Common Astronomy Software Applications Research Articles

An optimized transient detection pipeline for the ASKAP Variables and Slow Transients (VAST) survey

ABSTRACT In this paper, we present an optimized version of the detection pipeline for the ASKAP Variables and Slow Transients (VAST) survey, offering significant performance improvement. The key to this optimization is the replacement of the original w-projection algorithm integrated in the Common Astronomy Software Applications package with the w-stacking algorithm implemented in the WSClean software. Our experiments demonstrate that this optimization improves the overall processing efficiency of the pipeline by approximately a factor of 3. Moreover, the residual images generated by the optimized pipeline exhibit lower noise levels and fewer artefact sources, suggesting that our optimized pipeline not only enhances detection accuracy but also improves imaging fidelity. This optimized VAST detection pipeline is integrated into the Data Activated Liu Graph Engine (DALiuGE) execution framework, specifically designed for SKA-scale big data processing. Experimental results show that the performance and scalability advantages of the pipeline using DALiuGE over traditional MPI or BASH techniques increase with the data size. In summary, the optimized transient detection pipeline significantly reduces runtime, increases operational efficiency, and decreases implementation costs, offering a practical optimization solution for other ASKAP imaging pipelines as well.

The ALMA Interferometric Pipeline Heuristics

We describe the calibration and imaging heuristics developed and deployed in the Atacama Large Millimeter/submillimeter Array (ALMA) interferometric data processing pipeline, as of ALMA Cycle 9 operations. The pipeline software framework is written in Python, with each data reduction stage layered on top of tasks and toolkit functions provided by the Common Astronomy Software Applications package. This framework supports a variety of tasks for observatory operations, including science data quality assurance, observing mode commissioning, and user reprocessing. It supports ALMA and Very Large Array interferometric data along with ALMA and NRO 45 m single dish data, via different stages and heuristics. In addition to producing calibration tables, calibrated measurement sets, and cleaned images, the pipeline creates a WebLog which serves as the primary interface for verifying the quality assurance of the data by the observatory and for examining the contents of the data by the user. Following the adoption of the pipeline by ALMA Operations in 2014, the heuristics have been refined through annual prioritized development cycles, culminating in a new pipeline release aligned with the start of each ALMA Cycle of observations. Initial development focused on basic calibration and flagging heuristics (Cycles 2–3), followed by imaging heuristics (Cycles 4–5). Further refinement of the flagging and imaging heuristics, including the introduction of parallel processing, proceeded for Cycles 6–7. In the 2020 release, the algorithm to identify channels to use for continuum subtraction and imaging was substantially improved by the addition of a moment difference analysis. A spectral renormalization stage was added for the 2021 release (Cycle 8) to correct high spectral resolution visibility data acquired on targets exhibiting strong celestial line emission in their autocorrelation spectra. The calibration heuristics used in the low signal-to-noise regime were improved for the 2022 release (Cycle 9). In the two most recent Cycles, 97% of ALMA data sets were calibrated and imaged with the pipeline, ensuring long-term automated reproducibility of results. We conclude with a brief description of plans for future additions, including a self-calibration stage, support for multi-configuration imaging, and complete calibration and imaging of full polarization data.

Challenging interferometric imaging: Machine learning-based source localization from uv-plane observations

