2,611 research outputs found
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
Kondo-Dicke resonances in electronic transport through triple quantum dots
Electronic transport through a triple quantum dot system, with only a single
dot coupled directly to external leads, is considered theoretically. The model
includes Coulomb correlations in the central dot, while such correlations in
the two side-coupled dots are omitted. The infinite-U mean-field slave-boson
approach is used to obtain basic transport characteristics in the Kondo regime.
When tuning position of the side-coupled dots' levels, transition from
subradiant to superradiant like mode (and vice versa) has been found in the
spectral function, in analogy to the Dicke effect in atomic physics. Bias
dependence of the differential conductance and zero frequency shot noise is
also analysed.Comment: 8 pages, 10 figure
Dynamic instabilities in resonant tunneling induced by a magnetic field
We show that the addition of a magnetic field parallel to the current induces
self sustained intrinsic current oscillations in an asymmetric double barrier
structure. The oscillations are attributed to the nonlinear dynamic coupling of
the current to the charge trapped in the well, and the effect of the external
field over the local density of states across the system. Our results show that
the system bifurcates as the field is increased, and may transit to chaos at
large enough fields.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Letter
Time Dependent Current Oscillations Through a Quantum Dot
Time dependent phenomena associated to charge transport along a quantum dot
in the charge quantization regime is studied. Superimposed to the Coulomb
blockade behaviour the current has novel non-linear properties. Together with
static multistabilities in the negative resistance region of the I-V
characteristic curve, strong correlations at the dot give rise to
self-sustained current and charge oscillations. Their properties depend upon
the parameters of the quantum dot and the external applied voltages.Comment: 4 pages, 3 figures; to appear in PR
Molecular Gas, Dust and Star Formation in Galaxies: II. Dust properties and scalings in \sim\ 1600 nearby galaxies
We aim to characterize the relationship between dust properties. We also aim
to provide equations to estimate accurate dust properties from limited
observational datasets.
We assemble a sample of 1,630 nearby (z<0.1) galaxies-over a large range of
Mstar, SFR - with multi-wavelength observations available from wise, iras,
planck and/or SCUBA. The characterization of dust emission comes from SED
fitting using Draine & Li dust models, which we parametrize using two
components (warm and cold ). The subsample of these galaxies with global
measurements of CO and/or HI are used to explore the molecular and/or atomic
gas content of the galaxies.
The total Lir, Mdust and dust temperature of the cold component (Tc) form a
plane that we refer to as the dust plane. A galaxy's sSFR drives its position
on the dust plane: starburst galaxies show higher Lir, Mdust and Tc compared to
Main Sequence and passive galaxies. Starburst galaxies also show higher
specific Mdust (Mdust/Mstar) and specific Mgas (Mgas/Mstar). The Mdust is more
closely correlated with the total Mgas (atomic plus molecular) than with the
individual components. Our multi wavelength data allows us to define several
equations to estimate Lir, Mdust and Tc from one or two monochromatic
luminosities in the infrared and/or sub-millimeter.
We estimate the dust mass and infrared luminosity from a single monochromatic
luminosity within the R-J tail of the dust emission, with errors of 0.12 and
0.20dex, respectively. These errors are reduced to 0.05 and 0.10 dex,
respectively, if the Tc is used. The Mdust is correlated with the total Mism
(Mism \propto Mdust^0.7). For galaxies with Mstar 8.5<log(Mstar/Msun) < 11.9,
the conversion factor \alpha_850mum shows a large scatter (rms=0.29dex). The SF
mode of a galaxy shows a correlation with both the Mgass and Mdust: high
Mdust/Mstar galaxies are gas-rich and show the highest SFRs.Comment: 24 pages, 28 figures, 6 tables, Accepted for publication in A&
Correlation Effects in Side-Coupled Quantum Dots
Using Wilson's numerical renormalization group (NRG) technique we compute
zero-bias conductance and various correlation functions of a double quantum dot
(DQD) system. We present different regimes within a phase diagram of the DQD
system. By introducing a negative Hubbard U on one of the quantum dots, we
simulate the effect of electron-phonon coupling and explore the properties of
the coexisting spin and charge Kondo state. In a triple quantum dot (TQD)
system a multi-stage Kondo effect appears where localized moments on quantum
dots are screened successively at exponentially distinct Kondo temperatures.Comment: 13 pages, 10 figure
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