Allen Telescope Array Multi-Frequency Observations of the Sun

We present the first observations of the Sun with the Allen Telescope Array (ATA). We used up to six frequencies, from 1.43 to 6 GHz, and baselines from 6 to 300 m. To our knowledge, these are the first simultaneous multifrequency full-Sun maps obtained at microwave frequencies without mosaicing. The observations took place when the Sun was relatively quiet, although at least one active region was present each time. We present multi-frequency flux budgets for each sources on the Sun. Outside of active regions, assuming optically thin bremsstrahlung (free--free) coronal emission on top of an optically thick ~10 000 K chromosphere, the multi-frequency information can be condensed into a single, frequency-independent, "coronal bremsstrahlung contribution function" [EM/sqrt(T)] map. This technique allows the separation of the physics of emission as well as a measurement of the density structure of the corona. Deviations from this simple relationship usually indicate the presence of an additional gyroresonance-emission component, as is typical in active regions.Comment: 16 pages, 11 figures. Accepted for publication in Solar Physic

The first three-dimensional reconstruction of a celestial object at radio wavelengths: Jupiter's radiation belts

. For an object where the emission is optically thin, it is shown that the visibility measured by an interferometer is a sample of the three-dimensional Fourier transform of the object. If the object rotates, then it is possible to sample this three-dimensional Fourier space adequately, and so reconstruct the object in three dimensions. Using this principle, a reconstructions of Jupiter's synchrotron radiation belts can be formed. This paper considers the principle and practice of this reconstruction process. Key words: planets and satellites: individual: Jupiter -- techniques: interferometric -- methods: data analysis 1. Introduction Astronomical observations normally produce images which are projections of the true sources onto the celestial sphere. Losing the three-dimensionality makes interpreting the results more difficult. Indeed, going from three to two dimensions loses information, in much the same way as going from a two-dimensional image to a one-dimensional scan. Clearly, ..

Spectral, polarization and time-lag properties of GRS 1915+105 radio oscillations

We report high-sensitivity dual-frequency observations of radio oscillations from GRS 1915+ 105 following the decay of a major flare event in 2000 July. The oscillations are clearly observed at both frequencies, and the time-resolved spectral index traces the events between optically thin and thick states. While previously anticipated from sparse observations and simple theory, this is the first time a quasi-periodic signal has been seen in the radio spectrum, and is a clear demonstration that flat radio spectra can arise from the combination of emission from optically thick and thin regions. In addition, we measure the linear polarization of the oscillations, at both frequencies, at a level of about 1-2 per cent, with a flat spectrum. Cross-correlating the two light curves we find a mean delay, in the sense that the emission at 8640 MHz leads that at 4800 MHz, of around 600 s. Comparison with frequency-dependent time-delays reported in the literature reveals that this delay is variable between epochs. We briefly discuss possible origins for a varying time-delay, and suggest possible consequences