2,092 research outputs found
Turnstile behaviour of the Cooper-pair pump
We have experimentally studied the behaviour of the so-called Cooper pair
pump (CPP) with three Josephson junctions, in the limit of small Josephson
coupling EJ < EC. These experiments show that the CPP can be operated as a
traditional turnstile device yielding a gate-induced current 2ef in the
direction of the bias voltage, by applying an RF-signal with frequency f to the
two gates in phase, while residing at the degeneracy node of the gate plane.
Accuracy of the CPP during this kind of operation was about 3% and the
fundamental Landau-Zener limit was observed to lie above 20 MHz. We have also
measured the current pumped through the array by rotating around the degeneracy
node in the gate plane. We show that this reproduces the turnstile-kind of
behavior. To overcome the contradiction between the obtained e-periodic
DC-modulation and a pure 2e-behaviour in the RF-measurements, we base our
observations on a general principle that the system always minimises its
energy. It suggests that if the excess quasiparticles in the system have a
freedom to tunnel, they will organize themselves to the configuration yielding
the highest current.Comment: 29 pages, 16 figures, uses REVTeX and graphicx-packag
The Relation between Radio Polarization and Gamma-ray Emission in AGN Jets
We have compared the parsec-scale jet linear polarization properties of the
Fermi LAT-detected and non-detected sources in the complete
flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE
sources, 30 were detected by the LAT during its first three months of
operation. We find that during the era since the launch of Fermi, the
unresolved core components of the LAT-detected jets have significantly higher
median fractional polarization at 15 GHz. This complements our previous
findings that these LAT sources have higher apparent jet speeds, brightness
temperatures and Doppler factors, and are preferentially found in higher
activity states.Comment: 6 pages, 3 figures, to appear in the proceedings of "High Energy
Phenomena In Relativistic Outflows II" (Buenos Aires, Argentina, October
26-30, 2009) International Journal of Modern Physics
Multi-frequency VLBA study of the blazar S5 0716+714 during the active state in 2004: I. Inner jet kinematics
We observed the blazar \object{0716+714} with the
VLBA during its active state in 2003-2004. In this paper we discuss
multi-frequency analysis of the inner jet (first 1 mas) kinematics. The
unprecedentedly dense time sampling allows us to trace jet components without
misidentification and to calculate the component speeds with good accuracy. In
the smooth superluminal jet we were able to identify and track three components
over time moving outwards with relatively high apparent superluminal speeds
(8.5-19.4 ), which contradicts the hypothesis of a stationary oscillating
jet in this source. Component ejections occur at a relatively high rate (once
in two months), and they are accompanied by mm-continuum outbursts.
Superluminal jet components move along wiggling trajectories, which is an
indication of actual helical motion. Fast proper motion and rapid decay of the
components suggest that this source should be observed with the VLBI at a rate
of at least once in one or two months in order to trace superluminal jet
components without confusion.Comment: 4 pages, 3 figures, Astronomy & Astrophysics Letter, language
corrections adde
Causal connection in parsec-scale relativistic jets: results from the MOJAVE VLBI survey
We report that active galactic nucleus (AGN) jets are causally connected on
parsec scales, based on 15 GHz Very Long Baseline Array (VLBA) data from a
sample of 133 AGN jets. This result is achieved through a new method for
measuring the product of the jet Lorentz factor and the intrinsic opening angle
Gamma*theta_j from measured apparent opening angles in flux density limited
samples of AGN jets. The Gamma*theta_j parameter is important for jet physics
because it is related to the jet-frame sidewise expansion speed and causal
connection between the jet edges and its symmetry axis. Most importantly, the
standard model of jet production requires that the jet be causally connected
with its symmetry axis, implying that Gamma*theta_j < 1. When we apply our
method to the MOJAVE flux density limited sample of radio loud objects, we find
Gamma*theta_j = 0.2, implying that AGN jets are causally connected. We also
find evidence that AGN jets viewed very close to the line of sight effectively
have smaller intrinsic opening angles compared with jets viewed more off-axis,
which is consistent with Doppler beaming and a fast inner spine/slow outer
sheath velocity field. Notably, gamma-ray burst (GRB) jets have a typical
Gamma*theta_j that is two orders of magnitude higher, suggesting that different
physical mechanisms are at work in GRB jets compared to AGN jets. A useful
application of our result is that a jet's beaming parameters can be derived.
Assuming Gamma*theta_j is approximately constant in the AGN jet population, an
individual jet's Doppler factor and Lorentz factor (and therefore also its
viewing angle) can be determined using two observable quantities: apparent jet
opening angle and the apparent speed of jet components.Comment: 9 pages, 4 figure
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