3,711 research outputs found
Detection of Linear Modulations in the Presence of Strong Phase and Frequency Instabilities
Noncoherent sequence detection algorithms, recently proposed by the authors, have a performance which approaches that of coherent detectors and are robust to phase and frequency instabilities. These schemes exhibit a negligible performance loss in the presence of a frequency offset, provided this offset does not exceed an order of 1 % of the signaling frequency. For higher values, the performance rapidly degrades. In this paper, detection schemes are proposed, characterized by high robustness to frequency offsets and capable of tolerating offset values up to 10 % of the signaling frequency. More generally, these detection schemes are very robust to rapidly varying phase and frequency instabilities. The general case of coded linear modulations is addressed, with explicit reference to-ary phase shift keying and quadrature amplitude modulation
Near infrared flares of Sagittarius A*: Importance of near infrared polarimetry
We report on the results of new simulations of near-infrared (NIR)
observations of the Sagittarius A* (Sgr A*) counterpart associated with the
super-massive black hole at the Galactic Center. The observations have been
carried out using the NACO adaptive optics (AO) instrument at the European
Southern Observatory's Very Large Telescope and CIAO NIR camera on the Subaru
telescope (13 June 2004, 30 July 2005, 1 June 2006, 15 May 2007, 17 May 2007
and 28 May 2008). We used a model of synchrotron emission from relativistic
electrons in the inner parts of an accretion disk. The relativistic simulations
have been carried out using the Karas-Yaqoob (KY) ray-tracing code. We probe
the existence of a correlation between the modulations of the observed flux
density light curves and changes in polarimetric data. Furthermore, we confirm
that the same correlation is also predicted by the hot spot model. Correlations
between intensity and polarimetric parameters of the observed light curves as
well as a comparison of predicted and observed light curve features through a
pattern recognition algorithm result in the detection of a signature of
orbiting matter under the influence of strong gravity. This pattern is detected
statistically significant against randomly polarized red noise. Expected
results from future observations of VLT interferometry like GRAVITY experiment
are also discussed.Comment: 26 pages, 38 figures, accepted for publication by A&
Investigating Light Curve Modulation via Kernel Smoothing. I. Application to 53 fundamental mode and first-overtone Cepheids in the LMC
Recent studies have revealed a hitherto unknown complexity of Cepheid
pulsation. We implement local kernel regression to search for both period and
amplitude modulations simultaneously in continuous time and to investigate
their detectability, and test this new method on 53 classical Cepheids from the
OGLE-III catalog. We determine confidence intervals using parametric and
non-parametric bootstrap sampling to estimate significance and investigate
multi-periodicity using a modified pre-whitening approach that relies on
time-dependent light curve parameters. We find a wide variety of period and
amplitude modulations and confirm that first overtone pulsators are less stable
than fundamental mode Cepheids. Significant temporal variations in period are
more frequently detected than those in amplitude. We find a range of modulation
intensities, suggesting that both amplitude and period modulations are
ubiquitous among Cepheids. Over the 12-year baseline offered by OGLE-III, we
find that period changes are often non-linear, sometimes cyclic, suggesting
physical origins beyond secular evolution. Our method more efficiently detects
modulations (period and amplitude) than conventional methods reliant on
pre-whitening with constant light curve parameters and more accurately
pre-whitens time series, removing spurious secondary peaks effectively.Comment: Re-submitted including revisions to Astronomy and Astrophysic
Neutrino emission characteristics of black hole formation in three-dimensional simulations of stellar collapse
Neutrinos are unique probes of core-collapse supernova dynamics, especially
in the case of black hole (BH) forming stellar collapses, where the
electromagnetic emission may be faint or absent. By investigating two 3D
hydrodynamical simulations of BH-forming stellar collapses of mass 40 and 75
M_sun, we identify the physical processes preceding BH formation through
neutrinos, and forecast the neutrino signal expected in the existing IceCube
and Super-Kamiokande detectors, as well as in the future generation DUNE
facility. Prior to the abrupt termination of the neutrino signal corresponding
to BH formation, both models develop episodes of strong and long-lasting
activity by the spiral standing accretion shock instability (SASI). We find
that the spiral SASI peak in the Fourier power spectrum of the neutrino event
rate will be distinguishable at 3 sigma above the detector noise for distances
up to O(30) kpc in the most optimistic scenario, with IceCube having the
highest sensitivity. Interestingly, given the long duration of the spiral SASI
episodes, the spectrograms of the expected neutrino event rate carry clear
signs of the evolution of the blue spiral SASI frequency as a function of time,
as the shock radius and post-shock fluid velocity evolve. Due to the high
accretion luminosity and its large-amplitude SASI-induced modulations, any
contribution from asymmetric (dipolar or quadrupolar) neutrino emission
associated with the lepton emission self-sustained asymmetry (LESA) is far
subdominant in the neutrino signal.Comment: 25 pages, including 19 figures. Discussion on LESA expanded;
conclusions unchanged. Matches version accepted for publication in Phys. Rev.
