Radio Astronomy Techniques Research Articles

Multiscale and multidirectional very long baseline interferometry imaging with CLEAN

Context. Very long baseline interferometry (VLBI) is a radio-astronomical technique whereby the correlated signal from various baselines is combined into an image of the highest possible angular resolution. Due to the sparsity of the measurements, this imaging procedure constitutes an ill-posed inverse problem. For decades, the CLEAN algorithm has been the standard choice in VLBI studies, despite it bringing on some serious disadvantages and pathologies that are brought on by the requirements of modern frontline VLBI applications. Aims. We developed a novel multiscale CLEAN deconvolution method (DoB-CLEAN) based on continuous wavelet transforms that address several pathologies in CLEAN imaging. We benchmarked this novel algorithm against CLEAN reconstructions on synthetic data and reanalyzed BL Lac observations of RadioAstron with DoB-CLEAN. Methods. The DoB-CLEAN method approaches the image via multiscalar and multidirectional wavelet dictionaries. Two different dictionaries were used: 1) a difference of elliptical spherical Bessel functions dictionary fitted to the uv-coverage of the observation that is used to sparsely represent the features in the dirty image; 2) a difference of elliptical Gaussian wavelet dictionary that is well suited to represent relevant image features cleanly. The deconvolution was performed by switching between the dictionaries. Results. DoB-CLEAN achieves a super-resolution compared to CLEAN and remedies the spurious regularization properties of CLEAN. In contrast to CLEAN, the representation via basis functions has a physical meaning. Hence, the computed deconvolved image still fits the observed visibilities, in contrast to CLEAN. Conclusions. State-of-the-art multiscalar imaging approaches seem to outperform single-scalar standard approaches in VLBI and are well suited to maximize the extraction of information in ongoing frontline VLBI applications.

Discovery of Interstellar Isocyanogen (CNCN): Further Evidence that Dicyanopolyynes are Abundant in Space.

It is thought that dicyanopolyynes could be potentially abundant interstellar molecules, although their lack of dipole moment makes it impossible to detect them through radioastronomical techniques. Recently, the simplest member of this chemical family, cyanogen (NCCN), was indirectly probed for the first time in interstellar space through the detection of its protonated form toward the dense clouds L483 and TMC-1. Here we present a second firm evidence of the presence of NCCN in interstellar space, namely the detection of the metastable and polar isomer isocyanogen (CNCN). This species has been identified in L483 and tentatively in TMC-1 by observing various rotational transitions in the λ 3 mm band with the IRAM 30m telescope. We derive beam-averaged column densities for CNCN of 1.6 × 1012 cm-2 in L483 and 9 × 1011 cm-2 in TMC-1, which imply fractional abundances relative to H2 in the range (5 - 9) × 10-11. While the presence of NCCN in interstellar clouds seems out of doubt owing to the detection of NCCNH+ and CNCN, putting tight constraints on its abundance is still hampered by the poor knowledge of the chemistry that links NCCN with NCCNH+ and especially with CNCN. We estimate that NCCN could be fairly abundant, in the range 10-9-10-7 relative to H2, as other abundant nitriles like HCN and HC3N.

Study of the magnetospheres of active regions on the sun by radio astronomy techniques

In the 1990s, based on detailed studies of the structure of active regions (AR), the concept of the magnetosphere of the active region was proposed. This includes almost all known structures presented in the active region, ranging from the radio granulation up to noise storms, the radiation of which manifests on the radio waves. The magnetosphere concept, which, from a common point of view, considers the manifestations of the radio emission of the active region as a single active complex, allows one to shed light on the relation between stable and active processes and their interrelations. It is especially important to identify the basic ways of transforming nonthermal energy into thermal energy. A dominant role in all processes is attributed to the magnetic field, the measurement of which on the coronal levels can be performed by radio-astronomical techniques. The extension of the wavelength range and the introduction of new tools and advanced modeling capabilities makes it possible to analyze the physical properties of plasma structures in the AR magnetosphere and to evaluate the coronal magnetic fields at the levels of the chromosphere–corona transition zone and the lower corona. The features and characteristics of the transition region from the S component to the B component have been estimated.

