124,292 research outputs found
Searching with Measurement Dependent Noise
Consider a target moving with a constant velocity on a unit-circumference
circle, starting from an arbitrary location. To acquire the target, any region
of the circle can be probed for its presence, but the associated measurement
noise increases with the size of the probed region. We are interested in the
expected time required to find the target to within some given resolution and
error probability. For a known velocity, we characterize the optimal tradeoff
between time and resolution (i.e., maximal rate), and show that in contrast to
the case of constant measurement noise, measurement dependent noise incurs a
multiplicative gap between adaptive search and non-adaptive search. Moreover,
our adaptive scheme attains the optimal rate-reliability tradeoff. We further
show that for optimal non-adaptive search, accounting for an unknown velocity
incurs a factor of two in rate.Comment: Information Theory Workshop (ITW) 201
The BAT AGN Spectroscopic Survey -- XVIII. Searching for Supermassive Black Hole Binaries in the X-rays
Theory predicts that a supermassive black hole binary (SMBHB) could be
observed as a luminous active galactic nucleus (AGN) that periodically varies
on the order of its orbital timescale. In X-rays, periodic variations could be
caused by mechanisms including relativistic Doppler boosting and shocks. Here
we present the first systematic search for periodic AGNs using hard X-ray
light curves (14-195 keV) from the first 105 months of the Swift Burst Alert
Telescope (BAT) survey (2004-2013). We do not find evidence for periodic AGNs
in Swift-BAT, including the previously reported SMBHB candidate
MCG+1111032. We find that the null detection is consistent with the
combination of the upper-limit binary population in AGNs in our adopted model,
their expected periodic variability amplitudes, and the BAT survey
characteristics. We have also investigated the detectability of SMBHBs against
normal AGN X-ray variability in the context of the eROSITA survey. Under our
assumptions of a binary population and the periodic signals they produce which
have long periods of hundreds of days, up to % true periodic binaries can
be robustly distinguished from normal variable AGNs with the ideal uniform
sampling. However, we demonstrate that realistic eROSITA sampling is likely to
be insensitive to long-period binaries because longer observing gaps reduce
their detectability. In contrast, large observing gaps do not diminish the
prospect of detecting binaries of short, few-day periods, as 19% can be
successfully recovered, the vast majority of which can be identified by the
first half of the survey.Comment: 17 pages, including 8 figures and 4 tables. Accepted for publication
in Ap
Comparison of alternatives to amplitude thresholding for onset detection of acoustic emission signals
Acoustic Emission (AE) monitoring can be used to detect the presence of damage as well as determine its location in Structural Health Monitoring (SHM) applications. Information on the time difference of the signal generated by the damage event arriving at different sensors in an array is essential in performing localisation. Currently, this is determined using a fixed threshold which is particularly prone to errors when not set to optimal values. This paper presents three new methods for determining the onset of AE signals without the need for a predetermined threshold. The performance of the techniques is evaluated using AE signals generated during fatigue crack growth and compared to the established Akaike Information Criterion (AIC) and fixed threshold methods. It was found that the 1D location accuracy of the new methods was within the range of <1–7.1%<1–7.1% of the monitored region compared to 2.7% for the AIC method and a range of 1.8–9.4% for the conventional Fixed Threshold method at different threshold levels
Searching for reflected light from Bootis b with high-resolution ground-based spectroscopy: Approaching the contrast barrier
It is challenging to measure the starlight reflected from exoplanets because
of the extreme contrast with their host stars. For hot Jupiters, this contrast
is in the range of to , depending on their albedo, radius
and orbital distance. Searches for reflected light have been performed since
the first hot Jupiters were discovered, but with very limited success because
hot Jupiters tend to have low albedo values due to the general absence of
reflective cloud decks. The aim of this study is to search for reflected light
from Boo b, a hot Jupiter with one of the brightest host stars. Since
its discovery in 1997, it has been the subject of several reflected-light
searches using high-dispersion spectroscopy. Here we aim to combine these data
in to a single meta-analysis. We analysed more than 2,000 archival
high-dispersion spectra obtained with the UVES, ESPaDOnS, NARVAL UES and
HARPS-N spectrographs during various epochs between 1998 and 2013. Each
spectrum was first cleaned of the stellar spectrum and subsequently
cross-correlated with a PHOENIX model spectrum. These were then Doppler shifted
to the planet rest-frame and co-added in time, weighted according to the
expected signal-to-noise of the planet signal. We reach a 3 upper limit
of the planet to star contrast of . Assuming a planet
radius of 1.15 , this corresponds to an optical albedo of 0.12 between
400-700 nm. This low albedo is in line with secondary eclipse and phase curve
observations of other hot Jupiters using space-based observatories, as well as
theoretical predictions of their reflective properties.Comment: 15 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Searching for solar-like oscillations in the delta Scuti star rho Puppis
Despite the shallow convective envelopes of delta Scuti pulsators, solar-like
oscillations are theoretically predicted to be excited in those stars as well.
