1,574 research outputs found
A Comparative Analysis of the Supernova Legacy Survey Sample with {\Lambda}CDM and the Universe
The use of Type~Ia SNe has thus far produced the most reliable measurement of
the expansion history of the Universe, suggesting that CDM offers the
best explanation for the redshift--luminosity distribution observed in these
events. But the analysis of other kinds of source, such as cosmic chronometers,
gamma ray bursts, and high- quasars, conflicts with this conclusion,
indicating instead that the constant expansion rate implied by the Universe is a better fit to the data. The central difficulty with the
use of Type~Ia SNe as standard candles is that one must optimize three or four
nuisance parameters characterizing supernova luminosities simultaneously with
the parameters of an expansion model. Hence in comparing competing models, one
must reduce the data independently for each. We carry~out such a comparison of
CDM and the Universe, using the Supernova Legacy Survey
(SNLS) sample of 252 SN~events, and show that each model fits its individually
reduced data very well. But since has only one free parameter
(the Hubble constant), it follows from a standard model selection technique
that it is to be preferred over CDM, the minimalist version of which
has three (the Hubble constant, the scaled matter density and either the
spatial curvature constant or the dark-energy equation-of-state parameter). We
estimate by the Bayes Information Criterion that in a pairwise comparison, the
likelihood of is , compared with only for
a minimalist form of CDM, in which dark energy is simply a
cosmological constant. Compared to , versions of the standard
model with more elaborate parametrizations of dark energy are judged to be even
less likely.Comment: 31 Pages, 5 Figures, 1 Table. Accepted for publication in A
Semiparametric Inference and Lower Bounds for Real Elliptically Symmetric Distributions
This paper has a twofold goal. The first aim is to provide a deeper
understanding of the family of the Real Elliptically Symmetric (RES)
distributions by investigating their intrinsic semiparametric nature. The
second aim is to derive a semiparametric lower bound for the estimation of the
parametric component of the model. The RES distributions represent a
semiparametric model where the parametric part is given by the mean vector and
by the scatter matrix while the non-parametric, infinite-dimensional, part is
represented by the density generator. Since, in practical applications, we are
often interested only in the estimation of the parametric component, the
density generator can be considered as nuisance. The first part of the paper is
dedicated to conveniently place the RES distributions in the framework of the
semiparametric group models. The second part of the paper, building on the
mathematical tools previously introduced, the Constrained Semiparametric
Cram\'{e}r-Rao Bound (CSCRB) for the estimation of the mean vector and of the
constrained scatter matrix of a RES distributed random vector is introduced.
The CSCRB provides a lower bound on the Mean Squared Error (MSE) of any robust
-estimator of mean vector and scatter matrix when no a-priori information on
the density generator is available. A closed form expression for the CSCRB is
derived. Finally, in simulations, we assess the statistical efficiency of the
Tyler's and Huber's scatter matrix -estimators with respect to the CSCRB.Comment: This paper has been accepted for publication in IEEE Transactions on
Signal Processin
A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*
The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the
existence of a compact emitting component within several Schwarzschild radii,
, of the nucleus at the Galactic Center. This is interesting in view of
the predicted circularized flow within , based on detailed
multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this
unusual object. In this paper, we examine the physics of magnetic field
generation by a Keplerian dynamo subject to the conditions pertaining to Sgr
A*, and show that the sub-mm bump can be produced by thermal synchrotron
emission in this inner region. This spectral feature may therefore be taken as
indirect evidence for the existence of this circularization. In addition, the
self-Comptonization of the sub-mm bump appears to produce an X-ray flux
exceeding that due to bremsstrahlung from this region, which may account for
the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However,
the required accretion rate in the Keplerian flow is orders of magnitude
smaller than that predicted by the Bondi-Hoyle simulations. We speculate that
rapid evaporation, in the form of a wind, may ensue from the heating associated
with turbulent mixing of gas elements with large eccentricity as they settle
down into a more or less circular (i.e., low eccentricity) trajectory. The
spectrum of Sgr A* longward of mm may be generated outside of the
Keplerian flow, where the gas is making a transition from a quasi-spherical
infall into a circularized pattern.Comment: 40 pages, 9 figure
Pharmacokinetic profiles of the active metamizole metabolites in healthy horses
Metamizole (MT) is an analgesic and antipyretic drug labelled for use in humans, horses, cattle, swine and dogs. MT is rapidly hydrolysed to the active primary metabolite 4-methylaminoantipyrine (MAA). MAA is formed in much larger amounts compared with other minor metabolites. Among other secondary metabolites, 4-aminoantipyrine (AA) is also relatively active. The aim of this research was to evaluate the pharmacokinetic profiles of MAA and AA after dose of 25 mg/kg MT by intravenous (i.v.) and intramuscular (i.m.) routes in healthy horses. Six horses were randomly allocated to two equally sized treatment groups according to a 2 9 2 crossover study design. Blood was collected at predetermined times within 24 h, and plasma was analysed by a validated HPLC-UV method. No behavioural changes or alterations in health parameters were observed in the i.v. or i.m. groups of animals during or after (up to 7 days) drug administration. Plasma concentrations of MAA after i.v. and i.m. administrations of MT were detectable from 5 min to 10 h in all the horses. Plasma concentrations of AA were detectable in the same range of time, but in smaller amounts. Maximum concentration (Cmax), time to maximum concentration (Tmax) and AUMC0-last of MAA were statistically different between the i.v. and i.m. groups. The AUCIM/AUCIV ratio of MAA was 1.06. In contrast, AUC0-last of AA was statistically different between the groups (P < 0.05) with an AUCIM/AUCIV ratio of 0.54.
