5,458 research outputs found
Correlations in the Spatial Power Spectrum Inferred from Angular Clustering: Methods and Application to APM
We reconsider the inference of spatial power spectra from angular clustering
data and show how to include correlations in both the angular correlation
function and the spatial power spectrum. Inclusion of the full covariance
matrices loosens the constraints on large-scale structure inferred from the APM
survey by over a factor of two. We present a new inversion technique based on
singular value decomposition that allows one to propagate the covariance matrix
on the angular correlation function through to that of the spatial power
spectrum and to reconstruct smooth power spectra without underestimating the
errors. Within a parameter space of the CDM shape Gamma and the amplitude
sigma_8, we find that the angular correlations in the APM survey constrain
Gamma to be 0.19-0.37 at 68% confidence when fit to scales larger than k=0.2h
Mpc^-1. A downturn in power at k<0.04h Mpc^-1 is significant at only 1-sigma.
These results are optimistic as we include only Gaussian statistical errors and
neglect any boundary effects.Comment: 37 pages, LaTex, 9 figures. Submitted to Ap
On the Nature of X-ray Variability in Ark 564
We use data from a recent long ASCA observation of the Narrow Line Seyfert 1
Ark 564 to investigate in detail its timing properties. We show that a thorough
analysis of the time series, employing techniques not generally applied to AGN
light curves, can provide useful information to characterize the engines of
these powerful sources.We searched for signs of non-stationarity in the data,
but did not find strong evidences for it. We find that the process causing the
variability is very likely nonlinear, suggesting that variability models based
on many active regions, as the shot noise model, may not be applicable to Ark
564. The complex light curve can be viewed, for a limited range of time scales,
as a fractal object with non-trivial fractal dimension and statistical
self-similarity. Finally, using a nonlinear statistic based on the scaling
index as a tool to discriminate time series, we demonstrate that the high and
low count rate states, which are indistinguishable on the basis of their
autocorrelation, structure and probability density functions, are intrinsically
different, with the high state characterized by higher complexity.Comment: 13 pages, 13 figures, accepted for publication in A&
Replicating financial market dynamics with a simple self-organized critical lattice model
We explore a simple lattice field model intended to describe statistical
properties of high frequency financial markets. The model is relevant in the
cross-disciplinary area of econophysics. Its signature feature is the emergence
of a self-organized critical state. This implies scale invariance of the model,
without tuning parameters. Prominent results of our simulation are time series
of gains, prices, volatility, and gains frequency distributions, which all
compare favorably to features of historical market data. Applying a standard
GARCH(1,1) fit to the lattice model gives results that are almost
indistinguishable from historical NASDAQ data.Comment: 20 pages, 33 figure
XMM-Newton View of PKS 2155-304: Characterizing the X-ray Variability Properties with EPIC-PN
Starting from XMM-Newton EPIC-PN data, we present the X-ray variability
characteristics of PKS 2155-304 using a simple analysis of the excess variance,
\xs, and of the fractional rms variability amplitude, fvar. The scatter in \xs\
and \fvar, calculated using 500 s long segments of the light curves, is smaller
than the scatter expected for red noise variability. This alone does not imply
that the underlying process responsible for the variability of the source is
stationary, since the real changes of the individual variance estimates are
possibly smaller than the large scatters expected for a red noise process. In
fact the averaged \xs and \fvar, reducing the fluctuations of the individual
variances, chang e with time, indicating non-stationary variability. Moreover,
both the averaged \sqxs (absolute rms variability amplitude) and \fvar show
linear correlation with source flux but in an opposite sense: \sqxs correlates
with flux, but \fvar anti-correlates with flux. These correlations suggest that
the variability process of the source is strongly non-stationary as random
scatters of variances should not yield any correlation. \fvar spectra were
constructed to compare variability amplitudes in different energy bands. We
found that the fractional rms variability amplitude of the source, when
significant variability is observed, increases logarithmically with the photon
energy, indicating significant spectral variability. The point-to-point
variability amplitude may also track this trend, suggesting that the slopes of
the power spectral density of the source are energy-independent. Using the
normalized excess variance the black hole mass of \pks was estimated to be
about . This is compared and contrasted with the
estimates derived from measurements of the host galaxies.Comment: Accepted for publication in The Astrophysical Journa
Mind the Gap: Tightening the Mass-Richness Relation with Magnitude Gaps
We investigate the potential to improve optical tracers of cluster mass by
exploiting measurements of the magnitude gap, m12, defined as the difference
between the r-band absolute magnitude of the two brightest cluster members. We
find that in a mock sample of galaxy groups and clusters constructed from the
Bolshoi simulation, the scatter about the mass-richness relation decreases by
15-20% when magnitude gap information is included. A similar trend is evident
in a volume-limited, spectroscopic sample of galaxy groups observed in the
Sloan Digital Sky Survey (SDSS). We find that SDSS groups with small magnitude
gaps are richer than large-gap groups at fixed values of the one-dimensional
velocity dispersion among group members sigma_v, which we use as a mass proxy.
