3,555 research outputs found
Extreme Value GARCH modelling with Bayesian Inference
RePEC Working Paper Series No: 05/2009Extreme value theory is widely used financial applications such as risk analysis, forecasting and pricing models. One of the major difficulties in the applications to finance and economics is that the assumption of independence of time series observations is generally not satisfied, so that the dependent extremes may not necessarily be in the
domain of attraction of the classical generalised extreme value distribution. This study
examines a conditional extreme value distribution with the added specification that the extreme values (maxima or minima) follows a conditional autoregressive heteroscedasticity process. The dependence has been modelled by allowing the location and scale parameters of the extreme distribution to vary with time. The resulting combined
model, GEV-GARCH, is developed by implementing the GARCH volatility mechanism in these extreme value model parameters. Bayesian inference is used for the estimation of parameters and posterior inference is available through the Markov Chain Monte Carlo (MCMC) method. The model is firstly applied to relevant simulated data to verify model
stability and reliability of the parameter estimation method. Then real stock returns are
used to consider evidence for the appropriate application of the model. A comparison is
made between the GEV-GARCH and traditional GARCH models. Both the GEV-GARCH and GARCH show similarity in the resulting conditional volatility estimates, however the GEV-GARCH model differs from GARCH in that it can capture and explain extreme
quantiles better than the GARCH model because of more reliable extrapolation of the tail behaviour
X-ray flares in Orion young stars. I. Flare characteristics
Pre-main sequence (PMS) stars are known to produce powerful X-ray flares
which resemble magnetic reconnection solar flares scaled by factors up to 10^4.
However, numerous puzzles are present including the structure of X-ray emitting
coronae and magnetospheres, effects of protoplanetary disks, and effects of
stellar rotation. To investigate these issues in detail, we examine 216 of the
brightest flares from 161 PMS stars observed in the Chandra Orion Ultradeep
Project (COUP). These constitute the largest homogeneous dataset of PMS, or
indeed stellar flares at any stellar age, ever acquired. Our effort is based on
a new flare spectral analysis technique that avoids nonlinear parametric
modeling. It can be applied to much weaker flares and is more sensitive than
standard methods. We provide a catalog with >30 derived flare properties and an
electronic atlas for this unique collection of stellar X-ray flares. The
current study (Paper I) examines the flare morphologies, and provides general
comparison of COUP flare characteristics with those of other active X-ray stars
and the Sun. Paper II will concentrate on relationships between flare behavior,
protoplanetary disks, and other stellar properties. Several results are
obtained. First, the COUP flares studied here are among the most powerful,
longest, and hottest stellar X-ray flares ever studied. Second, no significant
statistical differences in peak flare luminosity or temperature distributions
are found among different morphological flare classes, suggesting a common
underlying mechanism for all flares. Third, comparison with the general
solar-scaling laws indicates that COUP flares may not fit adequately proposed
power-temperature and duration-temperature solar-stellar fits. Fourth, COUP
super-hot flares are found to be brighter but shorter than ... ABRIDGEDComment: Accepted for publication in ApJ (07/11/08); 63 pages, 16 figures, 4
table
Compressive Earth Observatory: An Insight from AIRS/AMSU Retrievals
We demonstrate that the global fields of temperature, humidity and
geopotential heights admit a nearly sparse representation in the wavelet
domain, offering a viable path forward to explore new paradigms of
sparsity-promoting data assimilation and compressive recovery of land
surface-atmospheric states from space. We illustrate this idea using retrieval
products of the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave
Sounding Unit (AMSU) on board the Aqua satellite. The results reveal that the
sparsity of the fields of temperature is relatively pressure-independent while
atmospheric humidity and geopotential heights are typically sparser at lower
and higher pressure levels, respectively. We provide evidence that these
land-atmospheric states can be accurately estimated using a small set of
measurements by taking advantage of their sparsity prior.Comment: 12 pages, 8 figures, 1 tabl
Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars
(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.Comment: 11 pages, 11 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pd
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