Kazemi S.,University of Groningen |
Monthly Notices of the Royal Astronomical Society | Year: 2013
A major stage of radio interferometric data processing is calibration or the estimation of systematic errors in the data and the correction for such errors. A stochastic error (noise) model is assumed, and in most cases, this underlying model is assumed to be Gaussian. However, outliers in the data due to interference or due to errors in the sky model would have adverse effects on processing based on a Gaussian noise model. Most of the shortcomings of calibration such as the loss in flux or coherence, and the appearance of spurious sources, could be attributed to the deviations of the underlying noise model. In this paper, we propose to improve the robustness of calibration by using a noise model based on Student's t-distribution. Student's t-noise is a special case of Gaussian noise when the variance is unknown. Unlike Gaussian-noise-model-based calibration, traditional least-squares minimization would not directly extend to a case when we have a Student's t-noise model. Therefore, we use a variant of the expectation-maximization algorithm, called the expectation-conditional maximization either algorithm, when we have a Student's t-noise model and use the Levenberg-Marquardt algorithm in the maximization step. We give simulation results to show the robustness of the proposed calibration method as opposed to traditional Gaussian-noise-model-based calibration, especially in preserving the flux of weaker sources that are not included in the calibration model. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
News Article | November 13, 2016
This study was conducted following a previous discovery of multiple jet-driven winds in IC5063, which are linked to the supermassive black hole in its center (see The jet of a black hole drives multiple winds in a nearby galaxy). About 160 million years ago, charged particles (electrons/protons) that were inflowing toward the black hole were caught in magnetic field lines and ejected outward in the shape of a beam with high velocities. The beam of particles, also known as jet, propagated through the galaxy for more than 3000 light years. It went through a gas disk, driving strong winds at the points where it collided with interstellar clouds. The winds lasted for more than a half-million years, as indicated by ESO Very Large Telescope data. The scientists analyzed the ALMA data aiming to determine whether the gas in the winds has different properties than the gas in the rest of the clouds. For this purpose, they targeted emission lines of CO, originating from molecules in dense interstellar clouds, where the formation of new stars is often taking place, and where the temperature of the gas is typically ~10K. They showed that the molecular gas impacted by the black hole jet is heated, with temperatures often in the range 30K to 100K. The importance of this result lies in the impediments it poses for star formation—the increased thermal and turbulent motions of the gas delay its gravitational collapse. The gravitational collapse is further delayed by the dispersion of the clouds as the impact of the jet removes gas from dense clouds and disperses it into tenuous winds. The mass of the molecular gas in the winds is at least 2 million solar masses. Because of the energy deposited by the jet, the molecular gas is more highly excited in the winds than in the rest of the clouds. This result is encouraging for future studies in the field, as it indicates that the detection of molecular winds will be easier than previously thought for distant galaxies, which can only be observed in high excitation CO lines. Consequently, scientists can evaluate the role of the winds driven by black hole jets in the sizes of the observed galaxies over cosmological scales. This study was published in the peer-reviewed journal Astronomy & Astrophysics on November 1, 2016. Explore further: The jet of a black hole drives multiple winds in a nearby galaxy More information: K. M. Dasyra et al. ALMA reveals optically thin, highly excited CO gasin the jet-driven winds of the galaxy IC 5063, Astronomy & Astrophysics (2016). DOI: 10.1051/0004-6361/201629689 , On Arxiv: https://arxiv.org/abs/1609.03421 The team of the astrophysicists who worked on the study: Drs. K. Dasyra (National and Kapodistrian University of Athens, Greece), F. Combes (College de France, Observatory of Paris, France), T. Oosterloo, R. Morganti (ASTRON and the University of Groningen, The Netherlands), R. Oonk (ASTRON and Leiden University, The Netherlands), P. Salome (Observatory of Paris, France), and N. Vlahakis (National and Kapodistrian University of Athens, Greece).
