Watt beam and a 5 arc-minute field. Spatial variation of retardance is a limitation to calibration of the full field. Three polarimeters operate seven cameras simultaneously in narrow bandpasses from 380 nm to 1800 nm. The DKIST polarization calibration optics must be 120 mm in diameter at Gregorian focus to pass the beam and operate under high heat load, U…
We present the design and characterization of a ground-based absolute polarization angle calibrator accurate to better than 0.1 degrees for use with polarization sensitive cosmic microwave background (CMB) experiments. The calibrator's accuracy requirement is driven by the need to reduce upper limits on cosmic polarization rotation, which is expected to be zero in a large class of cosmological models. Cosmic polarization effects such as cosmic birefringence and primordial magnetic fields can generate spurious B-modes that result in non-zero CMB TB and EB correlations that are degenerate with a…
A Formalism for Calibrating the Instrumental Polarization of Radio Interferometric Arrays at Meter Wavelengths using Unpolarized Sky: A Demonstration using the MWA Observations
Radio interferometers can measure the full polarization state of incoming waves by cross--correlating all combinations of two orthogonal polarizations at each antenna. The independent sets of electronics used to detect the two polarization states will introduce a differential instrumental delay between the two data streams. The usual technique of separate calibration of the parallel--hand sets of visibilities still allows for an arbitrary offset in group delay and phase between the two parallel systems. In order to use the cross--polarized visibilities, this instrumental offset must be determi…
QUIJOTE-TFGI polarization calibration -- Ground characterization and on-sky validation with Tau A and the Moon
Instituto de Astrofísica de Canarias（加那利天体物理学研究所）；Universidad de La Laguna (ULL)（拉古纳大学）；Instituto de Física de Cantabria (IFCA)（坎塔布里亚物理研究所）；Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo（东京大学前沿研究科 Kavli宇宙物理数学研究所）
We report about a 230 GHz very long baseline interferometry (VLBI) fringe finder observation of blazar 3C 279 with the APEX telescope in Chile, the phased submillimeter array (SMA), and the SMT of the Arizona Radio Observatory (ARO). We installed VLBI equipment and measured the APEX station position to 1 cm accuracy (1 sigma). We then observed 3C 279 on 2012 May 7 in a 5 hour 230 GHz VLBI track with baseline lengths of 2800 M$λ$ to 7200 M$λ$ and a finest fringe spacing of 28.6 micro-arcseconds. Fringes were detected on all baselines with SNRs of 12 to 55 in 420 s. The correlated flux density o…
Chromospheric magnetic fields are of paramount importance in understanding the dynamics of energetic events in the solar atmosphere. At the Kodaikanal Solar Observatory, several polarimeters were developed in the past to study the active region magnetic fields. A new polarimeter has been developed and installed at Kodaikanal Tower-tunnel Telescope to study the active regions at Chromospheric level, in Ca II $8542~Å$ spectral line. Design aspects of the instrument and polarimetry strategy are discussed. Telescope instrumental polarization has been revisited and possible ways to reduce it have b…
