30 research outputs found
Seyfert galaxies and "Unified Scheme"
From spectroscopic point of view Seyfert galaxies (as other Active Galactic Nuclei - AGN) basically are subdivided into two types: with and without broad permitted emission lines in their optical spectra (so called type I and type II Seyfert galaxies or AGNs). One of the most fundumental idea concerning AGN is that observed AGN type (I or II) is determined by inclination angle of AGN to the line of sight (LOS). At high inclination angles LOS crosses dusty torus which absorbs and scatters line emission. But in some recent papers the differences in close (< 100 kpc) environment of SyI and SyII (SyII have more close
companions), which are incompatible with Unification Scheme, were found and the possibility of physical (intrinsic) differences between Seyfert I and II was discussed. It was shown that this difference could be due to selection effects caused by the sample criteria. We sampled SyI and SyII galaxies from the Sloan Digital Sky Survey (SDSS) on the basis of their emission line properties thus excluding selection and discuss the properties of the environment of Seyfert galaxies
Circular polarization of AGNs on the parsec VLBI scales
Faraday effects possibly plays the major role in generation of circular polarization observed on Very Long Base Interferometry scales. Multi-frequency circular polarization measurementrs can become the desired breakthrough in understanding the active galactic nuclei jet physics and the only possibility to estimate some of their vital parameters. We review the possible mechanisms of circular polarization generation and their connection to the jet parameters. We throw a glimpse on the methods of data reduction and finally discuss our current observational progress and its possible interpretation
Radio Circular Polarization Produced in Helical Magnetic Fields in Eight Active Galactic Nuclei
Homan & Lister (2006) have recently published circular-polarization (CP)
detections for 34 objects in the MOJAVE sample - a set of bright, compact AGN
being monitored by the Very Long Baseline Array at 15 GHz. We report the
detection of 15-GHz parsec-scale CP in two more AGN (3C345 and 2231+114), and
confirm the MOJAVE detection of CP in 1633+382. It is generally believed that
the most likely mechanism for the generation of this CP is Faraday conversion
of linear polarization to CP. A helical jet magnetic-field (B-field) geometry
can facilitate this process - linearly polarized emission from the far side of
the jet is converted to CP as it passes through the magnetised plasma at the
front side of the jet on its way toward the observer. In this case, the sign of
the generated CP is essentially determined by the pitch angle and helicity of
the helical B field. We have determined the pitch-angle regimes and helicities
of the helical jet B fields in 8 AGN for which parsec-scale CP has been
detected, and used them to predict the expected CP signs for these AGN if the
CP is generated via conversion in these helical fields. We have obtained the
intriguing result that our predictions agree with the observed signs in all
eight cases, provided that the longitudinal B-field components in the jets
correspond to South magnetic poles. This clearly non-random pattern
demonstrates that the observed CP in AGN is directly associated with the
presence of helical jet B fields. These results suggest that helical B fields
are ubiquitous in AGN jets.Comment: 24 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
The 15-43 GHz parsec-scale circular polarization of 41 active galactic nuclei
We present the results of parsec-scale circular-polarization measurements based on Very Long Baseline Array data for a number of radio-bright, core-dominated active galactic nuclei obtained simultaneously at 15, 22 and 43 GHz. The degrees of circular polarization m(c) for the Very Long Baseline Interferometry (VLBI) core region at 15 GHz are similar to the values reported earlier at this wavelength, with typical values of a few tenths of a per cent. We find that mc as often rises as falls with increasing frequency between 15 and 22 GHz, while the degree of circular polarization at 43 GHz is in all cases higher than at 22 and 15 GHz. This behaviour seems contrary to expectations, since the degree of circular polarization from both synchrotron radiation and the Faraday conversion of linear to circular polarization - the two main mechanisms considered thus far in the literature - should decrease towards higher frequencies if the source is homogeneous. The increase in mc at 43 GHz may be due to the presence of regions of both positive and negative circular polarization with different frequency dependences ( but decreasing with increasing frequency) on small scales within the core region; alternatively, it may be associated with the intrinsic inhomogeneity of a Blandford-Konigl like jet. In several objects, the detected circular polarization appears to be near, but not coincident with, the core, although further observations are needed to confirm this. We find several cases of changes in sign with frequency, most often between 22 and 43 GHz. We find tentative evidence for transverse structure in the circular polarization of 1055+018 and 1334-127, that is consistent with their being generated by either the synchrotron mechanism or the Faraday conversion in a helical magnetic field. Our results confirm the earlier finding that the sign of the circular polarization at a given observing frequency is generally consistent across epochs separated by several years or more, suggesting stability of the magnetic-field orientation in the innermost jets
Do BL Lacs lack the circular polarization?
We present the first preliminary evidence of difference in the circular polarized emission between quasars and BL Lacs. BL Lac objects seem to lack the sources with high circular polarization degrees. It cannot be due to selection effects, so some intrinsic effects are involved. These possible differences can be attributed to jet composition, energy of particles or magnetic field properties. The first one can be the intriguing possibility of different central engine mechanisms of these objects. However, the small detection rate for BL Lac objects and therefore a small sample size still prevents us from making the firm conclusion, so further observations of BL Lacs are highly desired
Time-dependent radio emission from evolving jets
We investigated the time-dependent radiative and dynamical properties of
light supersonic jets launched into an external medium, using hydrodynamic
simulations and numerical radiative transfer calculations. These involved
various structural models for the ambient media, with density profiles
appropriate for galactic and extragalactic systems. The radiative transfer
formulation took full account of emission, absorption, re-emission, Faraday
rotation and Faraday conversion explicitly. High time-resolution intensity maps
were generated, frame-by-frame, to track the spatial hydrodynamical and
radiative properties of the evolving jets. Intensity light curves were computed
via integrating spatially over the emission maps. We apply the models to jets
in active galactic nuclei (AGN). From the jet simulations and the
time-dependent emission calculations we derived empirical relations for the
emission intensity and size for jets at various evolutionary stages. The
temporal properties of jet emission are not solely consequences of intrinsic
variations in the hydrodynamics and thermal properties of the jet. They also
depend on the interaction between the jet and the ambient medium. The
interpretation of radio jet morphology therefore needs to take account of
environmental factors. Our calculations have also shown that the environmental
interactions can affect specific emitting features, such as internal shocks and
hotspots. Quantification of the temporal evolution and spatial distribution of
these bright features, together with the derived relations between jet size and
emission, would enable us to set constraints on the hydrodynamics of AGN and
the structure of the ambient medium.Comment: 16 pages, 18 figures, MNRAS in press
Fiziologo-biokhimicheskie osobennosti mikroorganizmov-biodestruktorov polimernykh materialov
Abstract in Latvian and EnglishAvailable from Latvian Academic Library / LAL - Latvian Academic LibrarySIGLELVLatvi
Hepatotoxic and immune disorders in workers dealing with epoxy compounds, early revelation, correction and primary prophylaxis thereof
Available from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio