742 research outputs found
High-frequency VLBI observations of SgrA* during a multi-frequency campaign in May 2007
In May 2007 the compact radio source Sgr A* was observed in a global
multi-frequency monitoring campaign, from radio to X-ray bands. Here we present
and discuss first and preliminary results from polarization sensitive VLBA
observations, which took place during May 14-25, 2007. Here, Sgr A* was
observed in dual polarization on 10 consecutive days at 22, 43, and 86 GHz. We
describe the VLBI experiments, our data analysis, monitoring program and show
preliminary images obtained at the various frequencies. We discuss the data
with special regard also to the short term variability.Comment: 6 pages, 5 figures;necessary style files included; contribution for
the conference "The Universe under the Microscope" (AHAR 2008), held in Bad
Honnef (Germany) in April 2008, to be published in Journal of Physics:
Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart,
S. Pfalzner, and E. Ros (eds.
Evaluation of PCR primer selectivity and phylogenetic specificity by using amplification of 16S rRNA genes from betaproteobacterial ammonia-oxidizing bacteria in environmental samples
The effect of primer specificity for studying the diversity of ammonia-oxidizing betaproteobacteria (βAOB) was evaluated. βAOB represent a group of phylogenetically related organisms for which the 16S rRNA gene approach is especially suitable. We used experimental comparisons of primer performance with water samples, together with an in silico analysis of published sequences and a literature review of clone libraries made with four specific PCR primers for the βAOB 16S rRNA gene. With four aquatic samples, the primers NitA/NitB produced the highest frequency of ammonia-oxidizing-bacterium-like sequences compared to clone libraries with products amplified with the primer combinations βAMOf/βAMOr, βAMOf/Nso1255g, and NitA/Nso1225g. Both the experimental examination of ammonia-oxidizing-bacterium-specific 16S rRNA gene primers and the literature search showed that neither specificity nor sensitivity of primer combinations can be evaluated reliably only by sequence comparison. Apparently, the combination of sequence comparison and experimental data is the best approach to detect possible biases of PCR primers. Although this study focused on βAOB, the results presented here more generally exemplify the importance of primer selection and potential primer bias when analyzing microbial communities in environmental samples
Observations of Intrahour Variable Quasars: Scattering in our Galactic Neighbourhood
Interstellar scintillation (ISS) has been established as the cause of the
random variations seen at centimetre wavelengths in many compact radio sources
on timescales of a day or less. Observations of ISS can be used to probe
structure both in the ionized insterstellar medium of the Galaxy, and in the
extragalactic sources themselves, down to microarcsecond scales. A few quasars
have been found to show large amplitude scintillations on unusually rapid,
intrahour timescales. This has been shown to be due to weak scattering in very
local Galactic ``screens'', within a few tens of parsec of the Sun. The short
variability timescales allow detailed study of the scintillation properties in
relatively short observing periods with compact interferometric arrays. The
three best-studied ``intrahour variable'' quasars, PKS 0405-385, J1819+3845 and
PKS 1257-326, have been instrumental in establishing ISS as the principal cause
of intraday variability at centimetre wavelengths. Here we review the relevant
results from observations of these three sources.Comment: 10 pages, 4 figures, to appear in Astronomical and Astrophysical
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