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The merits of solar coronal at metric-wavelength (MW) radio have long been recognised (e.g. Pick and Vilmer, 2008). High-fidelity solar radio imaging at these frequencies has however remained challenging. On the one hand, dealing with the small spectral and temporal scales of variation in solar radio emission requires a data product capable of tracking the emission simultaneously across time, frequency and morphology. The Fourier imaging nature of interferometry, on the other hand, severely limits the instrumental ability to gather sufficient information to do this with the required fidelity and resolution. Benefiting from the enormous advances in technology the new generation of instruments, like the Murchison Widefield Array (MWA; Tingay et al. (2013), Bowman et al. (2013)), represent a quantum leap in our ability to gather data suitable for radio solar physics.
At low radio frequencies the solar corona is very dynamic in both spectral and temporal domains. To capture the fine details of this complex dynamics, imaging studies at high temporal and spectral resolution are necessary. The advent of the new instruments like the Murchison Widefield Array (MWA; Tingay et al. 2013, Bowman et al. 2013), is now making this possible.
We present the results of an approximately 6 100 deg2 104–196 MHz radio sky survey performed with the Murchison Widefield Array during instrument commissioning between 2012 September and 2012 December: the MWACS. The data were taken as meridian drift scans with two different 32-antenna sub-arrays that were available during the commissioning period. The survey covers approximately 20.5 h < RA < 8.5 h, − 58° < Dec < −14°over three frequency bands centred on 119, 150 and 180 MHz, with image resolutions of 6–3 arcmin. The catalogue has 3 arcmin angular resolution and a typical noise level of 40 mJy beam− 1, with reduced sensitivity near the field boundaries and bright sources. We describe the data reduction strategy, based upon mosaicked snapshots, flux density calibration, and source-finding method. We present a catalogue of flux density and spectral index measurements for 14 110 sources, extracted from the mosaic, 1 247 of which are sub-components of complexes of sources.
Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.
We present parsec-resolution spectral-line VLBI data for two epochs separated by 15 months as a precise new probe of the innermost regions of the nearby Ultraluminous Infrared Galaxy (ULIRG) Arp 220. This galaxy hosts a powerful starburst, with an associated supernova (SN) rate of order 4/yr. An extensive population of compact continuum sources interpreted as radio supernovae (RSNe) and young supernova remnants (SNR) has been imaged. We show here that many of the supernova-related radio continuum point sources exhibit clear evidence of OH absorption or maser emission in the intervening gas, and as such provide us with a sampling of conditions along very narrow and specific lines of sight through the nuclear environment. The OH gas along these lines of sight exhibits velocity dispersions of up to several tens of km/sec, and that in some cases, multiple distinct concentrations of masing gas at different radial velocities can be discerned. There is evidence for variability in the OH properties on ~1yr timescales. Our results are discussed in the context of the overall OH megamaser properties of Arp 220.
The history of OH megamaser (OHM) research is reviewed, and recent developments in the field are summarized. Particular attention is paid to results from VLBI, and the recognition of a wide range of maser properties within a single galaxy. A peculiar characteristic of compact parsec-scale OHM features is a very broad velocity width, which poses a challenge for radiative pumping models, and suggests filamentary geometries for the emitting clouds. The compact masers may be associated with shocks in a violent nuclear environment, and collisional pumping may play a role. A link between the compact masers and newly formed active galactic nuclei (AGNs) cannot be ruled out. The low-gain amplification model of the diffuse maser component is strongly supported by recent data, and detailed information about the parameters of the amplifying screen is starting to emerge. OHM can be used as powerful probes of dusty, obscured IR galaxy nuclei, and the prospects for these objects being detectable, and therefore useful as probes, at cosmological distances is discussed.
Since our unexpected discovery of 12 compact radio sources in one of the nuclei of Arp220 we have been monitoring the structure using global VLBI and the VLBA. With several epochs of data we can now demonstrate that the fluxes of the radio sources are, in general, decreasing sytematically but not smoothly. The sources also have steep spectra. Both facts support our earlier supposition that these are highly luminous radio supernovae. We initially predicted a radio supernova rate of approximately 2yr−1 Our most recent observations indicate that the occurrence rate of luminous RSN is several times lower than predicted, thus our model of the Arp220 starburst must be revised.
Luminous Infrared Galaxies (LIGs) are locally more numerous than normal galaxies, AGN, and QSOs above L ˜ 1011L⊙ and may be the evolutionary precursors of classical radio-quiet quasars. VLBI observations of a complete sample show that high-Tb radio cores are common, perhaps universal among LIGs. VLBI imaging shows that these radio cores may be produced by intense starbursts which generate luminous radio supernovae, as in the case of Arp 220 (Smith et al. 1998), or by a classical AGN core, as in the case of Mrk 231, which we interpret as a newly formed QSO emerging from a starburst. Compact OH 1667MHz maser emission appears to be common in LIGs and may be related to AGN activity. These results lend further support to the scenario suggested by Sanders et al (1988) in which mergers of gas-rich galaxies lead first to luminous starbursts which evolve into radio-quiet quasars.
Global VLBI imaging of the luminous IR galaxy Arp 220 reveals four major emission regions in the 1667 MHz line, each with complex spatial and velocity structure showing intriguing symmetries, suggestive of an AGN-related phenomenon. The continuum emission consists of about a dozen sub-mJy unresolved features which appear unrelated to the maser emission, and are interpreted as radio supernovae in an ongoing nuclear starburst.
We have written a suite of programs designed to deal with problems peculiar to short-wavelength VLBI. The most important difference between mm-VLBI and cm-VLBI is that one usually wishes to integrate the data for times which are long compared to the atmospheric coherence time. This causes difficulties with standard VLBI data reduction techniques, and mandates the use of the algorithms implemented in our package for accurate results.
The radio source 3C205 is identified with a quasar of redshift 1.53, and is, at first glance, a standard, powerful double radio source with hotspots. Upon closer examination (MERLIN, VLA A-array), the southern lobe is seen to consist of two distinct hotspots plus some more diffuse emission, some 50 kiloparsecs from the quasar (Ho ∼ 75). The arcsecond-scale morphology and polarization structure of this hotspot complex strongly suggests that the larger and weaker off-axis secondary hotspot has been formed by outflow of material from the more compact primary hotspot (Lonsdale and Barthel 1984,6). This interpretation implies a collision between the presumed energy supply beam and something capable of deflecting the flow energy towards the secondary. Because the flow is so energetic, this obstacle would have to be both massive and dense. The main constraint on the density of the obstacle comes from measurements of the internal energy density of the compact primary hotspot coupled with model-dependent estimates for the maximum advance velocity of this feature, thus yielding a balance with the ram-pressure exerted by the obstacle medium.
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