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At sufficiently low frequencies, no ground-based radio array will be able to produce high resolution images while looking through the ionosphere. A space-based array will be needed to explore the objects and processes which dominate the sky at the lowest radio frequencies. An imaging radio interferometer based on a large number of small, inexpensive satellites would be able to track solar radio bursts associated with coronal mass ejections out to the distance of Earth, determine the frequency and duration of early epochs of nonthermal activity in galaxies, and provide unique information about the interstellar medium. This would be a space-space VLBI mission, as only baselines between satellites would be used. Angular resolution would be limited only by interstellar and interplanetary scattering.
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High precision astrometric Space Very Long Baseline Interferometry (S-VLBI) at the low end of the conventional frequency range, i.e. 20cm, is a requirement for a number of high priority science goals. These are headlined by obtaining trigonometric parallax distances to pulsars in Pulsar--Black Hole pairs and OH masers anywhere in the Milky Way Galaxy and the Magellanic Clouds. We propose a solution for the most difficult technical problems in S-VLBI by the MultiView approach where multiple sources, separated by several degrees on the sky, are observed simultaneously. We simulated a number of challenging S-VLBI configurations, with orbit errors up to 8m in size and with ionospheric atmospheres consistant with poor conditions. In these simulations we performed MultiView analysis to achieve the required science goals. This approach removes the need for beam switching requiring a Control Moment Gyro, and the space and ground infrastructure required for high quality orbit reconstruction of a space-based radio telescope. This will dramatically reduce the complexity of S-VLBI missions which implement the phase-referencing technique.
The radio sky at lower frequencies, particularly below 20 MHz, is expected to be a combination of increasingly bright non-thermal emission and significant absorption from intervening thermal plasma. The sky maps at these frequencies cannot therefore be obtained by simple extrapolation of those at higher frequencies. However, due to severe constraints in ground-based observations, this spectral window still remains greatly unexplored. In this paper, we propose and study, through simulations, a novel minimal configuration for a space interferometer system which would enable imaging of the radio sky at frequencies well below 20 MHz with angular resolutions comparable to those achieved at higher radio frequencies in ground-based observations by using the aperture-synthesis technique. The minimal configuration consists of three apertures aboard Low Earth Orbit (LEO) satellites orbiting the Earth in mutually orthogonal orbits. Orbital periods for the satellites are deliberately chosen to differ from each other so as to obtain maximum (u, v) coverage in short time spans with baselines greater than 15000 km, thus, giving us angular resolutions finer than 10 arcsec even at these low frequencies. The sensitivity of the (u, v) coverage is assessed by varying the orbit and the initial phase of the satellites. We discuss the results obtained from these simulations and highlight the advantages of such a system.
Recent work has made it clear that the ``standard model of pulsar radio emission cannot be the full answer. Some fundamental assumptions about the magnetic field and plasma flow in the radio-loud region have been called into question by recent observational and theoretical work, but the solutions to the problems posed are far from clear. It is time to formulate and carry out new observational campaigns designed to address these problems; sensitive low-frequency observations will an important part of such a campaign. Because pulsars are strong at low frequencies, we believe there will be a good number of candidates even for high-time-resolution single pulse work, as well as mean profile and integrated spectrum measurements. Such data can push the envelope of current models, test competing theories of the radio loud region, and possibly provide direct measures of the state of the emitting plasma.
Interstellar scattering is known to broaden distant objects spatially and temporally. The latter aspect is difficult to analyse, unless the signals carry their own time stamps. Pulsars are so kind to do us this favour. Typically the signature is a broadened image with little or no substructure and a similarly smooth exponential scattering tail in the temporal profile. The case of the pulsar B1508+55 is special: The profile shows additional components that are moving relative to the main pulse with time. We use low-frequency VLBI with LOFAR to test the hypothesis that these components are actually such scattering-induced echoes, by trying to detect the expected angular offset. Using international stations (plus the Kilpisjarvi Atmospheric Imaging Receiver Array KAIRA) and the phased-up core of the LOFAR array, we can do interferometry at high resolution in time and space. This contribution presents a selection of results from an ongoing large-scale monitoring campaign. We can not only detect the offset, but even image a full string of echoes, and relate the positions with delays. What we find is apparently consistent with scattering by highly aligned components in a single screen at a distance of 120 pc. Further investigations will improve our understanding of the scattering process as basis of using the scattering-induced subimages as arms of a giant interstellar interferometer with insanely high resolution.
We present new, low-frequency images of the powerful FR I radio galaxy Hydra A (3C 218). Images were made with the Very Large Array (VLA) at frequencies of 1415, 330, and 74 MHz, with resolutions on the order of 20. The morphology of the source is seen to be more complex and even larger than previously known, and extends nearly 8 (530 kpc) in a North-South direction. The southern lobe is bent to the east and extends in that direction for nearly 3 (200 kpc). In addition, we find that the northern lobe has a flatter spectral slope than the southern lobe, consistent with the appearance of greater confinement to the south. We measure overall spectral indices alpha^{330}_{74} = -0.83 and alpha^{1415}_{330} = -0.89.
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