To search for such stochastic oscillations we organised a spectroscopic
multi-site campaign for the bright, metal-rich delta Sct star rho Puppis. We
obtained a total of 2763 high-resolution spectra using four telescopes. We
discuss the reduction and analysis with the iodine cell technique, developed
for searching for low-amplitude radial velocity variations, in the presence of
high-amplitude variability. Furthermore, we have determined the angular
diameter of rho Puppis to be 1.68 \pm 0.03 mas, translating into a radius of
3.52 \pm 0.07Rsun. Using this value, the frequency of maximum power of possible
solar-like oscillations, is expected at ~43 \pm 2 c/d (498 \pm 23 muHz). The
dominant delta Scuti-type pulsation mode of rho Puppis is known to be the
radial fundamental mode which allows us to determine the mean density of the
star, and therefore an expected large frequency separation of 2.73 c/d (31.6
muHz). We conclude that 1) the radial velocity amplitudes of the delta Scuti
pulsations are different for different spectral lines; 2) we can exclude
solar-like oscillations to be present in rho Puppis with an amplitude per
radial mode larger than 0.5 m/s.Comment: 14 pages, 12 figure, accepted for MNRA
A Search for Oxygen in the Low-Density Lyman-alpha Forest Using the Sloan Digital Sky Survey
We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search for
low-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption is
detected on a pixel-by-pixel basis by its correlation with Lyman-alpha forest
absorption. We have developed a novel Locally Calibrated Pixel (LCP) search
method that uses adjacent regions of the spectrum to calibrate interlopers and
spectral artifacts, which would otherwise limit the measurement of OVI
absorption. Despite the challenges presented by searching for weak OVI within
the Lyman-alpha forest in spectra of moderate resolution and signal-to-noise,
we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2
(the null hypothesis has a chi^2=80 for 9 data points).
We interpret our results using synthetic spectra generated from a lognormal
density field assuming a mixed quasar-galaxy photoionizing background (Haardt &
Madau 2001) and that it dominates the ionization fraction of detected OVI. The
LCP search data can be fit by a constant metallicity model with [O/H] =
-2.15_(-0.09)^(+0.07), but also by models in which low-density regions are
unenriched and higher density regions have a higher metallicity. The
density-dependent enrichment model by Aguirre et al. (2008) is also an
acceptable fit. All our successful models have similar mass-weighted oxygen
abundance, corresponding to [_MW] = -2.45+-0.06. This result can be used
to find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) =
1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on the
mass-weighted mean oxygen abundance and the cosmic oxygen density in the
Lyman-alpha forest to date and indicates that it contains approximately 16% of
metals produced by star formation (Bouch\'e et al. 2008) up to z = 3.Comment: 12 pages, 9 figures. Accepted by ApJ (minor changes
Fault-ignorant Quantum Search
We investigate the problem of quantum searching on a noisy quantum computer.
Taking a 'fault-ignorant' approach, we analyze quantum algorithms that solve
the task for various different noise strengths, which are possibly unknown
beforehand. We prove lower bounds on the runtime of such algorithms and thereby
find that the quadratic speedup is necessarily lost (in our noise models).
However, for low but constant noise levels the algorithms we provide (based on
Grover's algorithm) still outperform the best noiseless classical search
algorithm.Comment: v1: 15+8 pages, 4 figures; v2: 19+8 pages, 4 figures, published
version (Introduction section significantly expanded, presentation clarified,
results and order unchanged
Searching for chameleon-like scalar fields with the ammonia method
(Abridged) The ammonia method, which has been proposed to explore the
electron-to-proton mass ratio, mu = m_e/m_p, is applied to nearby dark clouds
in the Milky Way. This ratio, which is measured in different physical
environments of high (terrestrial) and low (interstellar) densities of baryonic
matter is supposed to vary in chameleon-like scalar field models, which predict
strong dependence of both masses and coupling constant on the local matter
density. High resolution spectral observations of molecular cores in lines of
NH3 (J,K) = (1,1), HC3N J = 2-1, and N2H+ J = 1-0 were performed at three radio
telescopes to measure the radial velocity offsets, DeltaV = V_rot - V_inv,
between the inversion transition of NH3 (1,1) and the rotational transitions of
other molecules with different sensitivities to the parameter dmm = (mu_obs -
mu_lab)/mu_lab. The measured values of DeltaV exhibit a statistically
significant velocity offset of 23 +/- 4_stat +/- 3_sys m/s. When interpreted in
terms of the electron-to-proton mass ratio variation, this infers that dmm =
(2.2 +/- 0.4_stat +/- 0.3_sys)x10^{-8}. If only a conservative upper bound is
considered, then the maximum offset between ammonia and the other molecules is
|DeltaV| <= 30 m/s. This gives the most accurate reference point at z = 0 for
dmm: |dmm| <= 3x10^{-8}.Comment: 23 pages, 11 figures, 6 tables. Accepted for publication in A&A.
Title and text corrected, references update
- …