2,973 research outputs found
Detection of TeV emission from the intriguing composite SNR G327.1-1.1
The shock wave of supernova remnants (SNRs) and the wind termination shock in
pulsar wind nebula (PWNe) are considered as prime candidates to accelerate the
bulk of Galactic cosmic ray (CR) ions and electrons. The SNRs hosting a PWN
(known as composite SNRs) provide excellent laboratories to test these
hypotheses. The SNR G327.1-1.1 belongs to this category and exhibits a shell
and a bright central PWN, both seen in radio and X-rays. Interestingly, the
radio observations of the PWN show an extended blob of emission and a curious
narrow finger structure pointing towards the offset compact X-ray source
indicating a possible fast moving pulsar in the SNR and/or an asymmetric
passage of the reverse shock. We report here on the observations, for a total
of 45 hours, of the SNR G327.1-1.1 with the H.E.S.S. telescope array which
resulted in the detection of TeV gamma-ray emission in spatial coincidence with
the PWN.Comment: Proceeding of the 32nd ICRC, August 11-18 2011, Beijing, Chin
Imprinting Patterns of Neutral Atoms in an Optical Lattice using Magnetic Resonance Techniques
We prepare arbitrary patterns of neutral atoms in a one-dimensional (1D)
optical lattice with single-site precision using microwave radiation in a
magnetic field gradient. We give a detailed account of the current limitations
and propose methods to overcome them. Our results have direct relevance for
addressing of planes, strings or single atoms in higher dimensional optical
lattices for quantum information processing or quantum simulations with
standard methods in current experiments. Furthermore, our findings pave the way
for arbitrary single qubit control with single site resolution.Comment: 9 pages, 7 figure
Precision preparation of strings of trapped neutral atoms
We have recently demonstrated the creation of regular strings of neutral
caesium atoms in a standing wave optical dipole trap using optical tweezers [Y.
Miroshnychenko et al., Nature, in press (2006)]. The rearrangement is realized
atom-by-atom, extracting an atom and re-inserting it at the desired position
with sub-micrometer resolution. We describe our experimental setup and present
detailed measurements as well as simple analytical models for the resolution of
the extraction process, for the precision of the insertion, and for heating
processes. We compare two different methods of insertion, one of which permits
the placement of two atoms into one optical micropotential. The theoretical
models largely explain our experimental results and allow us to identify the
main limiting factors for the precision and efficiency of the manipulations.
Strategies for future improvements are discussed.Comment: 25 pages, 18 figure
The Computational Power of Beeps
In this paper, we study the quantity of computational resources (state
machine states and/or probabilistic transition precision) needed to solve
specific problems in a single hop network where nodes communicate using only
beeps. We begin by focusing on randomized leader election. We prove a lower
bound on the states required to solve this problem with a given error bound,
probability precision, and (when relevant) network size lower bound. We then
show the bound tight with a matching upper bound. Noting that our optimal upper
bound is slow, we describe two faster algorithms that trade some state
optimality to gain efficiency. We then turn our attention to more general
classes of problems by proving that once you have enough states to solve leader
election with a given error bound, you have (within constant factors) enough
states to simulate correctly, with this same error bound, a logspace TM with a
constant number of unary input tapes: allowing you to solve a large and
expressive set of problems. These results identify a key simplicity threshold
beyond which useful distributed computation is possible in the beeping model.Comment: Extended abstract to appear in the Proceedings of the International
Symposium on Distributed Computing (DISC 2015
Adiabatic Quantum State Manipulation of Single Trapped Atoms
We use microwave induced adiabatic passages for selective spin flips within a
string of optically trapped individual neutral Cs atoms. We
position-dependently shift the atomic transition frequency with a magnetic
field gradient. To flip the spin of a selected atom, we optically measure its
position and sweep the microwave frequency across its respective resonance
frequency. We analyze the addressing resolution and the experimental robustness
of this scheme. Furthermore, we show that adiabatic spin flips can also be
induced with a fixed microwave frequency by deterministically transporting the
atoms across the position of resonance.Comment: 4 pages, 4 figure
Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model
International audienceCurrent theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N2] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions
Spatially Resolved Star Formation History Along the Disk of M82 Using Multi-Band Photometric Data
We present the results on the star formation history and extinction in the
disk of M82 over spatial scales of 10" (~180 pc). Multi-band photometric data
covering from the far ultraviolet to the near infrared bands were fitted to a
grid of synthetic spectral energy distributions. We obtained distribution
functions of age and extinction for each of the 117 apertures analyzed, taking
into account observational errors through Monte-Carlo simulations. These
distribution functions were fitted with gaussian functions to obtain the mean
ages and extinctions along with errors on them. The analyzed zones include the
high surface brightness complexes defined by O'Connell & Mangano (1978). We
found that these complexes share the same star formation history and extinction
as the field stellar populations in the disk. There is an indication that the
stellar populations are marginally older at the outer disk (450 Myr at ~3 kpc)
as compared to the inner disk (100 Myr at 0.5 kpc). For the nuclear regions
(radius less than 500 pc), we obtained an age of less than 10 Myr. The results
obtained in this work are consistent with the idea that the 0.5-3 kpc part of
the disk of M82 formed around 90% of the stellar mass in a star-forming episode
that started around 450 Myr ago lasting for about 350 Myr. We found that field
stars are the major contributors to the flux over the spatial scales analyzed
in this study, with stellar cluster contribution being 7% in the nucleus and
0.7% in the disk.Comment: 19 pages, 14 figures. Accepted for publication in The Astrophysical
Journa
K+ and K- production in heavy-ion collisions at SIS-energies
The production and the propagation of K+ and of K- mesons in heavy-ion
collisions at beam energies of 1 to 2 AGeV have systematically been
investigated with the Kaon Spectrometer KaoS at the SIS at the GSI. The ratio
of the K+ production excitation function for Au+Au and for C+C reactions
increases with decreasing beam energy, which is expected for a soft nuclear
equation-of-state. At 1.5 AGeV a comprehensive study of the K+ and of the K-
emission as a function of the size of the collision system, of the collision
centrality, of the kaon energy, and of the polar emission angle has been
performed. The K-/K+ ratio is found to be nearly constant as a function of the
collision centrality. The spectral slopes and the polar emission patterns are
different for K- and for K+. These observations indicate that K+ mesons
decouple earlier from the reaction zone than K- mesons.Comment: invited talk given at the SQM2003 conference in Atlantic Beach, USA
(March 2003), to be published in Journal of Physics G, 10pages, 7 figure
Direct observation of the effective bending moduli of a fluid membrane: Free-energy cost due to the reference-plane deformations
Effective bending moduli of a fluid membrane are investigated by means of the
transfer-matrix method developed in our preceding paper. This method allows us
to survey various statistical measures for the partition sum. The role of the
statistical measures is arousing much attention, since Pinnow and Helfrich
claimed that under a suitable statistical measure, that is, the local mean
curvature, the fluid membranes are stiffened, rather than softened, by thermal
undulations. In this paper, we propose an efficient method to observe the
effective bending moduli directly: We subjected a fluid membrane to a curved
reference plane, and from the free-energy cost due to the reference-plane
deformations, we read off the effective bending moduli. Accepting the
mean-curvature measure, we found that the effective bending rigidity gains even
in the case of very flexible membrane (small bare rigidity); it has been rather
controversial that for such non-perturbative regime, the analytical prediction
does apply. We also incorporate the Gaussian-curvature modulus, and calculated
its effective rigidity. Thereby, we found that the effective Gaussian-curvature
modulus stays almost scale-invariant. All these features are contrasted with
the results under the normal-displacement measure
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