News Article | December 16, 2016
The Netherlands and South Africa have set up a data science partnership to establish national and regional data centres in order to tackle one of the most significant challenges presented by the SKA telescope: how to manage, process, and make accessible the immense amount of data the telescope will generate. THE HAGUE, 16-Dec-2016 — /EuropaWire/ — The Square Kilometre Array (SKA) project is a global effort to build the world’s largest radio telescope. It will eventually have a collecting area of over a square kilometre, to be built in South Africa and Australia. The SKA will be used to conduct transformational science, addressing questions on how the first stars and galaxies formed just after the big bang, what the nature of dark energy could be and whether we are alone in the universe. The vast collection area of the SKA will produce staggering amounts of data, requiring key technological innovations in a number of areas such as big data and high-performance computing. The Netherlands and South Africa have set up a data science partnership between key institutions from both countries to address the question of how to deal with these large volumes of data. NWO, ASTRON and IBM have signed a Memorandum of Understanding (MoU) with SKA South Africa and the University of Cape Town to collaborate on a ground-breaking research project entitled Precursor Regional Science Data Centres for the SKA (SKA-RSDC). The MoU provides the vehicle for South African and Dutch partners to collaborate on policies, specifications, test-cases, proofs-of-concept and best-practices that will enable researchers to develop models for the efficient processing and analysis of the large quantity of data, based on the SKA pathfinders LOFAR (Low Frequency Array, NL) and MeerKAT (SA). These models will be an essential step in developing the necessary expertise for the SKA Regional Data Centres. The data centres will provide astronomers around the world with access to the large-scale data infrastructures and associated high-performance computing needed to make sense of the data. The techniques developed can, in turn, then be applied in other fields such as big data analytics, high-performance computing, green computing, and visualisation analytics. Watch the video below for more information on the SKA project. This trailer highlights the scale and ambition of the SKA project, the big questions it aims to answer, and the sheer complexity of the entire undertaking. Last week, the trailer won a European Excellence Award, a distinction that honours outstanding achievements in communication and PR.
News Article | January 13, 2016
The plans for a LOFAR station in Ireland have been around for a while, but now it's official: a LOFAR station will be built this year in Ireland. I-LOFAR, the Irish LOFAR consortium, has been awarded €1.4 million by Science Foundation Ireland (SFI). Together with €0.5 million in philanthropic grants plus contributions of I-LOFAR members, it is possible to build and exploit the LOFAR station, which will be constructed on the grounds of Birr Castle, located centrally in Ireland. Today, during a meeting at Birr Castle, Irish Ministers Bruton (Jobs, Enterprise and Innovation) and English (Education and Skills) announced the award for I-LOFAR, as one element of a €30 million investment by SFI in research infrastructures. LOFAR is a world-leading facility for astronomical studies, providing for highly sensitive and detailed scrutiny of the nearby and far-away Universe. LOFAR is designed and operated on behalf of the ILT by ASTRON, the Netherlands institute for Radio Astronomy. Dr. Rene Vermeulen, Director of the ILT, is delighted with the news: "The added Irish antenna station will be an excellent enhancement, extending the ILT to a pan-European fibre-connected network spanning nearly 2000 km. Such long distances allow exquisitely finely detailed sky imaging capability. And, at least as importantly, the Irish astronomical community will now add their expertise and effort to the "ILT family", in the pursuit of a great many cutting-edge science questions that LOFAR can answer. Topics range from the properties of the Earth's upper atmosphere, flaring of the Sun, out to the far reaches of the early Universe when the first stars and galaxies formed." According to Prof. Peter Gallagher, Head of I-LOFAR, "The Irish LOFAR station at Birr builds on Ireland's great scientific heritage of the Leviathan Telescope of Birr and will connect us to the largest low frequency radio telescope in the world. I-LOFAR will also inspire students to study science, engineering and computer science, and attract additional visitors to Birr. It will also act as a magnet to attract technology companies to the area." The International LOFAR Telescope is the largest connected radio telescope in the world. There are currently six partner countries: of the 50 antenna stations, 38 are located in the Netherlands, 6 in Germany, 3 in Poland, and 1 each in France, Sweden, and the United Kingdom. Together, these have many thousands of receiving elements. The new Irish station will increase the distances between antenna stations, thus providing finer image details. Explore further: New radio telescope will listen to the Universe on the FM-band More information: LOFAR brochure available here: dl.dropbox.com/u/3521586/i-lofar_brochure_final.pdf
News Article | March 3, 2016