D. Animated visualizations available at:
https://wwwmpa.mpa-garching.mpg.de/ccsnarchive/data/Walk2019
Enhancing Quantum Effects via Periodic Modulations in Optomechanical Systems
Parametrically modulated optomechanical systems have been recently proposed
as a simple and efficient setting for the quantum control of a micromechanical
oscillator: relevant possibilities include the generation of squeezing in the
oscillator position (or momentum) and the enhancement of entanglement between
mechanical and radiation modes. In this paper we further investigate this new
modulation regime, considering an optomechanical system with one or more
parameters being modulated over time. We first apply a sinusoidal modulation of
the mechanical frequency and characterize the optimal regime in which the
visibility of purely quantum effects is maximal. We then introduce a second
modulation on the input laser intensity and analyze the interplay between the
two. We find that an interference pattern shows up, so that different choices
of the relative phase between the two modulations can either enhance or cancel
the desired quantum effects.Comment: 10 pages, 4 figure
Follow-up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs
We present follow-up time-series photometric observations that confirm and
extend the results of the significant discovery made by Barlow et al.(2008)
that the Hot DQ white dwarfs SDSS J220029.08-074121.5 and SDSS
J234843.30-094245.3 are luminosity variable. These are the second and third
known members of a new class of pulsating white dwarfs, after the prototype
SDSS J142625.71+575218.3 (Montgomery et al. 2008). We find that the light curve
of SDSS J220029.08-074121.5 is dominated by an oscillation at 654.397+-0.056 s,
and that the light pulse folded on that period is highly nonlinear due to the
presence of the first and second harmonic of the main pulsation. We also
present evidence for the possible detection of two additional pulsation modes
with low amplitudes and periods of 577.576+-0.226 s and 254.732+-0.048 s in
that star. Likewise, we find that the light curve of SDSS J234843.30-094245.3
is dominated by a pulsation with a period of 1044.168+-0.012 s, but with no
sign of harmonic components. A new oscillation, with a low amplitude and a
period of 416.919+-0.004 s, is also probably detected in that second star. We
argue, on the basis of the very different folded pulse shapes, that SDSS
J220029.08-074121.5 is likely magnetic, while SDSS J234843.30-094245.3 is
probably not.Comment: 12 pages, 19 figures, accepted for publication in Ap
Fast time variations of supernova neutrino fluxes and their detectability
In the delayed explosion scenario of core-collapse supernovae (SNe), the
accretion phase shows pronounced convective overturns and a low-multipole
hydrodynamic instability, the standing accretion shock instability (SASI).
These effects imprint detectable fast time variations on the emerging neutrino
flux. Among existing detectors, IceCube is best suited to this task, providing
an event rate of ~1000 events per ms during the accretion phase for a fiducial
SN distance of 10 kpc, comparable to what could be achieved with a megaton
water Cherenkov detector. If the SASI activity lasts for several hundred ms, a
Fourier component with an amplitude of 1% of the average signal clearly sticks
out from the shot noise. We analyze in detail the output of axially symmetric
hydrodynamical simulations that predict much larger amplitudes up to
frequencies of a few hundred Hz. If these models are roughly representative for
realistic SNe, fast time variations of the neutrino signal are easily
detectable in IceCube or future megaton-class instruments. We also discuss the
information that could be deduced from such a measurement about the physics in
the SN core and the explosion mechanism of the SN.Comment: 14 pages, 11 figures. Final version accepted in PRD. Section on
astrophysical relevance and several references adde
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