| 1. |
- Tateyama, Claudio E., et al.
(författare)
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Global 3- and 7-mm VLBI Observations of OJ 287
- 1996
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Ingår i: Nippon Tenmon Gakkai obun kenkyu hokoku. - Japan : Astronomical Society of Japan. - 0004-6264. ; 48:1, s. 37-44
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Tidskriftsartikel (refereegranskat)abstract
- We present new results from global VLBI observations of the highly active blazar OJ287 at 7 and 3 mm wavelengths. We show the first map of the mas to sub-mas structure of OJ287 at 43 GHz and discuss this image in the context with models of the source structure seen at 100 GHz.
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| 2. |
- Marcaide, J. M., et al.
(författare)
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Discovery of shell-like radio-structure in SN1993J
- 1995
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Ingår i: Nature. - London : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 373:6509, s. 44-45
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Tidskriftsartikel (refereegranskat)abstract
- SUPERNOVA explosions are poorly understood, partly because of difficulties in modelling them theoretically(1), and partly because there have been no supernovae observed in our Galaxy since the invention of the telescope. But the recent discovery(2) of supernova SN1993J in the nearby galaxy M81 offers an opportunity to investigate the evolution of the remnant, and its interaction with the surrounding interstellar medium, at high resolution. Here we present radio observations of SN1993J, made using very-long-baseline interferometry, which show the development of a shell structure. This 8-month-old radio shell is the youngest ever discovered in a supernova. The data suggest that the supernova explosion and the expanding shell of the remnant have nearly spherical symmetry, with small deviations where some parts of the shell are brighter than others. If these deviations arise because of variations in the density of the shell, this may reconcile earlier reports of symmetric radio emission(3) with the observed optical asymmetry(4,5), as the density variations could easily cause the latter. We infer that the radio emission is generated at the interface(6-9), where the surrounding gas is shocked by the ejecta.
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| 3. |
- Rantakyrö, F.T., et al.
(författare)
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50 μas resolution VLBI images of AGN’s at λ3 mm
- 1998
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Ingår i: Astronomy and Astrophysics. - Berlin : Springer-Verlag. - 0004-6361 .- 1432-0746. ; 131, s. 451-467
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Tidskriftsartikel (refereegranskat)abstract
- We present 15 images from the global mm-VLBI sessions in 1990 April at 100 GHz and 1993 April at 86 GHz. These observations probe the central engines of the 16 observed AGN's with up to 50 mu as resolution. Among other sources previously observed with lambda 3 mm VLBI we present the first lambda 3 mm maps of 0735+178, 0748+126, 1055+018, 2145+067, and CTA102, in total we have been able to image 13 out of the 16 observed sources. 6 out of the 13 imaged sources observed exhibit curvature and rapid structural changes, although the low dynamic range in two thirds of the maps limits the detection of weak features. Most of the sources have unresolved cores even at this high resolution. There is substantial evidence that the observed sources can be grouped into two general groups: A misaligned population with parsec scale jets in the form of low pitch helices and an aligned population with straight jets with small changes in PA due to intrinsic bends.
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| 4. |
- Alberdi, A., et al.
(författare)
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The high-frequency compact radio structure of the peculiar quasar 4C 39.25
- 1997
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Ingår i: Astronomy and Astrophysics. - : Springer-Verlag. - 0004-6361 .- 1432-0746. ; 327:2, s. 513-521
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Tidskriftsartikel (refereegranskat)abstract
- We present new high angular resolution images of the compact non-thermal radio source 4C 39.25 obtained from VLBI observations at λ1.3cm, λ7mm, and λ3mm wavelengths. These maps and Gaussian model-fits show that the milli-arcsecond to sub-milliarcsecond structure of 4C 39.25 consists of a complex bent core-jet structure with embedded moving and stationary VLBI components. Facilitated by the small observing beams and high angular resolutions obtained at mm-wavelengths, we measured the relative positions of the jet components with an accuracy of a few hundred micro-arcseconds. This allows the detailed followup of the ongoing merging process of a westward superluminally moving component (b_) with a stationary component a_, located at ~2.9 mas east of the putative core d_. In contrast to the other components of the structure with steeper spectra, the westernmost component d_ exhibits an inverted spectrum peaking between λ7mm and λ3mm, thus further supporting its identification as the VLBI core, self-absorbed at longer wavelengths. From two VLBI maps obtained nearly simultaneously at λ7mm and λ1.3cm, we made the first spectral index map of 4C 39.25 in this wavelength regime. The main characteristics of the spectral index distribution of the jet are pronounced changes of the spectral index between orientations parallel and transverse to the jet axis. Near the merging components a_ and b_ the spectral index steepens with increasing separation from d_. However, in the bridge of emission c_, which connects d_ with a_ and b_, the spectral index gradient has a direction transverse to the jet axis, suggesting a frequency dependent jet curvature and edge-brightening. A brief discussion of this behaviour within current jet models is presented.
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| 5. |
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| 6. |
- Frostegård, Å, et al.
(författare)
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Changes in microbial community structure during long-term incubation in two soils experimentally contaminated with metals
- 1996
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Ingår i: Soil Biology and Biochemistry. - 0038-0717. ; 28:1, s. 55-63
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Tidskriftsartikel (refereegranskat)abstract
- The effects of Zn contamination on the microbial community structure of a forest humus and an arable soil, as estimated by phospholipid fatty acid (PLFA) analysis, were followed during 18 months. The soils were contaminated at 10 different metal concentrations and incubated in plastic jars at 22°C. In both soils effects of heavy metal contamination could be detected after 2 weeks. Qualitatively similar changes in the PLFA pattern were found at the later sampling occasions, although the changes became more pronounced with prolonged incubation. In the forest soil the double-unsaturated 18:2ω6, indicating fungi, increased proportionally due to the metal amendment, while there was a strong negative effect of incubation on the fungal biomass in all samples of this soil type. In the arable soil 18:2ω6 showed a strong increase in response to the Zn pollution. As in the forest soil, incubation decreased the mol% of 18:2ω6, although the effect was less pronounced than in the forest soil. The proportions of several individual bacterial PLFAs changed in both soils due to the treatments, indicating shifts within the bacterial community in the soils, but these shifts could not be interpreted in terms of changes in the proportional abundance of specific taxonomic groups of bacteria. The ratio of 16:1ω7t-to-16:1ω7c, which has been proposed as a starvation index, increased in the forest soil due to Zn contamination. In the high-metal samples this ratio decreased during incubation, while it remained unchanged in the uncontaminated control. In the arable soil no clear effect was found on the trans-to-cis ratio either in response to metal contamination or to incubation. The ATP content decreased during incubation. Little or no effect was found on the total amount of PLFAs or on the lipid phosphate content, except after 18 months when these biomass measurements decreased.
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| 7. |
- Rantakyrö, Fredrik T., et al.
(författare)
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Multiband VLBI observations of CTA 102
- 1996
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 310:1, s. 66-74
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Tidskriftsartikel (refereegranskat)abstract
- The source CTA102, known to exhibit low frequency variability, has been observed at six epochs (three at λ32cm, two at λ18cm, and one at λ1.3cm) with intercontinental VLBI arrays. On the basis of the changes observed in the structure, we believe that the flux density variations at these wavelengths are due to intrinsic processes and not due to interstellar scintillation. This source exhibits behaviour suggestive of being expanding with a very high apparent transverse velocity.
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