For years, scientists have tried to understand the nature of brief flashes of radio waves called fast radio bursts (FRBs) that are seemingly sent across the universe from an unknown source. While there have been many theories about the potential origin of FRBs, none of them have established a plausible explanation. This is because researchers have had a hard time knowing where to point their telescopes to observe these radio waves as they happen. A new study featured in the journal Nature, however, describes an area of the sky near the Auriga constellation where scientists at the Arecibo Observatory in Puerto Rico have been detected at least 11 instances of fast radio bursts over the past four years. These FRBs seem to come from the same astrophysical source in space. James Cordes, a researcher from Cornell University and one of the authors of the study, said that since the FRBs they examined tended to occur repeatedly, they can eliminate the possibility that these radio waves are simply one-offs formed as a result of evaporating black holes or any other potential sources described before. He said that it is more likely that fast radio bursts are created by massive energy releases from a neutron star. A fast radio burst is a phenomenon in space believed to be made of high energy, which often appears in the form of a transient radio pulse that lasts for only a few milliseconds. Some of the FRBs that have been detected in the past were bright, millisecond flashes that seemed to have come from areas of the sky beyond the Milky Way galaxy. The duration of an FRB depends on its wavelength, which results in a delay of the burst referred to by scientists as a dispersion measure. FRBs often have dispersion measures that are significantly larger than what researchers expect from a source within the Milky Way. They are also believed to be spread through the help of plasma. The first fast radio burst to be recorded was the Lorimer Burst. It was named after West Virginia University astrophysicist Duncan Lorimer, who led a team of researchers in studying the phenomenon in 2007. The researchers believe that the burst, which only lasted for about 5 milliseconds, could have originated from a part of the universe some 3 billion light-years away from Earth. It was discovered through data gathered by a radio survey of a dwarf galaxy known as the Small Magellanic Cloud in 2001. As for its possible source, Lorimer and his colleagues speculate that the Lorimer Burst could have been caused by either a collision between two neutron stars, the throes of a dying black hole, or something else they have yet to discover. It is estimated that as many as 10,000 fast radio bursts flash across the sky every day. However, the briefness of these flashes as well as their unpredictable arrival has made it difficult for scientists to study them. Almost all of the FRBs detected have also been single-flash events only. In 2012, researchers working at the Arecibo Observatory detected a multi-flash FRB event during a radio survey of the sky. While the repeating burst they found did not show any further activity following the initial observation, it had manifested 10 times in the past based on a review of archived surveys. Jason W. T. Hessels, an ASTRON scientist from the Netherlands Institute for Radio Astronomy and co-author of the study published in Nature, said that the repeating FRBs detected by Arecibo did not occur regularly. In one particular case, the researchers observed six successive bright pulses that occurred over 15-minute period. Hessels refers to this event as a "fantastic observation". The team also saw instances when they weren't able to detect any FRB activity for hours. Hessels and his colleagues believe that the nature of the repeating FRBs points to the possibility that some of the radio waves could originate from sources that can produce the bursts more than once. This contradicts the earlier notion that FRBs occur as a result of a cosmic collision between massive objects in space, which would leave them both destroyed in the aftermath.
An T.,Chinese Academy of Sciences |
Astrophysical Journal | Year: 2012
The evolution of symmetric extragalactic radio sources can be characterized by four distinct growth stages of the radio luminosity versus size of the source. The interaction of the jet with the ambient medium results in the formation and evolution of sources with non-standard (flaring) morphology. In addition, cessation or restarting of the jet power and obstruction of the jet will also result in distinct morphological structures. The radio source population may thus be classified in morphological types that indicate the prevailing physical processes. Compact symmetric objects (CSOs) occupy the earliest evolutionary phase of symmetric radio sources and their dynamical behavior is fundamental for any further evolution. Analysis of CSO dynamics is presented for a sample of 24 CSOs with known redshift and hotspot separation velocity and with a large range of radio power. Observables such as radio power, separation between two hotspots, hotspot separation velocity, and kinematic age of the source are found to be generally consistent with the self-similar predictions for individual sources that reflect the varying density structure of the ambient interstellar medium. Individual sources behave different from the group as a whole. The age and size statistics confirm that a large fraction of CSOs does not evolve into extended doubles. © 2012. The American Astronomical Society. All rights reserved..