Context. Rising interest in radio astronomy and upcoming projects in the field is expected to produce petabytes of data per day, questioning the applicability of traditional radio astronomy data analysis approaches under the new large-scale conditions. This requires new, intelligent, fast, and efficient methods that potentially involve less input from the domain expert. Aims. In our work, we examine, for the first time, the possibility of fast and efficient source localization directly from the uv-observations, omitting the recovering of the dirty or clean images. Methods. We propose a deep neural network-based framework that takes as its input a low-dimensional vector of sampled uv-data and outputs source positions on the sky. We investigated a representation of the complex-valued input uv-data via the real and imaginary and the magnitude and phase components. We provided a comparison of the efficiency of the proposed framework with the traditional source localization pipeline based on the state-of-the-art Python Blob Detection and Source Finder (PyBDSF) method. The investigation was performed on a data set of 9164 sky models simulated using the Common Astronomy Software Applications (CASA) tool for the Atacama Large Millimeter Array (ALMA) Cycle 5.3 antenna configuration. Results. We investigated two scenarios: (i) noise-free as an ideal case and (ii) sky simulations including noise representative of typical extra-galactic millimeter observations. In the noise-free case, the proposed localization framework demonstrates the same high performance as the state-of-the-art PyBDSF method. For noisy data, however, our new method demonstrates significantly better performance, achieving a completeness level that is three times higher for sources with uniform signal-to-noise ratios (S/N) between 1 and 10, and a high increase in completeness in the low S/N regime. Furthermore, the execution time of the proposed framework is significantly reduced (by factors ~30) as compared to traditional methods that include image reconstructions from the uv-plane and subsequent source detections. Conclusions. The proposed framework for obtaining fast and efficient source localization directly from uv-plane observations shows very encouraging results, which could open new horizons for interferometric imaging with existing and future facilities.

Resolving the Bow Shock and Tail of the Cannonball Pulsar PSR J0002+6216

We present X-ray and radio observations of the recently discovered bow-shock pulsar wind nebula (PWN) associated with PSR J0002+6216, characterizing the PWN morphology, which was unresolved in previous studies. The multifrequency, multiepoch Very Large Array (VLA) radio observations reveal a cometary tail trailing the pulsar and extending up to 5.′3, with multiple kinks along the emission. The presented radio continuum images from multiconfiguration broadband VLA observations are one of the first results from the application of multiterm multifrequency synthesis deconvolution in combination with the AWProject gridder implemented in the Common Astronomy Software Applications (CASA) package. The X-ray emission observed with Chandra extends to only 21″, fades quickly, and has some hot spots present along the extended radio emission. These kinks could indicate the presence of density variations in the local interstellar medium or turbulence. The bow-shock standoff distance estimates a small bow-shock region with a size of 0.003–0.009 pc, consistent with the pulsar spin-down power of = 1.51 × 1035 erg s−1 estimated from timing. The high-resolution radio image reveals the presence of an asymmetry in the bow-shock region, which is also present in the X-ray image. The broadband radio image shows an unusually steep spectrum along with a flat-spectrum sheath, which could indicate varying opacity or energy injection into the region. Spatially resolved X-ray spectra provide marginal evidence of synchrotron cooling along the extended tail. Our analysis of the X-ray data also shows that this pulsar has a low spin-down power and one of the lowest X-ray efficiencies observed in these objects.

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, and the Nobeyama 45 m 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 Astronomy Observatory (NRAO), the European Southern Observatory, the National Astronomical Observatory of Japan, and the Joint Institute for VLBI European Research Infrastructure Consortium (JIV-ERIC), under the guidance of NRAO.

Broad band flux-density monitoring of radio sources with the Onsala twin telescopes

The Onsala twin telescopes (OTT) are two 13 m telescopes located at the Onsala Space Observatory in Sweden. With dual linear polarized broad-band (3-14 GHz) receivers, they are part of the next generation Very Long Baseline Interferometry (VLBI) Global Observing System (VGOS) for geodesy and astrometry. In addition to purely geodetic data products, VGOS will regularly produce full-polarisation images of hundreds of radio sources. These rich monitoring data will be valuable for both astronomy and geodesy. In this pilot study we aimed at monitoring ten bright radio sources to search for flares or similar activity, and at testing the instrument calibration on long (months) and short (hours) time scales. Using the OTT as a standalone instrument, we observed and analysed 91 short (<30 min) sessions spanning seven months. We monitored seven potentially variable radio sources (0059+581, 0552+398, 1144+402, 1156+295, 1617+229, 3C418, OJ287) and three reference calibrators (3C147, 3C286, 3C295). We used the Common Astronomy Software Applications (CASA) package to fringe-fit, bandpass-correct and scale the data to obtain flux densities in the four standard VGOS bands: 3.0-3.5 GHz (band 1), 5.2-5.7 GHz (band 2), 6.3-6.8 GHz (band 3), and 10.2-10.7 GHz (band 4). We obtained simultaneous multi-frequency light curves for ten radio sources. A bright multi-frequency flare is observed in the radio source 0059+581. OJ287 and 1156+295 show significant long-term variability in their light curves. After correcting for instrumental biases, we determine the empirical flux density uncertainty as sim5 %.

