29,071 research outputs found
On line power spectra identification and whitening for the noise in interferometric gravitational wave detectors
In this paper we address both to the problem of identifying the noise Power
Spectral Density of interferometric detectors by parametric techniques and to
the problem of the whitening procedure of the sequence of data. We will
concentrate the study on a Power Spectral Density like the one of the
Italian-French detector VIRGO and we show that with a reasonable finite number
of parameters we succeed in modeling a spectrum like the theoretical one of
VIRGO, reproducing all its features. We propose also the use of adaptive
techniques to identify and to whiten on line the data of interferometric
detectors. We analyze the behavior of the adaptive techniques in the field of
stochastic gradient and in the
Least Squares ones.Comment: 28 pages, 21 figures, uses iopart.cls accepted for pubblication on
Classical and Quantum Gravit
SARAS 2: A Spectral Radiometer for probing Cosmic Dawn and the Epoch of Reionization through detection of the global 21 cm signal
The global 21 cm signal from Cosmic Dawn (CD) and the Epoch of Reionization
(EoR), at redshifts , probes the nature of first sources of
radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the
signal is predicted to be extremely weak, of wide fractional bandwidth, and
lies in a frequency range that is dominated by Galactic and Extragalactic
foregrounds as well as Radio Frequency Interference, detection of the signal is
a daunting task. Critical to the experiment is the manner in which the sky
signal is represented through the instrument. It is of utmost importance to
design a system whose spectral bandpass and additive spurious can be well
calibrated and any calibration residual does not mimic the signal. SARAS is an
ongoing experiment that aims to detect the global 21 cm signal. Here we present
the design philosophy of the SARAS 2 system and discuss its performance and
limitations based on laboratory and field measurements. Laboratory tests with
the antenna replaced with a variety of terminations, including a network model
for the antenna impedance, show that the gain calibration and modeling of
internal additives leave no residuals with Fourier amplitudes exceeding 2~mK,
or residual Gaussians of 25 MHz width with amplitudes exceeding 2~mK. Thus,
even accounting for reflection and radiation efficiency losses in the antenna,
the SARAS~2 system is capable of detection of complex 21-cm profiles at the
level predicted by currently favoured models for thermal baryon evolution.Comment: 44 pages, 17 figures; comments and suggestions are welcom
Multiwavelength campaign on Mrk 509 XII. Broad band spectral analysis
(Abridged) The simultaneous UV to X-rays/gamma rays data obtained during the
multi-wavelength XMM/INTEGRAL campaign on the Seyfert 1 Mrk 509 are used in
this paper and tested against physically motivated broad band models. Each
observation has been fitted with a realistic thermal comptonisation model for
the continuum emission. Prompted by the correlation between the UV and soft
X-ray flux, we use a thermal comptonisation component for the soft X-ray
excess. The UV to X-rays/gamma-rays emission of Mrk 509 can be well fitted by
these components. The presence of a relatively hard high-energy spectrum points
to the existence of a hot (kT~100 keV), optically-thin (tau~0.5) corona
producing the primary continuum. On the contrary, the soft X-ray component
requires a warm (kT~1 keV), optically-thick (tau~15) plasma. Estimates of the
amplification ratio for this warm plasma support a configuration close to the
"theoretical" configuration of a slab corona above a passive disk. An
interesting consequence is the weak luminosity-dependence of its emission, a
possible explanation of the roughly constant spectral shape of the soft X-ray
excess seen in AGNs. The temperature (~ 3 eV) and flux of the soft-photon field
entering and cooling the warm plasma suggests that it covers the accretion disk
down to a transition radius of 10-20 . This plasma could be the
warm upper layer of the accretion disk. On the contrary the hot corona has a
more photon-starved geometry. The high temperature ( 100 eV) of the
soft-photon field entering and cooling it favors a localization of the hot
corona in the inner flow. This soft-photon field could be part of the
comptonised emission produced by the warm plasma. In this framework, the change
in the geometry (i.e. ) could explain most of the observed flux and
spectral variability.Comment: 19 pages, 14 figures. Accepted for publication in A&
Optical constants of silicon carbide for astrophysical applications. II. Extending optical functions from IR to UV using single-crystal absorption spectra
Laboratory measurements of unpolarized and polarized absorption spectra of
various samples and crystal stuctures of silicon carbide (SiC) are presented
from 1200--35,000 cm ( 8--0.28 m) and used to improve
the accuracy of optical functions ( and ) from the infrared (IR) to the
ultraviolet (UV). Comparison with previous 6--20 m
thin-film spectra constrains the thickness of the films and verifies that
recent IR reflectivity data provide correct values for in the IR region. We
extract and needed for radiative transfer models using a new
``difference method'', which utilizes transmission spectra measured from two
SiC single-crystals with different thicknesses. This method is ideal for
near-IR to visible regions where absorbance and reflectance are low and can be
applied to any material. Comparing our results with previous UV measurements of
SiC, we distinguish between chemical and structural effects at high frequency.
