18,025 research outputs found
Neutrino emission from BL Lac objects: the role of radiatively inefficient accretion flows
The origin of the astrophysical high-energy neutrinos discovered by IceCube
is currently a major mystery. The recent detection of IceCube-170922A, a
300 TeV neutrino potentially correlated with the flaring -ray
source TXS 0506+056, directs attention toward BL Lac objects (BL Lacs), the
subclass of blazars with weak emission lines. While high-energy neutrinos can
be produced via photohadronic interactions between protons accelerated in their
jets and ambient low-energy photons, the density of the latter in such objects
had generally been thought to be too low for efficient neutrino emission. Here
we consider the role of radiatively inefficient accretion flows (RIAFs), which
can plausibly exist in the nuclei of BL Lacs, as the source of target photons
for neutrino production. Based on simple model prescriptions for the spectra of
RIAFs at different accretion rates, we find that they can be sufficienly
intense to allow appreciable neutrino emission for the class of
low-synchrotron-peak BL Lacs such as TXS 0506+056. In constrast, for
high-synchrotron-peak BL Lacs including Mkn 421 and Mkn 501, the contribution
of RIAFs is subdominant and their neutrino production efficiency can remain
low, consistent with their non-detection by IceCube to date.Comment: 5 pages, 4 figures, accepted to MNRAS as Lette
Exact diagonalization study of optical conductivity in two-dimensional Hubbard model
The optical conductivity \sigma(\omega) in the two-dimensional Hubbard model
is examined by applying the exact diagonalization technique to small square
clusters with periodic boundary conditions up to \sqrt{20} X \sqrt{20} sites.
Spectral-weight distributions at half filling and their doping dependence in
the 20-site cluster are found to be similar to those in a \sqrt{18} X \sqrt{18}
cluster, but different from 4 X 4 results. The results for the 20-site cluster
enable us to perform a systematic study of the doping dependence of the
spectral-weight transfer from the region of the Mott-gap excitation to
lower-energy regions. We discuss the dependence of the Drude weight and the
effective carrier number on the electron density at a large on-site Coulomb
interaction.Comment: 5 pages, 5 figure
Perturbation theory for localized solutions of sine-Gordon equation: decay of a breather and pinning by microresistor
We develop a perturbation theory that describes bound states of solitons
localized in a confined area. External forces and influence of inhomogeneities
are taken into account as perturbations to exact solutions of the sine-Gordon
equation. We have investigated two special cases of fluxon trapped by a
microresistor and decay of a breather under dissipation. Also, we have carried
out numerical simulations with dissipative sine-Gordon equation and made
comparison with the McLaughlin-Scott theory. Significant distinction between
the McLaughlin-Scott calculation for a breather decay and our numerical result
indicates that the history dependence of the breather evolution can not be
neglected even for small damping parameter
Generarized Cubic Model for BaTiO-like Ferroelectric Substance
We propose an order-disorder type microscopic model for BaTiO-like
Ferroelectric Substance. Our model has three phase transitions and four phases.
The symmetry and directions of the polarizations of the ordered phases agree
with the experimental results of BaTiO. The intermediate phases in our
model are known as an incompletely ordered phase, which appears in a
generalized clock model.Comment: 6 pages, 4figure
Detecting the Cold Spot as a Void with the Non-Diagonal Two-Point Function
The anomaly in the Cosmic Microwave Background known as the "Cold Spot" could
be due to the existence of an anomalously large spherical (few hundreds Mpc/h
radius) underdense region, called a "Void" for short. Such a structure would
have an impact on the CMB also at high multipoles l through Lensing. This would
then represent a unique signature of a Void. Modeling such an underdensity with
an LTB metric, we show that the Lensing effect leads to a large signal in the
non-diagonal two-point function, centered in the direction of the Cold Spot,
such that the Planck satellite will be able to confirm or rule out the Void
explanation for the Cold Spot, for any Void radius with a Signal-to-Noise ratio
of at least O(10).Comment: v1: 6 pages, 2 figures; v2: 6 pages, 2 figures, text improved, to
appear on JCA
Light Element Production in the Circumstellar Matter of Energetic Type Ic Supernovae
We investigate energetic type Ic supernovae as production sites for Li6 and
Be in the early stages of the Milky Way. Recent observations have revealed that
some very metal-poor stars with [Fe/H]<-2.5 possess unexpectedly high
abundances of Li6. Some also exbihit enhanced abundances of Be as well as N.
From a theoretical point of view, recent studies of the evolution of
metal-poor massive stars show that rotation-induced mixing can enrich the outer
H and He layers with C, N, and O (CNO) elements, particularly N, and at the
same time cause intense mass loss of these layers. Here we consider energetic
supernova explosions occurring after the progeniter star has lost all but a
small fraction of the He layer. The fastest portion of the supernova ejecta can
interact directly with the circumstellar matter (CSM), both composed of He and
CNO, and induce light element production through spallation and He-He fusion
reactions. The CSM should be sufficiently thick to energetic particles so that
the interactions terminate within its innermost regions. We calculate the
resulting Li6/O and Be9/O ratios in the ejecta+CSM material out of which the
very metal-poor stars may form. We find that they are consistent with the
observed values if the mass of the He layer remaining on the pre-explosion core
is 0.01-0.1 solar mass, and the mass fraction of N mixed in the He layer is
about 0.01. Further observations of Li6, Be and N at low metallicity should
provide critical tests of this production scenario.Comment: 12 pages, 2 figures, revised with referee suggestions, final version
accepted in ApJ Letter
Charge transport in two dimensional electron gas/superconductor junctions with Rashba spin-orbit coupling
We have studied the tunneling conductance in two dimensional electron gas /
insulator / superconductor junctions in the presence of Rashba spin-orbit
coupling (RSOC). It is found that for low insulating barrier the tunneling
conductance is suppressed by the RSOC while for high insulating barrier it is
almost independent of the RSOC. We also find the reentrant behavior of the
conductance at zero voltage as a function of RSOC for intermediate insulating
barrier strength. The results are essentially different from those predicted in
ferromagnet / superconductor junctions. The present derivation of the
conductance is applicable to arbitrary velocity operator with off-diagonal
components.Comment: 8 pages, 6 figure
Aerodynamic investigation of an air-cooled axial-flow turbine. Part 2: Rotor blade tip-clearance effects on overall turbine performance and internal gas flow conditions: Experimental results and prediction methods
Total turbine blade performance was investigated while changing the blade tip clearance in three ways. The internal flow at the moving blade outlet point was measured. Experimental results were compared with various theoretical methods. Increased blade clearance leads to decreased turbine efficiency
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