Offline Correction of Atmospheric Effects on Single-dish Radio Spectroscopy

We present a method to mitigate the atmospheric effects (residual atmospheric lines) in single-dish radio spectroscopy caused by the elevation difference between the target and reference positions. The method is developed as a script using the Atmospheric Transmission at Microwaves (ATM) library built into the Common Astronomy Software Applications (CASA) package. We apply the method to the data taken with the Total Power Array of the Atacama Large Millimeter/submillimeter Array. The intensities of the residual atmospheric (mostly O3) lines are suppressed by, typically, an order of magnitude for the tested cases. The parameters for the ATM model can be optimized to minimize the residual line and, for a specific O3 line at 231.28 GHz, a seasonal dependence of a best-fitting model parameter is demonstrated. The method will be provided as a task within the CASA package in the near future. The atmospheric removal method we developed can be used by any radio/millimeter/submillimeter observatory to improve the quality of its spectroscopic measurements.

CAPTURE: a continuum imaging pipeline for the uGMRT

We present the first fully automated pipeline for making images from the interferometric data obtained from the upgraded Giant Metrewave Radio Telescope (uGMRT) called CAsa Pipeline-cum-Toolkit for Upgraded Giant Metrewave Radio Telescope data REduction - CAPTURE. It is a python program that uses tasks from the NRAO Common Astronomy Software Applications (CASA) to perform the steps of flagging of bad data, calibration, imaging and self-calibration. The salient features of the pipeline are: i) a fully automatic mode to go from the raw data to a self-calibrated continuum image, ii) specialized flagging strategies for short and long baselines that ensure minimal loss of extended structure, iii) flagging of persistent narrow band radio frequency interference (RFI), iv) flexibility for the user to configure the pipeline for step-by-step analysis or special cases and v) analysis of data from the legacy GMRT. CAPTURE is available publicly on github (https://github.com/ruta-k/uGMRT-pipeline, release v1.0.0). The primary beam correction for the uGMRT images produced with CAPTURE is made separately available at https://github.com/ruta-k/uGMRTprimarybeam. We show examples of using CAPTURE on uGMRT and legacy GMRT data. In principle, CAPTURE can be tailored for use with radio interferometric data from other telescopes.

Simulating Imaging of Large Scale Radio Arrays on the Lunar Surface

In recent years there has been a renewed interest in returning to the Moon for reasons both scientific and exploratory in nature. The Moon provides the perfect training ground for building large scale bases that one may apply to other planets like Mars. The existence of a radio quiet zone on the lunar far side has promise for early universe studies and exoplanet searches, while the near side provides a stable base that may be used to observe low frequency emissions from Earth’s magnetosphere that may help gauge its response to incoming space weather. The construction of a large-scale radio array would provide large scientific returns as well as acting as a test of humanity’s ability to build structures on other planets. This work focuses on simulating the response of small to large-scale radio arrays on the Moon consisting of hundreds or thousands of antennas. The response of the array is dependent on the structure of the emission along with the configuration and sensitivity of the array. A set of locations are selected for the simulated radio receivers, using Digital Elevation Models from the Lunar Orbiter Laser Altimeter instrument on Lunar Reconnaissance Orbiter to characterize the elevation of the receiver locations. A custom Common Astronomy Software Applications code is described and used to process the data from the simulated receivers, aligning the lunar and sky coordinate frames using SPICE to ensure the proper projections are used for imaging. This simulation framework is useful for iterating array design for imaging any given scientific target in a small field of view. This framework does not currently support all sky imaging.