QUANTUM OPTIMALITY OF PHOTON COUNTING FOR TEMPERATURE MEASUREMENT OF THERMAL ASTRONOMICAL SOURCES

Using the quantum Cram\'{e}r-Rao bound from quantum estimation theory, we derive a fundamental quantum limit on the sensitivity of a temperature measurement of a thermal astronomical source. This limit is expressed in terms of the source temperature $T_s$, input spectral bandwidth $\Delta \nu$, and measurement duration $T$, subject to a long measurement time assumption $T\Delta \nu \gg 1$. It is valid for any measurement procedure that yields an unbiased estimate of the source temperature. The limit agrees with the sensitivity of direct detection or photon counting, and also with that of the ideal radiometer in the regime $kT_s/h \nu_0\gg 1$ for which the Rayleigh-Jeans approximation is valid, where $\nu_0$ is the center frequency at which the radiometer operates. While valid across the electromagnetic spectrum, the limit is especially relevant for radio astronomy in this regime, since it implies that no ingenious design or technological improvement can beat an ideal radiometer for temperature measurement. In this connection, our result refutes the recent claim of a radio astronomy technique with much-improved sensitivity over the radiometer (Lieu et al. 2015).

Probing non polar interstellar molecules through their protonated form: Detection of protonated cyanogen (NCCNH+).

Cyanogen (NCCN) is the simplest member of the series of dicyanopolyynes. It has been hypothesized that this family of molecules can be important constituents of interstellar and circumstellar media, although the lack of a permanent electric dipole moment prevents its detection through radioastronomical techniques. Here we present the first solid evidence of the presence of cyanogen in interstellar clouds through the detection of its protonated form toward the cold dark clouds TMC-1 and L483. Protonated cyanogen (NCCNH+) has been identified through the J = 5 - 4 and J = 10 - 9 rotational transitions using the 40m radiotelescope of Yebes and the IRAM 30m telescope. We derive beam averaged column densities for NCCNH+ of (8.6 ± 4.4) × 1010 cm-2 in TMC-1 and (3.9 ± 1.8) × 1010 cm-2 in L483, which translate to fairly low fractional abundances relative to H2, in the range (1-10) × 10-12. The chemistry of protonated molecules in dark clouds is discussed, and it is found that, in general terms, the abundance ratio between the protonated and non protonated forms of a molecule increases with increasing proton affinity. Our chemical model predicts an abundance ratio NCCNH+/NCCN of ~ 10-4, which implies that the abundance of cyanogen in dark clouds could be as high as (1-10) × 10-8 relative to H2, i.e., comparable to that of other abundant nitriles such as HCN, HNC, and HC3N.

Solar flare detection system based on tolerance near sets in a GPU–CUDA framework

This article presents a unique application of tolerance near sets (TNS) for detecting solar flare events in solar images acquired using radio astronomy techniques. In radio astronomy (RA) applications, the interferometric array processing of data streams presents algorithmic and response time challenges as well as a high volume of data. The radio interferometer is an RA instrument composed of an array of antennas. Radio signals emitted by a celestial object are captured by the antennas and are subsequently processed in such a way that each pair of antennas produces correlated data. The overall correlated data is then accumulated and, after an integration period, the spectral image of the observed object is obtained. The process of deconvolution of the spectral image produces the desired spatial image of the celestial object. The proposed solar flare detection system is embedded in a computational platform framework suitable for dealing with huge volumes of data, based on a cluster of CPU–GPU pairs. The experimental results presented in the paper include comparison of the TNS-based algorithm (implemented as the SOL-FLARE system) with the K-means algorithm using significant samples of test images to validate the detection system. The performances of both systems are comparatively analyzed using Receiver Operating Characteristic (ROC) curves. The images used in the experiments were selected from a data repository produced by the Nobeyama Radioheliograph, in Japan, during the years 2004 up to 2013. The main contribution of the article is a novel approach to solar flare detection in a GPU–CUDA framework.

Prospective work for Alma, Herschel, Sofia : submillimeter-wave spectroscopy of complex organic molecules

Laboratory analysis of spectra of potential interstellar molecules from the radio-frequency to the THz range is a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis allows us to derive molecular parameters that are able to model the frequency and intensity patterns of the transitions, as well as to predict with a high degree of precision the frequencies of transitions not measured in the laboratory.

