Deconvolution with hybrid parameterizations for radio emission reconstruction

Deconvolution is used to eliminate imperfections in the point spread function,such as sidelobes caused by incomplete sampling of radio telescopes,and is a key t...     

Compressed sensing imaging techniques for radio interferometry

Radio interferometry probes astrophysical signals through incomplete and noisy Fourier measurements. The theory of compressed sensing demonstrates that such measurements may actually suffice for accurate reconstruction of sparse or compressible signals. We propose new generic imaging techniques based on convex optimization for global minimization problems defined in this context. The versatility of the framework notably allows introduction of specific prior information on the signals, which offers the possibility of significant improvements of reconstruction relative to the standard local matching pursuit algorithm CLEAN used in radio astronomy. We illustrate the potential of the approach by studying reconstruction performances on simulations of two different kinds of signals observed with very generic interferometric configurations. The first kind is an intensity field of compact astrophysical objects. The second kind is the imprint of cosmic strings in the temperature field of the cosmic microwave background radiation, of particular interest for cosmology.     

Quark nova model for fast radio bursts

Fast radio bursts(FRBs) are puzzling, millisecond, energetic radio transients with no discernible source; observations show no counterparts in other frequency bands. The birth of a quark star from a parent neutron star experiencing a quark nova- previously thought undetectable when born in isolation- provides a natural explanation for the emission characteristics of FRBs. The generation of unstable r-process elements in the quark nova ejecta provides millisecond exponential injection of electrons into the surrounding strong magnetic field at the parent neutron star's light cylinder via β-decay. This radio synchrotron emission has a total duration of hundreds of milliseconds and matches the observed spectrum while reducing the inferred dispersion measure by approximately 200 cm~(-3)pc. The model allows indirect measurement of neutron star magnetic fields and periods in addition to providing astronomical measurements of β-decay chains of unstable neutron rich nuclei. Using this model, we can calculate expected FRB average energies(~10~(41)erg) and spectral shapes, and provide a theoretical framework for determining distances.     

Adaptive optics near-infrared imaging of NGC 2992 – unveiling core structures related to radio figure-of-8 loops

We present near-infrared adaptive optics, Very Large Array (VLA) radio and Hubble Space Telescope ( HST ) optical imaging of the nearby Seyfert galaxy NGC 2992. Spiral structure and an extension to the west are traced down to the core region at the limiting resolution of our near-infrared images. A faint, diffuse loop of near-infrared and radio emission is also observed to the north, embedded within the prominent 2-arcsec radio loop previously observed to the north-west. Near-infrared colour maps and CO narrow-band imaging are then used to identify which regions may not be purely reddened stellar populations. Our new data provide evidence that the VLA radio-loop morphology in the shape of a figure of 8 represents two components superimposed: (1) outflow bubbles out of the plane of the disc, coincident with the extended emission-line region (EELR); (2) star formation along the spiral arm within the galaxy disc and through the dust lane. The near-infrared continuum emission associated with the outflowing radio bubbles suggests that the radio loops are driven by the active nucleus.     

A torsional wave model for solar radio pulsations

One of the widely accepted models for solar radio pulsations invokes radial oscillations of a magnetic flux tube. Due to acoustic, radiative damping, this theory does not easily explain the long length of the pulse trains, the large modulation depths or the great stability of the pulse repetition rate often observed. Torsional waves efficiently modulate synchrotron emission, and since they do not undergo radiative damping, can produce stable pulse repetition rates and long pulse trains.     

Miyun metre-wave aperture synthesis radio telescope

The Miyun metre-wave aperture synthesis radiotelescope, working at frequency of 232 MHz, consists of an E-W array of 28 elements, each of 9 m aperture. 192 baselines are effected with a full coverage of the U-V plane (Fig. 2). The longest baseline is 1,164m. This instrument is designed for source survey and detection of peculiar sources in northern declinations. A set of observations completed in 2 × 12 hours gives a thermal noise limited sensititity of 0.05 Jy and a resolution of 3'8 × 3'8 csc δ. The field of view is 8° × 8°. This should enable us to complete an overall survey of the region δ ⩾ + 30° within two years, and to carry out monitoring of selected areas.Figures 1 and 2 show the main properties and general design of the instrument and Figures 3 and 4 give some preliminary results of sky mapping.     

