962 research outputs found
Variability and stability in optical blazar jets: the case of OJ287
OJ287 is a BL Lac object at redshift z=0.306 that has shown double-peaked
bursts at regular intervals of ~12 yr during the last ~ 40 yr. Due to this
behavior, it has been suggested that OJ287 might host a close supermassive
binary black hole. We present optical photopolarimetric monitoring data from
2005-2009, during which the latest double-peaked outburst occurred. We find a
stable component in the optical jet: the optical polarization core. The optical
polarization indicates that the magnetic field is oriented parallel to the jet.
Using historical optical polarization data, we trace the evolution of the
optical polarization core and find that it has showed a swing in the Stokes
plane indicating a reorientation of the jet magnetic field. We also find that
changes in the optical jet magnetic field seem tightly related to the
double-peaked bursts. We use our findings as a new constraint on possible
binary black hole models. Combining all available observations, we find that
none of the proposed binary black bole models is able to fully explain the
observations. We suggest a new approach to understanding OJ287 that is based on
the assumption that changes in the jet magnetic field drive the regular
outbursts.Comment: To appear in the proceedings of the meeting "Steady and Transient
Jets", held in Bonn, Germany (7-8 April 2010
Internal kinematics of spiral galaxies in distant clusters. Part II. Observations and data analysis
We have conducted an observing campaign with FORS at the ESO-VLT to explore
the kinematical properties of spiral galaxies in distant galaxy clusters. Our
main goal is to analyse transformation- and interaction processes of disk
galaxies within the special environment of clusters as compared to the
hierarchical evolution of galaxies in the field. Spatially resolved MOS-spectra
have been obtained for seven galaxy clusters at 0.3<z<0.6 to measure rotation
velocities of cluster members. For three of the clusters, Cl0303+17, Cl0413-65,
and MS1008-12, for which we presented results including a TF-diagram in Ziegler
et al. 2003, we describe here in detail the observations and data analysis.
Each of them was observed with two setups of the standard FORS MOS-unit.With
typical exposure times of >2 hours we reach an S/N>5 in the emission lines
appropriate for the deduction of the galaxies' internal rotation velocities
from [OII], Hbeta, or [OIII] profiles. Preselection of targets was done on the
basis of available redshifts as well as from photometric and morphological
information gathered from own observations, archive data, and from the
literature. Emphasis was laid on the definition of suitable setups to avoid the
typical restrictions of the standard MOS unit for this kind of observations. In
total we assembled spectra of 116 objects of which 50 turned out to be cluster
members. Position velocity diagrams, finding charts as well as tables with
photometric, spectral, and structural parameters of individual galaxies are
presented.Comment: 18 pages, 6 figures, accepted for publication in Astronomy and
Astrophysics. A version with full resolution figures can be downloaded from
http://www.uni-sw.gwdg.de/~vwgroup/publications.htm
Thermally Activated Magnetization and Resistance Decay during Near Ambient Temperature Aging of Co Nanoflakes in a Confining Semi-metallic Environment
We report the observation of magnetic and resistive aging in a self assembled
nanoparticle system produced in a multilayer Co/Sb sandwich. The aging decays
are characterized by an initial slow decay followed by a more rapid decay in
both the magnetization and resistance. The decays are large accounting for
almost 70% of the magnetization and almost 40% of the resistance for samples
deposited at 35 . For samples deposited at 50 the magnetization
decay accounts for of the magnetization and 50% of the resistance.
During the more rapid part of the decay, the concavity of the slope of the
decay changes sign and this inflection point can be used to provide a
characteristic time. The characteristic time is strongly and systematically
temperature dependent, ranging from x at 400K to x at 320K in samples deposited at . Samples deposited at 50
displayed a 7-8 fold increase in the characteristic time (compared to the samples) for a given aging temperature, indicating that this timescale may
be tunable. Both the temperature scale and time scales are in potentially
useful regimes. Pre-Aging, Scanning Tunneling Microscopy (STM) reveals that the
Co forms in nanoscale flakes. During aging the nanoflakes melt and migrate into
each other in an anisotropic fashion forming elongated Co nanowires. This aging
behavior occurs within a confined environment of the enveloping Sb layers. The
relationship between the characteristic time and aging temperature fits an
Arrhenius law indicating activated dynamics
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
Survey of Medium and Large Mammals in an Urban Park (Murray Park), Little Rock, Pulaski County, Arkansas
Because of increased environmental awareness by city planning commissions, there are more urban parks and greenbelt areas. These areas often result in increased human and wildlife contacts, thus resulting in the need for management plans regarding urban wildlife. From September 1998 to March 1999, we conducted mammal surveys of the urban greenspace Murray Park, Little Rock, Pulaski County, Arkansas. Surveys were conducted using five methods: direct observations; spot lighting; live trapping; animal sign; and scent posts. Species recorded included, opossum (Didelphis virginiana), nine-banded armadillo (Dasypus novemcinctus), fox squirrel (Sciurus niger), gray squirrel (Sciurus carolinensis), beaver (Castor canadensis), woodchuck (Marmota monax), muskrat (Ondatra zibethicus), eastern cottontail rabbit (Sylvilagus floridanus), swamp rabbit (Sylvilagus aquaticus), coyote (Canis latrans), red fox (Vulpes vulpes), raccoon (Procyon lotor), mink (Mustela vison), river otter (Lontra canadensis), bobcat (Lynx rufus), skunk sp., white-tailed deer (Odocoileus virginianus) and domestic dog (Canis familiaris) and cat (Felis sylvestris). These species represent 19 of the 23 mammals expected in surrounding natural areas. Management plans for urban wildlife need to include all mammals that potentially occur in the area
Qualification of electron-beam welded joints between copper and stainless steel for cryogenic application
Joints between copper and stainless steel are commonly applied in cryogenic systems. A relatively new and increasingly important method to combine these materials is electron-beam (EB) welding. Typically, welds in cryogenic applications need to withstand a temperature range from 300K down to 4 K, and pressures of several MPa. However, few data are available for classifying EB welds between OFHC copper and 316L stainless steel. A broad test program was conducted in order to qualify this kind of weld. The experiments started with the measurement of the hardness in the weld area. To verify the leak-tightness of the joints, integral helium leak tests at operating pressures of 16MPa were carried out at roomand at liquid nitrogen temperature. The tests were followed by destructive tensile tests at room temperature, at liquid nitrogen and at liquid helium temperatures, yielding information on the yield strength and the ultimate tensile strength of the welds at these temperatures. Moreover, nondestructive tensile tests up to the yield strength, i.e. the range in which the weld can be stressed during operation, were performed. Also, the behavior of the weld upon temperature fluctuations between room- and liquid nitrogen temperature was tested. The results of the qualification indicate that EB welded joints between OFHC copper and 316L stainless steel are reliable and present an interesting alternative to other technologies such as vacuum brazing or friction welding
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