We find that for all spectral regions, 3C (-SiC) and the polarization of 6H (a type of -SiC) have almost identical
optical functions that can be substituted for each other in modeling
astronomical environments. Optical functions for of 6H SiC
have peaks shifted to lower frequency, permitting identification of this
structure below m. The onset of strong UV absorption for pure
SiC occurs near 0.2 m, but the presence of impurities redshifts the rise
to 0.33 m. Optical functions are similarly impacted. Such large
differences in spectral characteristics due to structural and chemical effects
should be observable and provide a means to distinguish chemical variation of
SiC dust in space.Comment: 46 pages inc. 8 figures and 2 full tables. Also 6 electronic-only
data files. Accepted by Ap
Calibration of the EDGES High-Band Receiver to Observe the Global 21-cm Signature from the Epoch of Reionization
The EDGES High-Band experiment aims to detect the sky-average brightness
temperature of the -cm signal from the Epoch of Reionization (EoR) in the
redshift range . To probe this redshifted signal,
EDGES High-Band conducts single-antenna measurements in the frequency range
MHz from the Murchison Radio-astronomy Observatory in Western
Australia. In this paper, we describe the current strategy for calibration of
the EDGES High-Band receiver and report calibration results for the instrument
used in the observational campaign. We propagate uncertainties in
the receiver calibration measurements to the antenna temperature using a Monte
Carlo approach. We define a performance objective of ~mK residual RMS after
modeling foreground subtraction from a fiducial temperature spectrum using a
five-term polynomial. Most of the calibration uncertainties yield residuals of
~mK or less at confidence. However, current uncertainties in the
antenna and receiver reflection coefficients can lead to residuals of up to
mK even in low-foreground sky regions. These dominant residuals could be
reduced by 1) improving the accuracy in reflection measurements, especially
their phase 2) improving the impedance match at the antenna-receiver interface,
and 3) decreasing the changes with frequency of the antenna reflection phase.Comment: Updated to match version accepted by Ap
Relaxed micromorphic modeling of the interface between a homogeneous solid and a band-gap metamaterial: new perspectives towards meta-structural design
In the present paper, the material parameters of the isotropic relaxed
micromorphic model derived for a specific metamaterial in a previous
contribution are used to model its transmission properties. Specifically, the
reflection and transmission coefficients at an interface between a homogeneous
solid and the chosen metamaterial are analyzed by using both the relaxed
micromorphic model and a direct FEM implementation of the detailed
microstructure. The obtained results show an excellent agreement between the
transmission spectra derived via our enriched continuum model and those issued
by the direct FEM simulation. Such excellent agreement validates the indirect
measure of the material parameters and opens the way towards an efficient
meta-structural design.Comment: The paper has been already accepted in Mathematics and Mechanics of
Solids as it i
New Measurements of Fine-Scale CMB Polarization Power Spectra from CAPMAP at Both 40 and 90 GHz
We present new measurements of the cosmic microwave background (CMB)
polarization from the final season of the Cosmic Anisotropy Polarization MAPper
(CAPMAP). The data set was obtained in winter 2004-2005 with the 7 m antenna in
Crawford Hill, New Jersey, from 12 W-band (84-100 GHz) and 4 Q-band (36-45 GHz)
correlation polarimeters with 3.3' and 6.5' beamsizes, respectively. After
selection criteria were applied, 956 (939) hours of data survived for analysis
of W-band (Q-band) data. Two independent and complementary pipelines produced
results in excellent agreement with each other. A broad suite of null tests as
well as extensive simulations showed that systematic errors were minimal, and a
comparison of the W-band and Q-band sky maps revealed no contamination from
galactic foregrounds. We report the E-mode and B-mode power spectra in 7 bands
in the range 200 < l < 3000, extending the range of previous measurements to
higher l. The E-mode spectrum, which is detected at 11 sigma significance, is
in agreement with cosmological predictions and with previous work at other
frequencies and angular resolutions. The BB power spectrum provides one of the
best limits to date on B-mode power at 4.8 uK^2 (95% confidence).Comment: 19 pages, 17 figures, 2 tables, submitted to Ap
- …