An empirical model for the beams of radio pulsars

Motivated by recent results on the location of the radio emission in pulsar magnetospheres, we have developed a model which can account for the large diversity found in the average profile shapes of pulsars. At the centre of our model lies the idea that radio emission at a particular frequency arises from a wide range of altitudes above the surface of the star, and that it is confined to a region close to the last open field lines. We assert that the radial height range over which emission occurs is responsible for the complex average pulse shapes rather than the transverse (longitudinal) range proposed in most current models. By implementing an abrupt change in the height range to discriminate between young, short-period, highly energetic pulsars and their older counterparts, we obtain the observed transition between the simple and complex average pulse profiles observed in each group respectively. Monte Carlo simulations are used to demonstrate the match of our model to real observations.     

An asymmetric relativistic model for classical double radio sources

There is substantial observational evidence against the symmetric relativistic model of FR II radio sources. An asymmetric relativistic model is proposed which takes account of both relativistic effects and intrinsic/environmental asymmetries to explain the structural asymmetries of their radio lobes. A key parameter of the model is the jet-side of the double sources, which is estimated for 80 per cent of the FR II sources in the 3CRR complete sample. Statistical analyses of the properties of these sources show that the asymmetric model is in agreement with a wide range of observational data, and that the relativistic and intrinsic asymmetry effects are of comparable importance. Intrinsic/environmental asymmetry effects are more important at high radio luminosities and small physical scales. The mean translational speed of the lobes is found to be     consistent with the speeds found from spectral ageing arguments. According to a Gaussian model, the standard deviation of the distribution of v _lobe is σ _{v l}=0.04 c . The results are in agreement with an orientation-based unification scheme in which the critical angle separating the radio galaxies from quasars is about 45°.     

Exploring three faint source detections methods for aperture synthesis radio images

Wide-field radio interferometric images often contain a large population of faint compact sources. Due to their low intensity/noise ratio, these objects can be easily missed by automated detection methods, which have been classically based on thresholding techniques after local noise estimation. The aim of this paper is to present and analyse the performance of several alternative or complementary techniques to thresholding. We compare three different algorithms to increase the detection rate of faint objects. The first technique consists of combining wavelet decomposition with local thresholding. The second technique is based on the structural behaviour of the neighbourhood of each pixel. Finally, the third algorithm uses local features extracted from a bank of filters and a boosting classifier to perform the detections. The methods’ performances are evaluated using simulations and radio mosaics from the Giant Metrewave Radio Telescope and the Australia Telescope Compact Array. We show that the new methods perform better than well-known state of the art methods such as SExtractor, SAD and DUCHAMP at detecting faint sources of radio interferometric images.     

An empirical model for the polarization of pulsar radio emission

We present an empirical model for single pulses of radio emission from pulsars based on Gaussian probability distributions for relevant variables. The radiation at a specific pulse phase is represented as the superposition of radiation in two (approximately) orthogonally polarized modes (OPMs) from one or more subsources in the emission region of the pulsar. For each subsource, the polarization states are drawn randomly from statistical distributions, with the mean and the variance on the Poincaré sphere as free parameters. The intensity of one OPM is chosen from a lognormal distribution, and the intensity of the other OPM is assumed to be partially correlated, with the degree of correlation also chosen from a Gaussian distribution. The model is used to construct simulated data described in the same format as real data: distributions of the polarization of pulses on the Poincaré sphere and histograms of the intensity and other parameters. We concentrate on the interpretation of data for specific phases of PSR B0329+54 for which the OPMs are not orthogonal, with one well defined and the other spread out around an annulus on the Poincaré sphere at some phases. The results support the assumption that the radiation emerges in two OPMs with closely correlated intensities, and that in a statistical fraction of pulses one OPM is invisible.     

A relativistic mixing-layer model for jets in low-luminosity radio galaxies

We present an analytical model for jets in Fanaroff & Riley Class I (FR I) radio galaxies, in which an initially laminar, relativistic flow is surrounded by a shear layer. We apply the appropriate conservation laws to constrain the jet parameters, starting the model where the radio emission is observed to brighten abruptly. We assume that the laminar flow fills the jet there and that pressure balance with the surroundings is maintained from that point outwards. Entrainment continuously injects new material into the jet and forms a shear layer, which contains material from both the environment and the laminar core. The shear layer expands rapidly with distance until finally the core disappears, and all of the material is mixed into the shear layer. Beyond this point, the shear layer expands in a cone and decelerates smoothly. We apply our model to the well-observed FR I source 3C 31 and show that there is a self-consistent solution. We derive the jet power, together with the variations of mass flux and entrainment rate with distance from the nucleus. The predicted variation of bulk velocity with distance in the outer parts of the jets is in good agreement with model fits to Very Large Array observations. Our prediction for the shape of the laminar core can be tested with higher-resolution imaging.     