Rotational spectrum of13C2-methyl formate (HCOO13CH3) and detection of the two13C-methyl formate in Orion

Context. Laboratory measurements and analysis of the microwave and millimeter-wave spectra of potential interstellar molecules are a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis provides spectroscopic parameters that are used in the assignment procedure of the laboratory spectra, and that also predict the frequencies of transitions not measured in the laboratory with a high degree of precision. Aims. An experimental laboratory study and its theoretical analysis is presented for 13 C2-methyl formate (HCOO 13 CH3) allowing a search for this isotopologue in the Orion molecular cloud. The 13 C1-methyl formate (H 13 COOCH3) molecule was also searched for in this interstellar cloud, using previously published spectroscopic data. Methods. The experimental spectra of 13 C2-methyl formate were recorded in the microwave and sub-mm energy ranges (4–20 GHz, 8–80 GHz, 150–700 GHz). The spectra were analyzed using the Rho-Axis Method (RAM), which takes the CH3 internal rotation and the coupling between internal rotation and global rotation into account. Results. Twenty-seven spectroscopic constants of 13 C2-methyl formate have been obtained from a fit of 936 transitions of the ground torsional state with a standard (unitless) deviation of 1.08. A prediction of line positions and intensities is also produced. This prediction allowed us to identify 230 13 C2-methyl formate lines in the Orion interstellar molecular cloud. We refitted all previously published ground state transitions of the 13 C1-methyl formate molecule in order to provide a prediction of its ground state spectrum. 234 lines of 13 C1-methyl formate were detected in the Orion interstellar cloud using that prediction.

Radio astronomy in the future: impact on relativity

AbstractRadio astronomy has played an important part in the study of relativity. Famous examples include the discovery and exploitation of pulsars for precise binary pulsar tests, the proof of the existence of gravitational radiation and the discovery of the Cosmic Microwave background. In the future, radio astronomy will continue to play a decisive role, assisted by new and upcoming instruments like LOFAR and the SKA. In this review, I will present the revolution that is ongoing in radio astronomical techniques and outline the impact expected on the studies of relativity.

On the optimal frequency of observation of Cherenkov radiation in the radio astronomy method for measuring superhigh-energy cosmic-ray particle flux

Possible reasons for the absence of direct observations of individual events in measuring the super-high-energy particle flux by the radio astronomy technique are considered. One of these reasons is probably associated with the choice of extremely high frequencies (∼1.5 GHz) for detecting radio pulses. Calculations show that the radiation intensity attains its peak value at frequencies ∼500–600 MHz and then sharply decreases so that it becomes three orders of magnitude lower even at a frequency of ∼1.5 GHz. The effectiveness of particle detection in the range of high (∼600 MHz) and low (∼60 MHz) frequencies is analyzed.

Discovery of Interstellar Propylene (CH 2 CHCH 3 ): Missing Links in Interstellar Gas-Phase Chemistry

We report the discovery of propylene (also called propene, CH_2CHCH_3) with the IRAM 30-m radio telescope toward the dark cloud TMC-1. Propylene is the most saturated hydrocarbon ever detected in space through radio astronomical techniques. In spite of its weak dipole moment, 6 doublets (A and E species) plus another line from the A species have been observed with main beam temperatures above 20 mK. The derived total column density of propylene is 4 10^13 cm^-2, which corresponds to an abundance relative to H_2 of 4 10^-9, i.e., comparable to that of other well known and abundant hydrocarbons in this cloud, such as c-C_3H_2. Although this isomer of C_3H_6 could play an important role in interstellar chemistry, it has been ignored by previous chemical models of dark clouds as there seems to be no obvious formation pathway in gas phase. The discovery of this species in a dark cloud indicates that a thorough analysis of the completeness of gas phase chemistry has to be done.

The Viewpoint from Commission 40

AbstractAlthough Very Long Baseline Interferometry has resolved a few binary stellar systems, pulsars provide the main source of binary and multiple stars through radio astronomy techniques. There are about 85 binary pulsars and two multiple systems known. Currently there is no formal system of designating these companions.

