arXiv subjects

Mark Kettenis

Publications and source records attributed to Mark Kettenis.

9 recordsLinked to original sources

arXiv2023

Reference Array and Design Consideration for the next-generation Event Horizon Telescope

We describe the process to design, architect, and implement a transformative enhancement of the Event Horizon Telescope (ngEHT). This program - the next-generation Event Horizon Telescope (ngEHT) - will form a networked global array of radio dishes capable of making high-fidelity real-time movies of supermassive black holes (SMBH) and their emanating jets. This builds upon the EHT principally by deploying additional modest-diameter dishes to optimized geographic locations to enhance the current global mm/submm wavelength Very Long Baseline Interferometric (VLBI) array, which has, to date, utilized mostly pre-existing radio telescopes. The ngEHT program further focuses on observing at three frequencies simultaneously for increased sensitivity and Fourier spatial frequency coverage. Here, the concept, science goals, design considerations, station siting and instrument prototyping are discussed, and a preliminary reference array to be implemented in phases is described.

astro-ph.IM↗

arXiv2022

CASA on the fringe -- Development of VLBI processing capabilities for CASA

New functionality to process Very Long Baseline Interferometry (VLBI) data has been implemented in the CASA package. This includes two new tasks to handle fringe fitting and VLBI-specific amplitude calibration steps. Existing tasks have been adjusted to handle VLBI visibility data and calibration meta-data properly. With these updates, it is now possible to process VLBI continuum and spectral line observations in CASA. This article describes the development and implementation, and presents an outline for the workflow when calibrating European VLBI Network or Very Long Baseline Array data in CASA. Though the CASA VLBI functionality has already been vetted extensively as part of the Event Horizon Telescope data processing, in this paper we compare results for the same dataset processed in CASA and AIPS. We find identical results for the two packages and conclude that CASA in some cases performs better, though it cannot match AIPS for single-core processing time. The new functionality in CASA allows for easy development of pipelines or Jupyter notebooks, and thus contributes to raising VLBI data processing to present day standards for accessibility, reproducibility, and reusability.

astro-ph.IM↗

arXiv2022

CASA, the Common Astronomy Software Applications for Radio Astronomy

CASA, the Common Astronomy Software Applications, is the primary data processing software for the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), and is frequently used also for other radio telescopes. The CASA software can handle data from single-dish, aperture-synthesis, and Very Long Baseline Interferometery (VLBI) telescopes. One of its core functionalities is to support the calibration and imaging pipelines for ALMA, VLA, VLA Sky Survey (VLASS), and the Nobeyama 45m telescope. This paper presents a high-level overview of the basic structure of the CASA software, as well as procedures for calibrating and imaging astronomical radio data in CASA. CASA is being developed by an international consortium of scientists and software engineers based at the National Radio Astronomical Observatory (NRAO), the European Southern Observatory (ESO), the National Astronomical Observatory of Japan (NAOJ), and the Joint Institute for VLBI European Research Infrastructure Consortium (JIV-ERIC), under the guidance of NRAO.

astro-ph.IM↗

arXiv2020

Gravitational Test Beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.

gr-qc↗

arXiv2020

A probabilistic approach to phase calibration: I. Effects of source structure on fringe-fitting

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 and fringe-fitting related model parameters. We also compare the Bayesian posteriors with those obtained using widely-used VLBI data reduction packages such as CASA and AIPS, by fringe-fitting 200 Monte Carlo simulations of each source model with different noise realisations, to obtain distributions of the Maximum A Posteriori (MAP) estimates. We find that, in the presence of resolved asymmetric source structure and a given array geometry, the traditional practice of fringe-fitting with a point source model yields appreciable offsets in the estimated phase residuals, potentially biasing or limiting the dynamic range of the starting model used for self-calibration. Simultaneously estimating the source structure earlier in the calibration process with formal uncertainties improves the precision and accuracy of fringe-fitting and establishes the potential of the available data especially when there is little prior information. We also note the potential applications of this method to astrometry and geodesy for specific science cases and the planned improvements to the computational performance and analyses of more complex source distributions.

astro-ph.IM↗

arXiv2019

rPICARD: A CASA-based Calibration Pipeline for VLBI Data. Calibration and imaging of 7 mm VLBA observations of the AGN jet in M87

(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) implementation ensures that large bandwidth data from future arrays can be processed within reasonable computing times. 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. In this work we demonstrate the capabilities of rPICARD by calibrating and imaging 7 mm VLBA data of the central radio source in the M87 galaxy. The reconstructed jet image reveals a complex collimation profile and edge-brightened structure. 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.

astro-ph.IM↗

arXiv2019

CASA on the fringe: VLBI data processing in the CASA software package

In recent years new functionality for VLBI data processing has been added to the CASA package. This paper presents the new CASA tasks 'fringefit' and 'accor', which are closely matched to their AIPS counterparts FRING and ACCOR. Several CASA tasks received upgrades to handle VLBI specific metadata. With the current CASA release VLBI data processing is possible, and functionality will be expanded in the upcoming release. Longer term developments include fringe fitting of broad, non-continuous frequency bands and dispersive delays, which will ensure that the number of use cases for VLBI calibration will increase in future CASA releases.

astro-ph.IM↗

arXiv2018

Minimal Re-computation for Exploratory Data Analysis in Astronomy

We present a technique to automatically minimise the re-computation when a data analysis program is iteratively changed, or added to, as is often the case in exploratory data analysis in astronomy. A typical example is flagging and calibration of demanding or unusual observations where visual inspection suggests improvement to the processing strategy. The technique is based on memoization and referentially transparent tasks. We describe the implementation of this technique for the CASA radio astronomy data reduction package. We also propose a technique for optimising efficiency of storage of memoized intermediate data products using copy-on-write and block level de-duplication and measure their practical efficiency. We find the minimal recomputation technique improves the efficiency of data analysis while reducing the possibility for user error and improving the reproducibility of the final result. It also aids exploratory data analysis on batch-schedule cluster computer systems.

astro-ph.IM↗

arXiv2017

Minimal Re-computation for Exploratory Data Analysis in Astronomy

We present a technique to automatically minimise the re-computation when a data processing program is iteratively changed, or added to, as is often the case in exploratory data analysis in radio astronomy. A typical example is flagging and calibration of demanding or unusual observations where visual inspection suggests improvement to the processing strategy. The technique is based on memoization and referentially transparent tasks. We describe a prototype implementation for the CASA data reduction package. This technique improves the efficiency of data analysis while reducing the possibility for user error and improving the reproducibility of the final result.

astro-ph.IM↗