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Early large scale radio surveys of the sky were made with instruments with poor imaging quality and were limited to measuring positions and brightnesses of discrete sources. In recent decades radio interferometric arrays have dramatically increased their speed, sensitivity and ability to image the sky and several large scale radio surveys are currently being made with imaging instruments. One of these surveys is discussed in this paper.
The accumulation of evidence now strongly favours interstellar scintillation (ISS) as the principal mechanism causing intra-day variability (IDV) at cm wavelengths. While ISS reduces the implied brightness temperatures, they remain uncomfortably high. The distance to the scattering screen is an important parameter in determining the actual brightness temperature encountered. The high brightness temperatures, the presence of strong and variable circular polarization and the observed lifetimes of a decade or more for several IDV sources, pose significant problems for synchrotron theory.
“The fault, dear Brutus, is not in our stars, but in ourselves, that we are underlings.” William Shakespeare, Julius Caesar
Intra-day variability (IDV) of active galactic nuclei (AGN) has been detected from gamma-ray energies to radio wavelengths. At high energies, such variability appears to be intrinsic to the sources themselves. However, at radio wavelengths, brightness temperatures as high as 1018 to 1021 K are encountered if the IDV is intrinsic to the source. We discuss here the accumulating evidence showing that, at radio wavelengths where the highest brightness temperatures are encountered, interstellar scintillation (ISS) is the principal mechanism causing IDV. While ISS reduces the implied brightness temperatures, they still remain uncomfortably high.
The ALFA mission is designed to map the entire sky at frequencies between approximately 0.3 and 30 MHz with angular resolution limited by interstellar and interplanetary scattering. Most of this region of the spectrum is inaccessible from the ground because of absorption and refraction by the Earth’s ionosphere. A wide range of astrophysical questions concerning solar system, galactic, and extragalactic objects could be answered with high resolution images at low frequencies, where absorption effects and coherent emission processes become important and the synchrotron lifetimes of electrons are comparable to the age of the universe.
We present multi-wavelength observations of the dark GRB 051008. The burst was not
detected in the optical bands, however we discover the host galaxy and secured the
redshift of the host with following campaign of multicolor observations of Shajn, NOT,
Gemini North and Keck telescopes. We provide arguments that the galaxy could be in a
complex of gravitationally bound galaxies. Our investigation of the GRB 051008 also
confirms a tendency of host galaxies of dark bursts to be more dusty.
The Christmas burst, GRB 101225A, was one of the most controversial bursts in the last few years. Its exceptionally long duration but bright X-ray emission showing a thermal component followed by a strange afterglow with a thermal SED lead to two different interpretations. We present here our model ascribing this strange event to a new type of GRB progenitor consisting of a neutron star and an evolved main-sequence star in a very faint galaxy at redshift 0.33 while Campana et al. (2011) proposed a Galactic origin. New observations at several wavelengths might resolve the question between the two models in the near future.
The activation of Si+ and Be+ ions implanted into InGaP, InGaAs or InAlAs grown by GSMBE and OMVPE was investigated as a function of ion dose and annealing temperature. Activation efficiencies close to 100% were obtained in InGaP and InGaAs for Be doses up to ∼1014 cm−2 and annealing temperatures of 700–850°C. Activation of Be was less efficient in InAlAs. By contrast, implanted Si displayed a saturation in active sheet electron densities at 1–3 × 1013 cm−2 and required higher annealing temperatures for optimum activation efficiency. High sheet resistance (≤108 μ/□) regions were created by O+ implantation into n+ InGaP or InAlAs, with hopping conduction dominating carrier transport in the bombarded material. For post-implant annealing temperatures above 750°C, the conductivity was restored to its initial value. No evidence was found for the creation of electrically active oxygen-related deep levels in either material.
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