A model for solar radio pulsations at short centimetric band

In this paper, the observed solar radio pulsations during the bursts at 9.375 GHz are considered to be excited by some plasma instability. Under the condition of the conservation of energy in the wave-particle interaction, the saturation time of plasma instabilities is inversely proportional to the initial radiation intensity, which may explain why the repetition rate of the pulsations is directly proportional to the radio burst flux at 9.375 GHz as well as 15 GHz and 22 GHz. It is also predicted that the energy released in an individual pulse increases with increasing the flux of radio bursts, the modularity of the pulsations decreases with increasing the flux of radio bursts, these predictions are consistent with the statistical results at 9.375 GHz in different events. The energy density of the non-thermal particles in these events is estimated from the properties of pulsation. For the typical values of the ambient plasma density (10⁹ cm^–3) and the ratio between the nonthermal and ambient electrons (10^–4), the order of magnitude of the energy density and the average energy of the nonthermal electrons is 10^–4 erg/cm³ and 10 kev, respectively. It is interesting that there are two branches in a statistical relation between the repetition rate and the radio burst flux in a special event on March 11–17, 1989, which just corresponds to two different orders of magnitude for the quasi-quantized energy released in these five bursts. This result may be explained by the different ratios between the thermal and the nonthermal radiations.     

The Ooty synthesis radio telescope: First results

A 4-km synthesis radio telescope has recently been commissioned at Ootacamund, India for operation at 327 MHz. It consists of the Ooty Radio Telescope (530 m × 30 m) and 7 small antennas which are distributed over an area of about 4 km × 2 km. It has a coverage of about ± 40‡ in declination δ. The beam-width is about 40 arcsec × 90 arcsec at δ = 0‡ and about 40 arcsec × 50 arcsec at δ = 40‡. The sensitivity attained for a 5:1 signal-to-noise ratio is about 15 m Jy after a 10-hour integration. The observational programmes undertaken and some of the results obtained recently are summarized. The radio halo around the edge-on spiral NGC 4631 is found to have a larger scale-height at 327 MHz than is known at higher frequencies. Mapping of interesting radio galaxies at 327 MHz is being carried out; preliminary results for 0511-305 (∼2 Mpc) and 1333-337 (∼750 kpc) are summarized. The very-steep-spectrum radio source in the Abell cluster A85 is found to be resolved; since it has no obvious optical counterpart, it is conceivable that it is a remnant of past activity of a galaxy that has drifted away in about 10⁹ years.     

A self-consistent model for the storm radio emission from the Sun

A self-consistent theoretical model for storm continuum and bursts is presented. We propose that the Langmuir waves are emitted spontaneously by an anisotropic loss-cone distribution of electrons trapped in the magnetic field above active regions. These high-frequency electrostatic waves are assumed to coalesce with lower-hybrid waves excited either by the trapped protons or by weak shocks, making the observed brightness temperature equal to the effective temperature of the Langmuir waves.It is shown that whenever the collisional damping ( _c ) is more than the negative damping (- _A ) due to the anisotropic distribution, there is a steady emission of Langmuir waves responsible for the storm continuum. The type I bursts are generated randomly whenever the collisional damping ( _c ) is balanced by the negative damping (- _A ) at the threshold density of the trapped particles, since it causes the effective temperature of Langmuir waves to rise steeply. The number density of the particles responsible for the storm radiation is estimated. The randomness of type I bursts, brightness temperature, bandwidth and transition from type I to type III storm are self-consistently explained.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Few projections astrotomography: radio astronomical approach to 3D reconstruction

We develop a radio astronomical approach to 3D‐reconstruction in few projections tomography. It is based on the 2‐CLEAN DSA method which consists of two clean algorithms by using a synthesized beam. In complex cases two extreme solutions are used for the analysis of the image structure. These solutions determine the limits of permissible energy redistribution on the image among the components of small and large scales. Two variants of 3D‐reconstruction proceeding from a set of two‐dimensional projections (3D_2D) and from a set of one‐dimensional ones (3D_1D) are considered. It is shown that the quality of 3D_2D‐reconstruction should be similar to the quality of 2D_1D‐reconstruction if the same number of equally spaced scans is used. But a doubled number of projections is required for 3D_1D‐reconstruction. We have simulated 3D‐reconstruction of an optically thin emitting object. The present technique is a component of astrotomography and it has good prospects for a wide range of remote sensing. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)