10,690 research outputs found
Electrophilic Aromatic Nitrosation. Isolation and X-ray Crystallography of the Metastable NO\u3csup\u3e+\u3c/sup\u3e Complex With Nitrosoarene
Isolation of the unstable 1â¶1 complex of 4-nitrosoanisole with NO+PF6â allows its precise X-ray structural characterization. The charge-transfer crystal is formed via strong NâŻN coordination [the distance of 1.938(5) Ă
corresponding to a Ï-bond order of â0.2] in the mean plane of the planar 4-nitrosoanisole donor. Thorough analysis of its molecular geometry in terms of valence resonance and MO schemes reveals a strong charge polarization with a local negative charge localized on the nitroso group and a local positive charge distributed over the adjacent p-methoxybenzyl moiety. Such a charge distribution accommodates the well-known passivation of nitrosoarenes to multiple nitrosation and explains the ease of demethylation of the complex. Comparison of a variety of nitroso- and nitroarene structures has shown that the nitrosoarene experiences a much stronger quinoidal distortion of the aromatic ring as compared with the latter. This indicates a stronger electron-withdrawing effect of the nitroso group relative to that of the nitro group. The weakened aromatic resonance in the nitrosoarenes could be responsible for the observed slower rate and the measurable isotope effect in electrophilic nitrosation as opposed to nitration
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
Transient gamma-ray emission from Cygnus X-3
The high-mass microquasar Cygnus X-3 has been recently detected in a flaring
state by the gamma-ray satellites Fermi and Agile. In the present contribution,
we study the high-energy emission from Cygnus X-3 through a model based on the
interaction of clumps from the Wolf-Rayet wind with the jet. The clumps inside
the jet act as obstacles in which shocks are formed leading to particle
acceleration and non-thermal emission. We model the high energy emission
produced by the interaction of one clump with the jet and briefly discus the
possibility of many clumps interacting with the jet. From the characteristics
of the considered scenario, the produced emission could be flare-like due to
discontinuous clump penetration, with the GeV long-term activity explained by
changes in the wind properties.Comment: Contribution to the proceedings of the 25th Texas Symposium on
Relativistic Astrophysics - TEXAS 2010, December 06-10, Heidelberg, German
Gamma rays from cloud penetration at the base of AGN jets
Dense and cold clouds seem to populate the broad line region surrounding the
central black hole in AGNs. These clouds could interact with the AGN jet base
and this could have observational consequences. We want to study the gamma-ray
emission produced by these jet-cloud interactions, and explore under which
conditions this radiation would be detectable. We investigate the
hydrodynamical properties of jet-cloud interactions and the resulting shocks,
and develop a model to compute the spectral energy distribution of the emission
generated by the particles accelerated in these shocks. We discuss our model in
the context of radio-loud AGNs, with applications to two representative cases,
the low-luminous Centaurus A, and the powerful 3C 273. Some fraction of the jet
power can be channelled to gamma-rays, which would be likely dominated by
synchrotron self-Compton radiation, and show typical variability timescales
similar to the cloud lifetime within the jet, which is longer than several
hours. Many clouds can interact with the jet simultaneously leading to fluxes
significantly higher than in one interaction, but then variability will be
smoothed out. Jet-cloud interactions may produce detectable gamma-rays in
non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A,
and steady in the case of powerful objects of FR II type.Comment: Accepted for publication in A&A (9 pages, 7 figures
High-energy flares from jet-clump interactions
High-mass microquasars are binary systems composed by a massive star and a
compact object from which relativistic jets are launched. Regarding the
companion star, observational evidence supports the idea that winds of hot
stars are formed by clumps. Then, these inhomogeneities may interact with the
jets producing a flaring activity. In the present contribution we study the
interaction between a jet and a clump of the stellar wind in a high-mass
microquasar. This interaction produces a shock in the jet, where particles may
be accelerated up to relativistic energies. We calculate the spectral energy
distributions of the dominant non-thermal processes: synchrotron radiation,
