11,390 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 1∶1 complex of 4-nitrosoanisole with NO+PF6− allows its precise X-ray structural characterization. The charge-transfer crystal is formed via strong N⋯N coordination [the distance of 1.938(5) Å corresponding to a σ-bond order of ≈0.2] 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
A model for high-mass microquasar jets under the influence of a strong stellar wind
Context. High-mass microquasars (HMMQs) are systems from which relativistic jets are launched. At the scales of several times the binary system size, the jets are expected to follow a helical path caused by the interaction with a strong stellar wind and orbital motion. Such a trajectory has its influence on the non-thermal emission of the jets, which also depends strongly on the observing angle due to Doppler boosting effects. Aims: We explore how the expected non-thermal emission of HMMQ jets at small scales is affected by the impact of the stellar wind and the orbital motion on the jet propagation. Methods: We studied the broadband non-thermal emission, from radio to gamma rays, produced in HMMQ jets up to a distance of several orbital separations, taking into account a realistic jet trajectory, different model parameters, and orbital modulation. The jet trajectory is computed by considering momentum transfer with the stellar wind. Electrons are injected at the position where a recollimation shock in the jets is expected due to the wind impact. Their distribution along the jet path is obtained assuming local acceleration at the recollimation shock, and cooling via adiabatic, synchrotron, and inverse Compton processes. The synchrotron and inverse Compton emission is calculated taking into account synchrotron self-absorption within the jet, free-free absorption with the stellar wind, and absorption by stellar photons via pair production. Results: The spectrum is totally dominated by the jet over the counter-jet due to Doppler boosting. Broadband emission from microwaves to gamma rays is predicted, with radio emission being totally absorbed. This emission is rather concentrated in the regions close to the binary system and features strong orbital modulation at high energies. Asymmetric light curves are obtained owing to the helical trajectory of the jets.Fil: Molina, E.. Universidad de Barcelona; EspañaFil: del Palacio, Santiago. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Bosch Ramon, Valentí. Universidad de Barcelona; Españ
Gamma-ray emission from massive stars interacting with AGN jets
Dense populations of stars surround the nuclear regions of galaxies. In
active galactic nuclei, these stars can interact with the relativistic jets
launched by the supermasive black hole. In this work, we study the interaction
of early-type stars with relativistic jets in active galactic nuclei. A
bow-shaped double-shock structure is formed as a consequence of the interaction
of the jet and the stellar wind of each early-type star. Particles can be
accelerated up to relativistic energies in these shocks and emit high-energy
radiation. We compute, considering different stellar densities of the galactic
core, the gamma-ray emission produced by non-thermal radiative processes. This
radiation may be significant in some cases, and its detection might yield
valuable information on the properties of the stellar population in the galaxy
nucleus, as well as on the relativistic jet. This emission is expected to be
particularly relevant for nearby non-blazar sources.Comment: Accepted for publication on MNRAS (15 pages, 9 figures
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
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
High-energy emission from jet-clump interactions in microquasars
High-mass microquasars are binary systems consisting of a massive star and an
accreting compact object from which relativistic jets are launched. There is
considerable observational evidence that winds of massive stars are clumpy.
Individual clumps may interact with the jets in high-mass microquasars to
produce outbursts of high-energy emission. Gamma-ray flares have been detected
in some high-mass X-ray binaries, such as Cygnus X-1, and probably in LS 5039
and LS I+61 303. We predict the high-energy emission produced by the
interaction between a jet and a clump of the stellar wind in a high-mass
microquasar. Assuming a hydrodynamic scenario for the jet-clump interaction, we
calculate the spectral energy distributions produced by the dominant
non-thermal processes: relativistic bremsstrahlung, synchrotron and inverse
Compton radiation, for leptons, and for hadrons, proton-proton collisions.
Significant levels of emission in X-rays (synchrotron), high-energy gamma rays
(inverse Compton), and very high-energy gamma rays (from the decay of neutral
pions) are predicted, with luminosities in the different domains in the range ~
10^{32}-10^{35} erg/s. The spectral energy distributions vary strongly
depending on the specific conditions. Jet-clump interactions may be detectable
at high and very high energies, and provide an explanation for the fast TeV
variability found in some high-mass X-ray binary systems.
Our model can help to infer information about the properties of jets and
clumpy winds by means of high-sensitivity gamma-ray astronomy.Comment: Accepted for publication in A&A (10 pages, 8 figures
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