10,271 research outputs found
Estimation of trend in state-space models: Asymptotic mean square error and rate of convergence
The focus of this paper is on trend estimation for a general state-space
model , where the th difference of the trend
is assumed to be i.i.d., and the error sequence
is assumed to be a mean zero stationary process. A fairly precise asymptotic
expression of the mean square error is derived for the estimator obtained by
penalizing the th order differences. Optimal rate of convergence is
obtained, and it is shown to be "asymptotically equivalent" to a nonparametric
estimator of a fixed trend model of smoothness of order . The results of
this paper show that the optimal rate of convergence for the stochastic and
nonstochastic cases are different. A criterion for selecting the penalty
parameter and degree of difference is given, along with an application to
the global temperature data, which shows that a longer term history has
nonlinearities that are important to take into consideration.Comment: Published in at http://dx.doi.org/10.1214/08-AOS675 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
The evolution of faint AGN between z~1 and z~5 from the COMBO-17 survey
We present a determination of the optical/UV AGN luminosity function and its
evolution, based on a large sample of faint (R<24) QSOs identified in the
COMBO-17 survey. Using multi-band photometry in 17 filters within 350nm <
lambda_obs < 930nm, we could simultaneously determine photometric redshifts
with an accuracy of sigma_z<0.03 and obtain spectral energy distributions. The
redshift range covered by the sample is 1.2<z<4.8, which implies that even at
z~3, the sample reaches below luminosities corresponding to M_B = -23,
conventionally employed to distinguish between Seyfert galaxies and quasars. We
clearly detect a broad plateau-like maximum of quasar activity around z~2 and
map out the smooth turnover between z~1 and z~4. The shape of the LF is
characterised by some mild curvature, but no sharp `break' is present within
the range of luminosities covered. Using only the COMBO-17 data, the evolving
LF can be adequately described by either a pure density evolution (PDE) or a
pure luminosity evolution (PLE) model. However, the absence of a strong L*-like
feature in the shape of the LF inhibits a robust distinction between these
modes. We present a robust estimate for the integrated UV luminosity generation
by AGN as a function of redshift. We find that the LF continues to rise even at
the lowest luminosities probed by our survey, but that the slope is
sufficiently shallow that the contribution of low-luminosity AGN to the UV
luminosity density is negligible. Although our sample reaches much fainter flux
levels than previous data sets, our results on space densities and LF slopes
are completely consistent with extrapolations from recent major surveys such as
SDSS and 2QZ.Comment: 17 pages, 14 figures, Astronomy & Astrophysics, in print, revised
versio
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