Simulating Imaging of Large Scale Radio Arrays on the Lunar Surface.

In recent years there has been a renewed interest in returning to the Moon for reasons both scientific and exploratory in nature. The Moon provides the perfect training ground for building large scale bases that one may apply to other planets like Mars. The existence of a radio quiet zone on the lunar far side has promise for early universe studies and exoplanet searches, while the near side provides a stable base that may be used to observe low frequency emissions from Earth's magnetosphere that may help gauge its response to incoming space weather. The construction of a large-scale radio array would provide large scientific returns as well as acting as a test of humanity's ability to build structures on other planets. This work focuses on simulating the response of small to large-scale radio arrays on the Moon consisting of hundreds or thousands of antennas. The response of the array is dependent on the structure of the emission along with the configuration and sensitivity of the array. A set of locations are selected for the simulated radio receivers, using Digital Elevation Models from the Lunar Orbiter Laser Altimeter instrument on Lunar Reconnaissance Orbiter to characterize the elevation of the receiver locations. A custom Common Astronomy Software Applications code is described and used to process the data from the simulated receivers, aligning the lunar and sky coordinate frames using SPICE to ensure the proper projections are used for imaging. This simulation framework is useful for iterating array design for imaging any given scientific target in a small field of view. This framework does not currently support all sky imaging.

Measuring the Earth's Synchrotron Emission From Radiation Belts With a Lunar Near Side Radio Array

AbstractThe high kinetic energy electrons that populate the Earth's radiation belts emit synchrotron emissions because of their interaction with the planetary magnetic field. A lunar near side array would be uniquely positioned to image this emission and provide a near real time measure of how the Earth's radiation belts are responding to the current solar input. The Salammbô code is a physical model of the dynamics of the three‐dimensional phase‐space electron densities in the radiation belts, allowing the prediction of 1‐keV to 100‐MeV electron distributions trapped in the belts. This information is put into a synchrotron emission simulator that provides the brightness distribution of the emission up to 1 MHz from a given observation point. Using Digital Elevation Models from Lunar Reconnaissance Orbiter Lunar Orbiter Laser Altimeter data, we select a set of locations near the Lunar sub‐Earth point with minimum elevation variation over various‐sized patches where we simulate radio receivers to create a synthetic aperture. We consider all realistic noise sources in the low‐frequency regime. We then use a custom Common Astronomy Software Applications code to image and process the data from our defined array, using SPICE to align the lunar coordinates with the Earth. We find that for a moderate lunar surface electron density of 250/cm , the radiation belts may be detected every 12–24 hr with a 16,384‐element array over a 100‐km‐diameter circle. Changing electron density can make measurements 10 times faster at lunar night and 10 times slower at lunar noon.

Research on Parallel Algorithms for uv-faceting Imaging

The uv-faceting imaging is one of the widely used large field of view imaging technologies, and will be adopted for the data processing of the low-frequency array in the first stage of the Square Kilometre Array (SKA1). Due to the scale of the raw data of SKA1 is unprecedentedly large, the efficiency of data processing directly using the original uv-faceting imaging will be very low. Therefore, a uv-faceting imaging algorithm based on the MPI (Message Passing Interface)+OpenMP (Open Multi-Processing) and a uv-faceting imaging algorithm based on the MPI+CUDA (Compute Unified Device Architecture) are proposed. The most time-consuming data reading and gridding in the algorithm are optimized in parallel. The verification results show that the results of the proposed two algorithms are basically consistent with that obtained by the current mainstream data processing software CASA (Common Astronomy Software Applications), which indicates that the proposed two algorithms are basically correct. Further analysis of the accuracy and total running time shows that the MPI+CUDA method is better than the MPI+OpenMP method in both the correctness rate and running speed. The performance test results show that the proposed algorithms are effective and have certain extensibility.