Astrophysical Journal Letters | Year: 2012
The interstellar medium (ISM) in galaxies is directly affected by the mass and energy outflows originating in regions of star formation. Magnetic fields are an essential ingredient of the ISM, but their connection to the gaseous medium and its evolution remains poorly understood. Here, we present the detection of a gradient in Faraday rotation measure (RM), co-located with a hole in the neutral hydrogen (H I) distribution in the disk of the nearby spiral galaxy NGC6946. The gas kinematics in the same location show evidence for infall of cold gas. The combined characteristics of this feature point to a substantial vertical displacement of the initially plane-parallel-ordered magnetic field, driven by a localized star formation event. This reveals how the large-scale magnetic field pattern in galaxy disks is directly influenced by internal energetic phenomena. Conversely, magnetic fields are observed to be an important ingredient in disk-halo interactions, as predicted in MHD simulations. Turbulent magnetic fields at smaller spatial scales than the observed RM gradient will also be carried from the disk and provide a mechanism for the dynamo process to amplify the ordered magnetic field without quenching. We discuss the observational biases and suggest that this is a common feature of star-forming galaxies with active disk-halo flows. © 2012. The American Astronomical Society. All rights reserved..
van Diepen G.N.J.,ASTRON
Astronomy and Computing | Year: 2015
The Casacore Table Data System (CTDS) has been developed as part of the CASA (formerly AIPS++) package for (radio-)astronomical data processing. It offers a relational-like data model extended by the ability to store large arrays. A versatile query language can be used for data queries and manipulation. CTDS forms the basis of the MeasurementSet, the primary data format used for the data acquisition and data processing at various radio telescopes (e.g., WSRT, LOFAR, ASKAP, JVLA, ALMA). This paper discusses the most important features of CTDS, its performance, and how it is used. Also comparisons with other data formats are made. © 2015 Elsevier B.V.
Serra P.,ASTRON |
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010
We present an analysis of the neutral hydrogen and stellar populations of elliptical galaxies in the Tal et al. sample. Our aim is to test their conclusion that the continuing assembly of these galaxies at z ~ 0 is essentially gas free and not accompanied by significant star formation. In order to do so, we make use of H I data and line-strength indices available in the literature. We look for direct and indirect evidence of the presence of cold gas during the recent assembly of these objects and analyse its relation to galaxy morphological fine structure. We find that ≥25 per cent of ellipticals contain H I at the level of M(H I) > 10 8 M ⊙, and that M(HI) is of the order of a few per cent of the total stellar mass. Available data are insufficient to establish whether galaxies with a disturbed stellar morphology are more likely to contain HI. However, H I interferometry reveals very disturbed gas morphology/kinematics in all but one of the detected systems, confirming the continuing assembly of many ellipticals but also showing that this is not necessarily gas free. We also find that all very disturbed ellipticals have a single-stellar-population-equivalent age <4 Gyr. We interpret this as evidence that ~0.5-5 per cent of their stellar mass is contained in a young population formed during the past ~1 Gyr. Overall, a large fraction of ellipticals seem to have continued their assembly over the past few Gyr in the presence of a mass of cold gas of the order of 10 per cent of the galaxy stellar mass. This material is now observable as neutral hydrogen and young stars. © 2009 The Authors. Journal compilation © 2009 RAS.
Monthly Notices of the Royal Astronomical Society | Year: 2010
Radio transients are sporadic signals, requiring that the backends of radio telescopes be equipped with the appropriate hardware and software for their detection. Observational programmes for detecting transients can be dedicated to that purpose or can rely upon observations made by other programmes. It is the single-dish single-transient (non-periodical) mode which is considered in this paper. Because neither the width of a transient nor the time of its arrival is known, a sequential analysis in the form of a cumulative sum (cusum) algorithm is proposed here. Computer simulations and real observation data processing are included to demonstrate the performance of the cusum. The use of the Hough transform is proposed here for the purpose of non-coherent de-dispersion. It is possible that the detected transients could be radio frequency interferences (RFI), and a procedure is proposed here which can distinguish between celestial signals and man-made RFI. This procedure is based on an analysis of the statistical properties of the signals. © 2010 The Author. Journal compilation © 2010 RAS.