67 research outputs found
Investigations of Intraday Variable Blazar Cores and the Connected Interstellar Medium
The work presented here is an attempt to shed further light upon the phenomenon of short-term radio variations (Intraday Variability, IDV) often observed in compact blazar cores. Three different new observational approaches were performed with particular emphasis on the connected scattering material in the local ISM. I present the results of a new Effelsberg 4.85 GHz IDV monitoring over the period of 2.5 years aiming the detection of seasonal cycles in the variability time scale of "classical" type-II IDV sources. While the most promising new candidate turned out to be BL Lac 0954+658, the annual cycle in quasar 0917+624 proposed earlier could not be confirmed due to the sudden death of rapid variability in this source since 2000. Here, additional 15 GHz VLBI observations were performed in parallel to our IDV monitoring suggesting an extremely quenched scattering scenario and a possible connection between intrinsic structural changes on (sub-) mas-scale and the dramatic changes in IDV characteristics. The attempt to detect an intrinsic contribution to the IDV pattern using high frequency IDV observations revealed no significant variations in three sources including BL Lac 0716+714 for which IDV in the mm-band was detected earlier. The results of a short multi-frequency observation of the 'rapid scintillator' J1819+3845 are in agreement with weak interstellar scintillation of a point source due to enhanced turbulence in the local bubble wall at a distance of about 15pc. Finally, the possibility of a direct detection of the scattering medium in front of IDV and extreme scattering event (ESE) sources is examined and discussed with emphasis on the various components of the (local) ISM, such as the WIM, local bubble, adjacent bubble/loop structures and their turbulent interaction zones. First spectral line observations led to the detection of a high latitude molecular cloud intervening the line of sight to 0954+658. Here, several findings suggest that the observed scattering effects (IDV and ESEs) are produced by the ionized CII component provided by the photo-dissociation region surrounding the CO-cloud
Hafenstädte in globaler Perspektive.: Einleitung
Port Cities in Global Perspective: An Introduction
This volume examines the phenomena of mobility and migration in port cities at the height of the steamship era, ca. 1860-1930. The inherent contradiction that ports were the source of a city’s (or a country’s or an Empire’s) esteem in the world on the one hand and that on the other, the ports’ unwanted and uncontrollable processes prompted negative reactions, is the common thread of all articles. Our approach to the study of port cities focuses on the contact in ports between people of different origin. This contact situation has the potential to put into question one’s concept of the city, the nation, the religious community, civilization, or even time. By consequence, it can threaten existing power structures and has the capacity to alter or reaffirm urban and national identities
Physical Conditions and Variability Processes in AGN Jets through Multi-Frequency Linear and Circular Radio Polarization Monitoring
Radio polarimetry is an invaluable tool to investigate the physical
conditions and variability processes in active galactic nuclei (AGN) jets.
However, detecting their linear and circular polarization properties is a
challenging endeavor due to their low levels and possible depolarization
effects. We have developed an end-to-end data analysis methodology to recover
the polarization properties of unresolved sources with high accuracy. It has
been applied to recover the linear and circular polarization of 87 AGNs
measured by the F-GAMMA program from July 2010 to January 2015 with a mean
cadence of 1.3 months. Their linear polarization was recovered at four
frequencies between 2.64 and 10.45 GHz and the circular polarization at 4.85
and 8.35 GHz. The physical conditions required to reproduce the observed
polarization properties and the processes which induce their variability were
investigated with a full-Stokes radiative transfer code which emulates the
synchrotron emission of modeled jets. The model was used to investigate the
conditions needed to reproduce the observed polarization behavior for the
blazar 3C 454.3, assuming that the observed variability is attributed to
evolving internal shocks propagating downstream.Comment: 6 pages, 2 figure
Radio QPO in the -ray-loud X-ray binary LS I +61303
LS I +61303 is a -ray emitting X-ray binary with periodic
radio outbursts with time scales of one month. Previous observations have
revealed microflares superimposed on these large outbursts with periods ranging
from a few minutes to hours. This makes LS I +61303, along with Cyg
X-1, the only TeV emitting X-ray binary exhibiting radio microflares. To
further investigate these microflaring activity in LS I +61303 we
observed the source with the 100-m Effelsberg radio telescope at 4.85, 8.35,
and 10.45 GHz and performed timing analysis on the obtained data. Radio
oscillations of 15 hours time scales are detected at all three frequencies. We
also compare the spectral index evolution of radio data to that of the photon
index of GeV data observed by Fermi-LAT. We conclude that the observed QPO
could result from multiple shocks in a jet.Comment: 5 pages, 4 figures, accepted for publication in MNRA
Filling in the Gaps in the 4.85 GHz Sky
We describe a 4.85 GHz survey of bright, flat-spectrum radio sources
conducted with the Effelsberg 100 m telescope in an attempt to improve the
completeness of existing surveys, such as CRATES. We report the results of
these observations and of follow-up 8.4 GHz observations with the VLA of a
subset of the sample. We comment on the connection to the WMAP point source
catalog and on the survey's effectiveness at supplementing the CRATES sky
coverage.Comment: 13 pages, 3 figures, 2 tables. Accepted for publication in the
Astronomical Journal. Tables available in electronic form:
http://astro.stanford.edu/gaps
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