High precision pulsar astrometry and its applications

Very long baseline interferometry (VLBI) is a radio astronomy technique that allows imaging and astrometry at milliarcsecond or better resolution. The last decade has seen improvements in VLBI techniques and equipment which have enabled precision astrometry of pulsars. Most notably, VLBI arrays have become more sensitive and calibration has become more accurate. Pulsar astrometry using VLBI can readily yield a proper motion and in many cases a trigonometric parallax as well. These observables have have numerous applications which justify the difficulty of the measurements. The goals of pulsar astrometry include: 1. determination of the distribution of pulsar birth velocities, a critical probe of supernova asymmetry; 2. improvement in understanding of the ISM through observations of pulsar bow shocks, interstellar scattering, and timing dispersion measures; and 3. understanding characteristics of individual pulsars though knowledge of their distance and velocity. I will discuss these science cases, some recent results, and prospects for better astrometry in the near future.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

First results from the Cluster wideband plasma wave investigation

Abstract. In this report we present the first results from the Cluster wideband plasma wave investigation. The four Cluster spacecraft were successfully placed in closely spaced, high-inclination eccentric orbits around the Earth during two separate launches in July – August 2000. Each spacecraft includes a wideband plasma wave instrument designed to provide high-resolution electric and magnetic field wave-forms via both stored data and direct downlinks to the NASA Deep Space Network. Results are presented for three commonly occurring magnetospheric plasma wave phenomena: (1) whistlers, (2) chorus, and (3) auroral kilometric radiation. Lightning-generated whistlers are frequently observed when the spacecraft is inside the plasmasphere. Usually the same whistler can be detected by all spacecraft, indicating that the whistler wave packet extends over a spatial dimension at least as large as the separation distances transverse to the magnetic field, which during these observations were a few hundred km. This is what would be expected for nonducted whistler propagation. No case has been found in which a strong whistler was detected at one spacecraft, with no signal at the other spacecraft, which would indicate ducted propagation. Whistler-mode chorus emissions are also observed in the inner region of the magnetosphere. In contrast to lightning-generated whistlers, the individual chorus elements seldom show a one-to-one correspondence between the spacecraft, indicating that a typical chorus wave packet has dimensions transverse to the magnetic field of only a few hundred km or less. In one case where a good one-to-one correspondence existed, significant frequency variations were observed between the spacecraft, indicating that the frequency of the wave packet may be evolving as the wave propagates. Auroral kilometric radiation, which is an intense radio emission generated along the auroral field lines, is frequently observed over the polar regions. The frequency-time structure of this radiation usually shows a very good one-to-one correspondence between the various spacecraft. By using the microsecond timing available at the NASA Deep Space Net-work, very-long-baseline radio astronomy techniques have been used to determine the source of the auroral kilometric radiation. One event analyzed using this technique shows a very good correspondence between the inferred source location, which is assumed to be at the electron cyclotron frequency, and a bright spot in the aurora along the magnetic field line through the source.Key words. Ionosphere (wave-particle interactions; wave propagation) – Magnetospheric physics (plasma waves and instabilities; instruments and techniques)

Oceanic Effects on Earth's Rotation Rate

Changes in the mass distribution of water in Earth9s oceans in turn leads to variations in its angular momentum. In his Perspective, Wilson discusses results presented in the same issue by Marcus et al . in which techniques of radio astronomy were used to measure changes in Earth9s angular momentum. The variations were observed by measuring changes in the length of day, and therefore Earth9s rotation rate. Measurements such as these may serve as an indicator of oceanic behavior, much as other indices are used to keep track of atmospheric climate.

Earth rotation measurement yields valuable information about the dynamics of the Earth system

Intensive work in recent years has demonstrated the value of high precision and high temporal resolution in collecting Earth rotation data, paving the way for continuous measurement that will open exciting new research areas in geophysics as the new millennium unfolds. Very long baseline interferometry (VLBI) will continue to be a principal technique. However, it will have greatly improved precision and temporal resolution, making possible more in‐depth studies of geophysical phenomena affecting Earth rotation. Originally developed as a radio astronomical technique for high‐resolution mapping of distant radio sources, VLBI, when “turned around,” has become a valuable tool in geodesy. Because of its past success, a new VLBI project, Continuous Observation of the Rotation of the Earth (CORE), was launched last year. It is being implemented in phases by the NASA Goddard Space Flight Center in collaboration with the U.S. Naval Observatory (USNO) [see Ma et al., 1997].

