2,974 research outputs found
Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
The standard approach for time-resolved X-ray spectral analysis of
thermonuclear bursts involves subtraction of the pre-burst emission as
background. This approach implicitly assumes that the persistent flux remains
constant throughout the burst. We reanalyzed 332 photospheric radius expansion
bursts observed from 40 sources by the Rossi X-ray Timing Explorer, introducing
a multiplicative factor to the persistent emission contribution in our
spectral fits. We found that for the majority of spectra the best-fit value of
is significantly greater than 1, suggesting that the persistent emission
typically increases during a burst. Elevated values were not found solely
during the radius expansion interval of the burst, but were also measured in
the cooling tail. The modified model results in a lower average value of the
fit statistic, indicating superior spectral fits, but not yet to the
level of formal statistical consistency for all the spectra.
We interpret the elevated values as an increase of the mass accretion
rate onto the neutron star during the burst, likely arising from the effects of
Poynting-Robertson drag on the disk material. We measured an inverse
correlation of with the persistent flux, consistent with theoretical
models of the disc response. We suggest that this modified approach may provide
more accurate burst spectral parameters, as well as offering a probe of the
accretion disk structure.Comment: 15 pages, 9 figure
Lattice Models of Quantum Gravity
Standard Regge Calculus provides an interesting method to explore quantum
gravity in a non-perturbative fashion but turns out to be a CPU-time demanding
enterprise. One therefore seeks for suitable approximations which retain most
of its universal features. The -Regge model could be such a desired
simplification. Here the quadratic edge lengths of the simplicial complexes
are restricted to only two possible values , with
, in close analogy to the ancestor of all lattice theories, the
Ising model. To test whether this simpler model still contains the essential
qualities of the standard Regge Calculus, we study both models in two
dimensions and determine several observables on the same lattice size. In order
to compare expectation values, e.g. of the average curvature or the Liouville
field susceptibility, we employ in both models the same functional integration
measure. The phase structure is under current investigation using mean field
theory and numerical simulation.Comment: 4 pages, 1 figure
The Clustering Of Galaxies Around Radio-Loud AGNs
We examine the hypothesis that mergers and close encounters between galaxies
can fuel AGNs by increasing the rate at which gas accretes towards the central
black hole. We compare the clustering of galaxies around radio-loud AGNs with
the clustering around a population of radio-quiet galaxies with similar masses,
colors and luminosities. Our catalog contains 2178 elliptical radio galaxies
with flux densities greater than 2.8 mJy at 1.4 GHz from the 6dFGS survey. We
find that radio AGNs with more than 200 times the median radio power have, on
average, more close (r<160 kpc) companions than their radio-quiet counterparts,
suggestive that mergers play a role in forming the most powerful radio
galaxies. For ellipticals of fixed stellar mass, the radio power is not a
function of large-scale environment nor halo mass, consistent with the radio
powers of ellipticals varying by orders of magnitude over billions of years.Comment: 12 pages, 6 figure
Quantum simulation of lattice gauge theories using Wilson fermions
Quantum simulators have the exciting prospect of giving access to real-time
dynamics of lattice gauge theories, in particular in regimes that are difficult
to compute on classical computers. Future progress towards scalable quantum
simulation of lattice gauge theories, however, hinges crucially on the
efficient use of experimental resources. As we argue in this work, due to the
fundamental non-uniqueness of discretizing the relativistic Dirac Hamiltonian,
the lattice representation of gauge theories allows for an optimization that up
to now has been left unexplored. We exemplify our discussion with lattice
quantum electrodynamics in two-dimensional space-time, where we show that the
formulation through Wilson fermions provides several advantages over the
previously considered staggered fermions. Notably, it enables a strongly
simplified optical lattice setup and it reduces the number of degrees of
freedom required to simulate dynamical gauge fields. Exploiting the optimal
representation, we propose an experiment based on a mixture of ultracold atoms
trapped in a tilted optical lattice. Using numerical benchmark simulations, we
demonstrate that a state-of-the-art quantum simulator may access the Schwinger
mechanism and map out its non-perturbative onset.Comment: 19 pages, 11 figure
Quantum control of spin-correlations in ultracold lattice gases
We demonstrate that it is possible to prepare a lattice gas of ultracold
atoms with a desired non-classical spin-correlation function using atom-light
interaction of the kind routinely employed in quantum spin polarization
spectroscopy. Our method is based on quantum non-demolition (QND) measurement
and feedback, and allows in particular to create on demand exponentially or
algebraically decaying correlations, as well as a certain degree of
multi-partite entanglement.Comment: 2 figure
Dephasing of quantum dot exciton polaritons in electrically tunable nanocavities
We experimentally and theoretically investigate dephasing of zero dimensional
microcavity polaritons in electrically tunable single dot photonic crystal
nanocavities. Such devices allow us to alter the dot-cavity detuning in-situ
and to directly probe the influence on the emission spectrum of varying the
incoherent excitation level and the lattice temperature. By comparing our
results with theory we obtain the polariton dephasing rate and clarify its
dependence on optical excitation power and lattice temperature. For low
excitation levels we observe a linear temperature dependence, indicative of
phonon mediated polariton dephasing. At higher excitation levels, excitation
induced dephasing is observed due to coupling to the solid-state environment.
The results provide new information on coherence properties of quantum dot
microcavity polaritons.Comment: Figure 2, panel (b) changed to logarithmic + linear scal
Enhanced photoluminescence emission from two-dimensional silicon photonic crystal nanocavities
We present a temperature dependent photoluminescence study of silicon optical
nanocavities formed by introducing point defects into two-dimensional photonic
crystals. In addition to the prominent TO phonon assisted transition from
crystalline silicon at ~1.10 eV we observe a broad defect band luminescence
from ~1.05-1.09 eV. Spatially resolved spectroscopy demonstrates that this
defect band is present only in the region where air-holes have been etched
during the fabrication process. Detectable emission from the cavity mode
persists up to room-temperature, in strong contrast the background emission
vanishes for T > 150 K. An Ahrrenius type analysis of the temperature
dependence of the luminescence signal recorded either in-resonance with the
cavity mode, or weakly detuned, suggests that the higher temperature stability
may arise from an enhanced internal quantum efficiency due to the
Purcell-effect
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