Simulations of imaging extended sources using the GMRT and the U-GMRT

Astrophysical sources such as radio halos and relics in galaxy clusters, supernova remnants and radio galaxies have angular sizes from a few to several $10$s of arcminutes. In radio interferometric imaging of such sources, the largest angular size of the source that can be imaged is limited by the shortest projected baseline towards the source. It is essential to determine the limitations of the recovery of the extended features on various angular scales in order to interpret the radio image. We simulated observations of a model extended source of Gaussian shape with the Giant Metrewave Radio Telescope (GMRT) using Common Astronomy Software Applications (CASA). The recovery in flux density and in morphology of the model source was quantified in a variety of observing cases with changing source properties and the uv-coverage. If $\theta_{lar}$ is the largest angular scale sampled in an observation with the GMRT, then $>80\%$ recovery of a source of size $0.3\times\theta_{lar}$ is possible. The upgraded GMRT (U-GMRT) providing 200 MHz instantaneous bandwidth between 300 - 500 MHz will allow a factor of two better recovery of a source of size $\theta_{lar}$ as compared to the GMRT at 300 MHz with 33 MHz bandwidth. We provide quantitative estimates for the improvement in extended source recovery in observations at low elevations and long durations. The presented simulations can be carried out for future radio telescopes such as the Square Kilometre Array (SKA) for optimisation of observing strategies to image extended radio sources.

EXtended CASA Line Analysis Software Suite (XCLASS)

The eXtended CASA Line Analysis Software Suite (XCLASS) is a toolbox for the Common Astronomy Software Applications package (CASA) containing new functions for modeling interferometric and single dish data. Among the tools is the myXCLASS program which calculates synthetic spectra by solving the radiative transfer equation for an isothermal object in one dimension, whereas the finite source size and dust attenuation are considered as well. Molecular data required by the myXCLASS program are taken from an embedded SQLite3 database containing entries from the Cologne Database for Molecular Spectroscopy CDMS) and JPL using the Virtual Atomic and Molecular Data Center (VAMDC) portal. Additionally, the toolbox provides an interface for the model optimizer package Modeling and Analysis Generic Interface for eXternal numerical codes (MAGIX), which helps to find the best description of observational data using myXCLASS (or another external model program), i.e., finding the parameter set that most closely reproduces the data.

A database of phase calibration sources and their radio spectra for the Giant Metrewave Radio Telescope

We are pursuing a project to build a database of phase calibration sources suitable for Giant Metrewave Radio Telescope (GMRT). Here we present the first release of 45 low frequency calibration sources at 235 MHz and 610 MHz. These calibration sources are broadly divided into quasars, radio galaxies and unidentified sources. We provide their flux densities, models for calibration sources, (u,v) plots, final deconvolved restored maps and clean-component lists/files for use in the Astronomical Image Processing System (AIPS) and the Common Astronomy Software Applications (CASA). We also assign a quality factor to each of the calibration sources. These data products are made available online through the GMRT observatory website. In addition we find that (i) these 45 low frequency calibration sources are uniformly distributed in the sky and future efforts to increase the size of the database should populate the sky further, (ii) spectra of these calibration sources are about equally divided between straight, curved and complex shapes, (iii) quasars tend to exhibit flatter radio spectra as compared to the radio galaxies or the unidentified sources, (iv) quasars are also known to be radio variable and hence possibly show complex spectra more frequently, and (v) radio galaxies tend to have steeper spectra, which are possibly due to the large redshifts of distant galaxies causing the shift of spectrum to lower frequencies.