Interstellar silicon-nitrogen chemistry. I. The microwave and the infrared signatures of the HSiN, HNSi, HSiNH2, HNSiH2 and HSiNH+ species

Abstract The experimental and the theoretical interests for the silicon chemistry have been renewed by the recent detection of SiN in space. In this contribution a theoretical study of the HSiN, HNSi, HSiNH 2 and HNSiH 2 molecular systems is presented that aims to help in the interpretation of available experimental results as well as in the attribution of new interstellar lines. The main goal of this report remains, however, the calibration of ab initio calculations on still-unknown silicon-nitrogen systems: the infrared and the microwave signatures of the HSiNH + cation are reported as a direct application. The signatures of the five molecules under investagation have been computed at increasing levels of post-Hartree-Fock theories, using up to a 6–311 + + G ∗∗ atomic orbital expansion. Accurate geometries and B e rotational constants have been determined at the Moller-Plesset MP n ( n = 2, 3, 4), CASSCF and CCSD(T) theoretical plateaus for HNSi. The comparison with experimental data allows then to derive the scaling factors needed to obtain accurate rotational constants for related species: they are applied as such on the crude constants determined for HSiN, HSiNH 2 , HNSiH 2 , and finally HSiNH 2 in its floppy linear singlet ground state and in its lowest cis-bent a 3 A′ state as well. Dipole moments are reported in order to assess the feasability for these species to be detected owing to their rotational signatures either in the laboratory or in space using millimetric radioastronomy techniques. Infrared (IR) signatures are computed at the same levels of theory and compared to the recent matrix isolation experiments devoted to HSiN, HNSi, HSiNH 2 and HNSiH 2 . The calculations unambiguosly confirm that all these species have been effectively produced and observed. They also lead to the determination of accurate IR scaling factors that are significantly larger than the usual ones. Such an approach allows then to quantitatively predict the IR spectra of the still-unknown HSiNH + entity. The study of the IR spectra furthermore points out the failure of single-reference correlation methods to obtain predictive IR signatures in some cases, as is unambigously illustrated in the case of the HSiN species.

An ancient revisits cosmology.

In this after-dinner speech, a somewhat light-hearted attempt is made to view the observational side of physical cosmology as a subdiscipline of astrophysics, still in an early stage of sophistication and in need of more theoretical understanding. The theoretical side of cosmology, in contrast, has its deep base in general relativity. A major result of observational cosmology is that an expansion of the Universe arose from a singularity some 15 billion years ago. This has had an enormous impact on the public's view of both astronomy and theology. It places on cosmologists an extra responsibility for clear thinking and interpretation. Recently, gravitational physics caused another crisis from an unexpected observational result that nonbaryonic matter appears to dominate. Will obtaining information about this massive nonbaryonic component require that astronomers cease to rely on measurement of photons? But 40 years ago after radio astronomical techniques uncovered the high-energy universe, we happily introduced new subfields, with techniques from physics and engineering still tied to photon detection. Another historical example shows how a subfield of cosmology, big bang nucleosynthesis, grew in complexity from its spectroscopic astrophysics beginning 40 years ago. Determination of primordial abundances of lighter nuclei does illuminate conditions in the Big Bang, but the observational results faced and overcame many hurdles on the way.

Book Review: Interferometry in Radioastronomy and Radar Techniques

Book Review: Interferometry in Radioastronomy and Radar Techniques

Propagation of coherence in scattering: an experiment using interplanetary scintillation

It is well known that high-time-resolution imaging of the optical seeing disk reveals the existence of speckles inside the envelope. Since analogs of seeing disks occur in many different fields, one might expect that analogs of speckles also exist. Using radio-astronomical techniques, we have studied one such example in depth: interplanetary scintillation. By using a radio-interferometric array, we measured directly the coherence of the radiation after scattering by the solar wind and confirmed the existence of fine-scale structure similar to speckles. In order to understand further the behavior of the coherence function after scattering, we adapted and extended the theory previously developed by Cornwell and Napier [in Proceedings of the National Radio Astronomy Observatory Workshop on Radio Astronomy in Space ( National Radio Astronomy Observatory, Greenbank, W. Va., 1986); Radio Sci.23, 739 ( 1988)] for the case of coherence measurements at the focal plane of a single-reflector radio telescope. We develop and demonstrate a test for coherence of the scattered radiation, and we discuss the prospects for high-angular-resolution imaging of objects. This treatment also leads to new insights into the effect of spatial coherence of emitters on interferometric measurements.