636 research outputs found
Long term Ultra-Violet Variability of Seyfert galaxies
Flux variability is one of the defining characteristics of Seyfert galaxies,
a class of active galactic nuclei (AGN). Though these variations are observed
over a wide range of wavelengths, results on their flux variability
characteristics in the ultra-violet (UV) band are very limited. We present here
the long term UV flux variability characteristics of a sample of fourteen
Seyfert galaxies using data from the International Ultraviolet Explorer
acquired between 1978 and 1995. We found that all the sources showed flux
variations with no statistically significant difference in the amplitude of UV
flux variation between shorter and longer wavelengths. Also, the flux
variations between different near-UV (NUV, 1850 - 3300 A) and far-UV (FUV, 1150
- 2000 A) passbands in the rest frames of the objects are correlated with no
time lag. The data show indications of (i) a mild negative correlation of UV
variability with bolometric luminosity and (ii) weak positive correlation
between UV variability and black hole mass. At FUV, about 50% of the sources
show a strong correlation between spectral indices and flux variations with a
hardening when brightening behaviour, while for the remaining sources the
correlation is moderate. In NUV, the sources do show a harder when brighter
trend, however, the correlation is either weak or moderate.Comment: Accepted by Journal of Astrophysics and Astronom
The Lamb shift contribution of very light millicharged particles
The leading order vacuum polarization contribution of very light millicharged
fermions and scalar (spin-0) particles with charge \epsilon e and mass \mu to
the Lamb shift of the hydrogen atom is shown to imply universal, i.e.
\mu-independent, upper bounds on \epsilon: \epsilon \lsim 10^{-4} for \mu \lsim
1 keV in the case of fermions, and for scalars this bound is increased by a
factor of 2. This is in contrast to expectations based on the commonly used
approximation to the Uehling potential relevant only for conventionally large
fermion (and scalar) masses.Comment: 10 pages including 3 figures, version to appear in Physical Review D
(Rapid Communications
Optimisation of Retrofit Wall Insulation: An Irish case study
Ireland has one of the highest rates of emissions per capita in the world and its residential sector is responsible for approximately 10% of total national CO2 emissions. Therefore, reducing the CO2 emissions in this sector will play a decisive role in achieving EU targets of reducing emissions by 40% by 2030. To better inform decisions regarding retrofit of the existing building stock, this study proposes Optimum Insulation Thicknesses (OIT) for typical walls in 25 regions in Ireland. The calculation of OIT includes annual heat energy expenditure, CO2 emissions, and material payback period. The approach taken is based on Heating Degree Day (HDD) and life cycle cost analysis methods for different combinations of insulation material, heat energy type, and Irish wall configuration. Results indicate that OIT increases with increased HDD and varies up to 30% from lower to higher HDD regions in Ireland. The type of wall materials, configuration, insulation type, and heat energy type all have a significant impact on annual cost saving potential (up to 170 €/m2) and carbon emission (up to 50 kg/m2). The Analysis of Variance (ANOVA) technique is used to compare the significant mean difference between combinations under OIT and cost savings
Quantum anomalous Hall phase in synthetic bilayers via twistronics without a twist
We recently proposed quantum simulators of "twistronic-like" physics based on
ultracold atoms and syntheticdimensions [Phys. Rev. Lett. 125, 030504 (2020)].
Conceptually, the scheme is based on the idea that aphysical monolayer optical
lattice of desired geometry is upgraded to a synthetic bilayer system by
identifyingthe internal states of the trapped atoms with synthetic spatial
dimensions. The couplings between the internalstates, i.e. between sites on the
two layers, can be exquisitely controlled by laser induced Raman transitions.By
spatially modulating the interlayer coupling, Moir\'e-like patterns can be
directly imprinted on the latticewithout the need of a physical twist of the
layers. This scheme leads practically to a uniform pattern across thelattice
with the added advantage of widely tunable interlayer coupling strengths. The
latter feature facilitates theengineering of flat bands at larger "magic"
angles, or more directly, for smaller unit cells than in conventionaltwisted
materials. In this paper we extend these ideas and demonstrate that our system
exhibits topologicalband structures under appropriate conditions. To achieve
non-trivial band topology we consider imanaginarynext-to-nearest neighbor
tunnelings that drive the system into a quantum anomalous Hall phase. In
particular,we focus on three groups of bands, whose their Chern numbers triplet
can be associated to a trivial insulator(0,0,0), a standard non-trivial
(-1,0,1) and a non-standard non-trivial (-1,1,0). We identify regimes of
parameterswhere these three situations occur. We show the presence of an
anomalous Hall phase and the appearance oftopological edge states. Our works
open the path for experiments on topological effects in twistronics without
atwistComment: 11 pages, 10 figure
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