inverse Compton scattering, and proton-proton collisions. Significant levels of
X- and gamma-ray emission are predicted, with luminosities in the different
domains up to ~ 10^{34} - 10^{35} erg/s on a timescale of about ~ 1 h. Finally,
jet-clump interactions in high-mass microquasars could be detectable at high
energies. These phenomena may be behind the fast TeV variability found in some
high-mass X-ray binary systems, such as Cygnus X-1, LS 5039 and LS I+61 303. In
addition, our model can help to derive information on the properties of jets
and clumpy winds.Comment: Proceeding of the conference "High Energy Phenomena in Massive
Stars". Jaen (Spain), 2-5 February 200
On the nature of the AGILE galactic transient sources
The Italian gamma-ray satellite AGILE has recently reported the detection of
some variable high-energy sources likely of galactic origin. These sources do
not have any obvious counterpart at lower energies. We propose that these
sources are produced in proton-dominated jets of galactic microquasars. We
develop a model for microquasar jets that takes into account both primary
leptons and protons and all relevant radiative processes, including secondary
particle emission and gamma-ray attenuation due to pair creation in the jet. We
obtain spectral energy distributions that correspond to what is observed by
AGILE, with most of the power concentrated between 100 MeV and 10 GeV and
reaching luminosities of erg s. We make detailed spectral
predictions that can be tested by the Fermi gamma-ray telescope in the
immediate future. We conclude that hadronic jets in galactic accreting sources
can be responsible for the variable unidentified gamma-ray sources detected by
AGILE.Comment: 4 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysics (Letters
Dozens of compact and high velocity-dispersion early-type galaxies in Sloan Digital Sky Survey
Aims. We aim at finding candidates of potential survivors of high-redshift
compact galaxies in SDSS, as targets for more detailed follow-up observations.
Methods. From the virial theorem it is expected that for a given mass,
compact galaxies have stellar velocity dispersion higher than the mean due to
their smaller sizes. Therefore velocity dispersion coupled with size (or mass)
is an appropriate method to select relics, independent of the stellar
population properties. Based on these consideration we design a set of criteria
using distribution of early-type galaxies from SDSS on the
log(R)-log() plane to find the most extreme
objects on it.
Results. We find 76 galaxies at 0.05 < z < 0.2, which have properties similar
to the typical quiescent galaxies at high redshift. We study how well these
galaxies fit on well-known local universe relations of early-type galaxies such
as the fundamental plane, the red sequence or mass-size relations. As expected
from the selection criteria, the candidates are located in an extreme corner of
mass-size plane. However, they do not extend as deeply into the so-called zone
of exclusion as some of the high-redshift compact galaxies ('red nuggets')
found at high redshift, being a factor 2-3 less massive at a given intrinsic
scale size. Our candidates are systematically offset from scaling relations of
average early-type galaxies, while being in the mass-size range expected for
passive evolution of the red nuggets from their high redshift to the present.
Conclusions. The 76 selected candidates form a well suited set of objects for
further follow-up observations. We argue that selecting a high velocity
dispersion is the best way to find analogues of compact high redshift galaxies
in the local universe.Comment: 37 pages, 24 figures, accepted for publication in A&
Photoinduced Coupling of Acetylenes and Quinone in the Solid State as Preorganized DonorâAcceptor Pairs
Crystalline electron donorâacceptor (EDA) complexes of various diarylacetylenes (DA) and dichlorobenzoquinone (DB) are isolated and structurally characterized by X-ray crystallography. Deliberate excitation of either the DB acceptor at λDB = 355 nm or the 1:2 [DA, 2DB] complex at λCT = 532 nm in the solid state leads to [2 + 2] cycloaddition and identical (isomeric) mixtures of the quinone methide products. Time-resolved (ps) diffuse reflectance spectroscopy identifies the ion-radical pair [DAâą+, DBâą-] as the reactive intermediate derived by photoinduced electron transfer in both photochemical procedures. The effects of crystal-lattice control on the subsequent ion-radical pair dynamics are discussed in comparison with the same photocouplings of acetylenes and quinone previously carried out in solution
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