10,568 research outputs found
Large Area Crop Inventory Experiment (LACIE). Evaluation of multitemporal data enchancements for the identification of winter wheat fields
There are no author-identified significant results in this report
Venus and Mars nominal natural environment for advanced manned planetary mission programs
Nominal natural environment values for advanced manned planetary space flights to Venus and Mar
Intra-day variability observations and the VLBI structure analysis of quasar S4 0917+624
The IDV observations of S4 0917+624 were carried out monthly, from August
2005 to January 2010, with the Urumqi 25m radio telescope at 4.8 GHz. The
quasar S4 0917+624 exhibits only very weak or no IDV during our 4.5 year
observing interval. Prior to the year 2000, the source S4 0917+624 was one of
the most prominent IDV sources. Our new data indicate that the previous strong
IDV has ceased. We analyzed the long-term VLBI structural variability using
Gaussian model-fitting. From this we obtained the flux densities and the
deconvolved sizes of core and inner-jet components of the source. We studied
the properties such as core fraction, angular size, spectral index, and
brightness temperature of VLBI core for S4 0917+624, as well as the time delay
between 5 and 15 GHz variations, and compared them with the IDV properties of
S4 0917+624. The source shows ejection of several jet components that are
suspected to have partially reduced the IDV amplitude of S4 0917+624. However,
during 2005-2006, the VLBI core size was comparable to the size before the year
2000, but no strong IDV was detected in the period, suggesting that the
quenching effect due to source size changes may not be responsible for the lack
of strong IDV after the year 2000. The refractive scattering properties for the
strong IDV phase of S4 0917+624 before the year 2000 are discussed. The
disappearance of strong IDV in S4 0917+624 after the year 2000 is a mystery and
cannot be explained via the quenching effect by changes in the observable VLBI
structure. However, it may be caused by changes in the interstellar medium,
i.e. by interstellar weather, which induces changes in the scintillation
pattern on timescales of several years. Further coordinated multi-frequency
observations will be required to distinguish between the effect of
source-intrinsic variability and changing properties of the interstellar
medium.Comment: 8 pages, 6 figures, accepted for publication in A&
Time scale, objectivity and irreversibility in quantum mechanics
It is argued that setting isolated systems as primary scope of field theory
and looking at particles as derived entities, the problem of an objective
anchorage of quantum mechanics can be solved and irreversibility acquires a
fundamental role. These general ideas are checked in the case of the Boltzmann
description of a dilute gas.Comment: 13 pages, latex, no figures, to appear in the Proceedings of the XXI
International Colloquium on Group Theoretical Methods in Physics, 1996
(Goslar, Germany
Decoherence in an exactly solvable qubit model with initial qubit-environment correlations
We study a model of dephasing (decoherence) in a two-state quantum system
(qubit) coupled to a bath of harmonic oscillators. An exact analytic solution
for the reduced dynamics of a two-state system in this model has been obtained
previously for factorizing initial states of the combined system. We show that
the model admits exact solutions for a large class of correlated initial states
which are typical in the theory of quantum measurements. We derive exact
expressions for the off-diagonal elements of the qubit density matrix, which
hold for an arbitrary strength of coupling between the qubit and the bath. The
influence of initial correlations on decoherence is considered for different
bath spectral densities. Time behavior of the qubit entropy in the decoherence
process is discussed.Comment: 10 pages, 5 figure
FERMI transient J1544-0649: a flaring radio-weak BL Lac
On May 15th, 2017, the \emph{FERMI}/LAT gamma-ray telescope observed a
transient source not present in any previous high-energy catalogue: J1544-0649.
It was visible for two consecutive weeks, with a flux peak on May 21st.
Subsequently observed by a \emph{Swift}/XRT follow-up starting on May 26, the
X-ray counterpart position was coincident with the optical transient
ASASSN-17gs = AT2017egv, detected on May 25, with a potential host galaxy at
=0.171. We conducted a 4-months follow-up in radio (Effelsberg-100m) and
optical (San Pedro M\'artir, 2.1m) bands, in order to build the overall
Spectral Energy Distribution (SED) of this object. The radio data from 5 to 15
GHz confirmed the flat spectrum of the source, favoring a line of sight close
to jet axis, not showing significant variability in the explored post-burst
time-window. The Rx ratio, common indicator of radio loudness, gives a value at
the border between the radio-loud and radio-quiet AGN populations. The
Ca H\&K break value (0.290.05) is compatible with the range
expected for the long-sought intermediate population between BL Lacs and FRI
radio galaxies. An overall SED fitting from Radio to -ray band shows
properties typical of a low-power BL Lac. As a whole, these results suggest
that this transient could well be a new example of the recently discovered
class of radio-weak BL Lac, showing for the first time a flare in the
gamma/X-ray bands.Comment: Accepted for publication in ApJ Letter
Optimal copying of entangled two-qubit states
We investigate the problem of copying pure two-qubit states of a given degree
of entanglement in an optimal way. Completely positive covariant quantum
operations are constructed which maximize the fidelity of the output states
with respect to two separable copies. These optimal copying processes hint at
the intricate relationship between fundamental laws of quantum theory and
entanglement.Comment: 13 pages, 7 figure
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