15,063 research outputs found
Clock spectroscopy of interacting bosons in deep optical lattices
We report on high-resolution optical spectroscopy of interacting bosonic
Yb atoms in deep optical lattices with negligible tunneling. We prepare
Mott insulator phases with singly- and doubly-occupied isolated sites and probe
the atoms using an ultra-narrow "clock" transition. Atoms in singly-occupied
sites undergo long-lived Rabi oscillations. Atoms in doubly-occupied sites are
strongly affected by interatomic interactions, and we measure their inelastic
decay rates and energy shifts. We deduce from these measurements all relevant
collisional parameters involving both clock states, in particular the intra-
and inter-state scattering lengths
Non-linear Relaxation of Interacting Bosons Coherently Driven on a Narrow Optical Transition
We study the dynamics of a two-component Bose-Einstein condensate (BEC) of
Yb atoms coherently driven on a narrow optical transition. The
excitation transfers the BEC to a superposition of states with different
internal and momentum quantum numbers. We observe a crossover with decreasing
driving strength between a regime of damped oscillations, where coherent
driving prevails, and an incoherent regime, where relaxation takes over.
Several relaxation mechanisms are involved: inelastic losses involving two
excited atoms, leading to a non-exponential decay of populations; Doppler
broadening due to the finite momentum width of the BEC and inhomogeneous
elastic interactions, both leading to dephasing and to damping of the
oscillations. We compare our observations to a two-component Gross-Pitaevskii
(GP) model that fully includes these effects. For small or moderate densities,
the damping of the oscillations is mostly due to Doppler broadening. In this
regime, we find excellent agreement between the model and the experimental
results. For higher densities, the role of interactions increases and so does
the damping rate of the oscillations. The damping in the GP model is less
pronounced than in the experiment, possibly a hint for many-body effects not
captured by the mean-field description.Comment: 7 pages, 4 figures; supplementary material available as ancillary
fil
Electromagnetic radiation initiated by hadronic jets from microquasars in the ISM
Microquasars are potential candidates to produce a non-negligible fraction of
the observed galactic cosmic rays. The protons accelerated at the jet
termination shock interact with the interstellar medium and may produce
detectable fluxes of extended emission at different energy bands: high-energy
and very high-energy gamma-rays produced by neutral pion-decay, synchrotron and
bremsstrahlung emission in a wide energy range generated by the secondary
electrons produced by charged pion-decay. We discuss the association between
this scenario and some of the unidentified EGRET sources in the galactic plane.Comment: (1)Universitat de Barcelona (2)Max Planck institute fur kernphysik,
11 pages, 14 figures, accepted for publication in Astronomy & Astrophysic
Broad-band electromagnetic radiation from microquasars interacting with ISM
Microquasars (MQs) are galactic objects with relativistic jets that
constitute a source population which can be responsible for production of a
non-negligible fraction of the observed galactic cosmic rays. These
relativistic protons, associated with the termination of the jet, interact with
the interstellar medium and, at certain surrounding conditions, may lead to
production of detectable fluxes of high-energy and very high-energy gamma-rays.
This radiation is accompanied by the broad-band emission of secondary electrons
from decays of -mesons produced through synchrotron, bremstrahlung and
inverse Compton process. The features of broad-band emission initiated by
proton-proton (pp) interactions in such a scenario is discussed in the context
of the strategy of search for counterparts of high-energy and very high-energy
gamma-ray sources in the galactic plane.Comment: High Energy Gamma-Ray Astronomy: 2nd International Symposium,
Proceedings of the conference held 26-30 July 2004 in Heidelberg (Germany).
Edited by Felix A. Aharonian, Heinz J. Volk, and Dieter Horns. AIP Conference
Proceedings, Volume 745. New York: American Institute of Physics, 2005.,
p.317-32
Leptonic emission from microquasar jets: from radio to very high-energy gamma-rays
Microquasars are sources of very high-energy gamma-rays and, very probably,
high-energy gamma-ray emitters. We propose a model for a jet that can allow to
give accurate observational predictions for jet emission at different energies
and provide with physical information of the object using multiwavelength data.Comment: 2 pages, 1 figure. Proceedings of the conference: "International
Astronomical Union Symposium No. 230: Populations of High Energy Sources in
Galaxies". Edited by Evert J.A. Meurs & Giuseppina Fabbian
Relative Riemann-Zariski spaces
In this paper we study relative Riemann-Zariski spaces attached to a morphism
of schemes and generalizing the classical Riemann-Zariski space of a field. We
prove that similarly to the classical RZ spaces, the relative ones can be
described either as projective limits of schemes in the category of locally
ringed spaces or as certain spaces of valuations. We apply these spaces to
prove the following two new results: a strong version of stable modification
theorem for relative curves; a decomposition theorem which asserts that any
separated morphism between quasi-compact and quasi-separated schemes factors as
a composition of an affine morphism and a proper morphism. (In particular, we
obtain a new proof of Nagata's compactification theorem.)Comment: 30 pages, the final version, to appear in Israel J. of Mat
Leptonic secondary emission in a hadronic microquasar model
Context: It has been proposed that the origin of the very high-energy photons
emitted from high-mass X-ray binaries with jet-like features, so-called
microquasars (MQs), is related to hadronic interactions between relativistic
protons in the jet and cold protons of the stellar wind. Leptonic secondary
emission should be calculated in a complete hadronic model that include the
effects of pairs from charged pion decays inside the jets and the emission from
pairs generated by gamma-ray absorption in the photosphere of the system. Aims:
We aim at predicting the broadband spectrum from a general hadronic microquasar
model, taking into account the emission from secondaries created by charged
pion decay inside the jet. Methods: The particle energy distribution for
secondary leptons injected along the jets is consistently derived taking the
energy losses into account. We also compute the spectral energy distribution
resulting from these leptons is calculated after assuming different values of
the magnetic field inside the jets. The spectrum of the gamma-rays produced by
neutral pion-decay and processed by electromagnetic cascades under the stellar
photon field. Results: We show that the secondary emission can dominate the
spectral energy distribution at low energies (~1 MeV). At high energies, the
production spectrum can be significantly distorted by the effect of
electromagnetic cascades. These effects are phase-dependent, and some
variability modulated by the orbital period is predicted.Comment: 8 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysic
Spatial Econometrics Revisited: A Case Study of Land Values in Roanoke County
Omitting spatial characteristics such as proximity to amenities from hedonic land value models may lead to spatial autocorrelation and biased and inefficient estimators. A spatial autoregressive error model can be used to model the spatial structure of errors arising from omitted spatial effects. This paper demonstrates an alternative approach to modeling land values based on individual and joint misspecification tests using data from Roanoke County in Virginia. Spatial autocorrelation is found in land value models of Roanoke County. Defining neighborhoods based on geographic and socioeconomics characteristics produces better estimates of neighborhood effects on land values than simple distance measures. Implementing a comprehensive set of individual and joint misspecification tests results in better correction for misspecification errors compared to existing practices.Land Economics/Use,
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