This study suggested that the differences in the MAA and AA plasma concentrations found after i.m. and i.v. administrations of MT might have minor consequences on the pharmacodynamics of the drug
Long-Term Radio Modulation in Sagittarius A* from Spin-Induced Disk Precession
There is some evidence, though yet unconfirmed, that Sagittarius A*--the
supermassive black hole at the Galactic center--emits its radio waves modulated
with a ~100-day period. What is intriguing about this apparent
quasi-periodicity is that, though the amplitude of the modulation increases
with decreasing wavelength (from 3.6 to 1.3 cm), the quasi-period itself does
not seem to depend on the frequency of the radiation. It is difficult to
imagine how a binary companion, were that the cause of this modulation, could
have escaped detection until now. Instead, it has been suggested that the
spin-induced precession of a disk surrounding a slowly rotating black hole
could have the right period to account for this behavior. In this paper, we
examine how Sagittarius A*'s light curve could be modulated by this mechanism.
We demonstrate that the partial occultation of a nonthermal halo by a compact,
radio-opaque disk does indeed produce the observed frequency-dependent
amplitude. This appears to be in line with other observational arguments
suggesting that Sagittarius A*'s mm/sub-mm spectrum is produced by a ~10
Schwarzschild-radius disk, whereas its cm-waves originate from a nonthermal
particle distribution in a halo extending out to over 20 Schwarzschild radii.
Interestingly, this model suggests that the observed period corresponds to half
the precession period and that a non-axisymmetric disk could produce a second
period roughly twice as long as the first.Comment: 17 pages, 5 figures, accepted to Ap
An Accretion-Induced X-ray Flare in Sgr A*
The recent detection of a three-hour X-ray flare from Sgr A* by Chandra
provides very strong evidence for a compact emitting region near this
supermassive black hole at the Galactic center. Sgr A*'s mm/sub-mm spectrum and
polarimetric properties, and its quiescent-state X-ray flux density, are
consistent with a model in which low angular momentum gas captured at large
radii circularizes to form a hot, magnetized Keplerian flow within tens of
Schwarzschild radii of the black hole's event horizon. In Sgr A*'s quiescent
state, the X-ray emission appears to be produced by self-Comptonization (SSC)
of the mm/sub-mm synchrotron photons emitted in this region. In this paper, we
show that the prominent X-ray flare seen in Sgr A* may be due to a sudden
enhancement of accretion through the circularized flow. Depending on whether
the associated response of the anomalous viscosity is to increase or decrease
in tandem with this additional injection of mass, the X-ray photons during the
outburst may be produced either via thermal bremsstrahlung (if the viscosity
decreases), or via SSC (if the viscosity increases). However, the latter
predicts a softer X-ray spectrum than was seen by Chandra, so it appears that a
bremsstrahlung origin for the X-ray outburst is favored. A strong correlation
is expected between the mm/sub-mm and X-ray fluxes when the flare X-rays are
produced by SSC, while the correlated variability is strongest between the
sub-mm/far-IR and X-rays when bremsstrahlung emission is dominant during the
flare. In addition, we shows that future coordinated multi-wavelength
observations planned for the 2002 and 2003 cycles may be able to distinguish
between the accretion and jet scenarios.Comment: 15 pages, 3 figures, To appear in ApJ Lette
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