DIRECT DETECTION OF PRECURSORS OF GAS GIANTS FORMED BY GRAVITATIONAL INSTABILITY WITH THE ATACAMA LARGE MILLIMETER/SUBMILLIMETER ARRAY

ABSTRACT Phases of gravitational instability are expected in the early phases of disk evolution, when the disk mass is still a substantial fraction of the mass of the star. Disk fragmentation into sub-stellar objects could occur in the cold exterior part of the disk. Direct detection of massive gaseous clumps on their way to collapse into gas giant planets would offer an unprecedented test of the disk instability model. Here we use state-of-the-art 3D radiation-hydro simulations of disks undergoing fragmentation into massive gas giants, post-processed with RADMC-3D to produce dust continuum emission maps. These are then fed into the Common Astronomy Software Applications (CASA) ALMA simulator. The synthetic maps show that both overdense spiral arms and actual clumps at different stages of collapse can be detected with the Atacama Large Millimeter/submillimeter Array (ALMA) in the full configuration at the distance of the Ophiuchus star forming region (125 pc). The detection of clumps is particularly effective at shorter wavelengths (690 GHz) combining two resolutions with multi-scale clean. Furthermore, we show that a flux-based estimate of the mass of a protoplanetary clump can be comparable to a factor of three higher than the gravitationally bound clump mass. The estimated mass depends on the assumed opacity, and on the gas temperature, which should be set using the input of radiation-hydro simulations. We conclude that ALMA has the capability to detect “smoking gun” systems that are a signpost of the disk instability model for gas giant planet formation.

On the Visibility of Prominence Fine Structures at Radio Millimeter Wavelengths

Prominence temperatures have so far mainly been determined by analyzing spectral line shapes, which is difficult when the spectral lines are optically thick. The radio spectra in the millimeter range offer a unique possibility to measure the kinetic temperature. However, studies in the past used data with insufficient spatial resolution to resolve the prominence fine structures. The aim of this article is to predict the visibility of prominence fine structures in the submillimeter/millimeter (SMM) domain, to estimate their brightness temperatures at various wavelengths, and to demonstrate the feasibility and usefulness of future high-resolution radio observations of solar prominences with ALMA (Atacama Large Millimeter-submillimeter Array). Our novel approach is the conversion of H\(\upalpha\) coronagraphic images into microwave spectral images. We show that the spatial variations of the prominence brightness both in the H\(\upalpha\) line and in the SMM domain predominantly depend on the line-of-sight emission measure of the cool plasma, which we derive from the integrated intensities of the observed H\(\upalpha\) line. This relation also offers a new possibility to determine the SMM optical thickness from simultaneous H\(\upalpha\) observations with high resolution. We also describe how we determine the prominence kinetic temperature from SMM spectral images. Finally, we apply the ALMA image-processing software Common Astronomy Software Applications (CASA) to our simulated images to assess what ALMA would detect at a resolution level that is similar to the coronagraphic H\(\upalpha\) images used in this study. Our results can thus help in preparations of first ALMA prominence observations in the frame of science and technical verification tests.

Trapping dust particles in the outer regions of protoplanetary disks

In order to explain grain growth to mm sized particles and their retention in outer regions of protoplanetary disks, as it is observed at sub-mm and mm wavelengths, we investigate if strong inhomogeneities in the gas density profiles can slow down excessive radial drift and can help dust particles to grow. We use coagulation/fragmentation and disk-structure models, to simulate the evolution of dust in a bumpy surface density profile which we mimic with a sinusoidal disturbance. For different values of the amplitude and length scale of the bumps, we investigate the ability of this model to produce and retain large particles on million years time scales. In addition, we introduced a comparison between the pressure inhomogeneities considered in this work and the pressure profiles that come from magnetorotational instability. Using the Common Astronomy Software Applications ALMA simulator, we study if there are observational signatures of these pressure inhomogeneities that can be seen with ALMA. We present the favorable conditions to trap dust particles and the corresponding calculations predicting the spectral slope in the mm-wavelength range, to compare with current observations. Finally we present simulated images using different antenna configurations of ALMA at different frequencies, to show that the ring structures will be detectable at the distances of the Taurus Auriga or Ophiucus star forming regions.