We demonstrate explicitly that m12 contains information about cluster mass that
supplements the information provided by group richness and the luminosity of
the brightest cluster galaxy, L_bcg. In so doing, we show that the luminosities
of the members of a group with richness N are inconsistent with the
distribution of luminosities that results from N random draws from the global
galaxy luminosity function. As the cosmological constraining power of galaxy
clusters is limited by the precision in cluster mass determination, our
findings suggest a new way to improve the cosmological constraints derived from
galaxy clusters.Comment: references adde
Re-visiting the relations: Galactic thin disc age-velocity dispersion relation
The velocity dispersion of stars in the solar neighbourhood thin disc
increases with time after star formation. Nordstrom et al. (2004) is the most
recent observational attempt to constrain the age-velocity dispersion relation.
They fitted the age-velocity dispersion relations of each Galactic cardinal
direction space velocity component, U (towards the Galactic centre), V (in the
direction of Galactic rotation) and W (towards the North Galactic Pole), with
power laws and interpreted these as evidence for continuous heating of the disc
in all directions throughout its lifetime. We re-visit these relations with
their data and use Famaey et al. (2005) to show that structure in the local
velocity distribution function distorts the in-plane (U and V) velocity
distributions away from Gaussian so that a dispersion is not an adequate
parametrization of their functions. The age-sigma(W) relation can however be
constrained because the sample is well phase-mixed vertically. We do not find
any local signature of the stellar warp in the Galactic disc. Vertical disc
heating does not saturate at an early stage. Our new result is that a power law
is not required by the data: disc heating models that saturate after ~ 4.5 Gyr
are equally consistent with observations.Comment: Accepted for publication in MNRAS, 24 pages, 20 figure
Large Scale Clustering of Sloan Digital Sky Survey Quasars: Impact of the Baryon Density and the Cosmological Constant
We report the first result of the clustering analysis of Sloan Digital Sky
Survey (SDSS) quasars. We compute the two-point correlation function (2PCF) of
SDSS quasars in redshift space at ,
with particular attention to its baryonic signature. Our sample consists of
19986 quasars extracted from the SDSS Data Release 4 (DR4). The redshift range
of the sample is (the mean redshift is )
and the reddening-corrected -band apparent magnitude range is . Due to the relatively low number density of the
quasar sample, the bump in the power spectrum due to the baryon density,
, is not clearly visible. The effect of the baryon density is,
however, to distort the overall shape of the 2PCF.The degree of distortion
makes it an interesting alternate measure of the baryonic signature. Assuming a
scale-independent linear bias and the spatially flat universe, i.e.,
, where
and denote the density parameters of dark matter and the
cosmological constant, we combine the observed quasar 2PCF and the predicted
matter 2PCF to put constraints on and . Our
result is fitted as at the 2 confidence level, which is consistent with
results from other cosmological observations such as WMAP. (abridged)Comment: 26 pages, 12 figures, Accepted for publication in the PAS
Observing the clustering properties of galaxy clusters in dynamical dark-energy cosmologies
We study the clustering properties of galaxy clusters expected to be observed
by various forthcoming surveys both in the X-ray and sub-mm regimes by the
thermal Sunyaev-Zel'dovich effect. Several different background cosmological
models are assumed, including the concordance CDM and various
cosmologies with dynamical evolution of the dark energy. Particular attention
is paid to models with a significant contribution of dark energy at early times
which affects the process of structure formation. Past light cone and selection
effects in cluster catalogs are carefully modeled by realistic scaling
relations between cluster mass and observables and by properly taking into
account the selection functions of the different instruments. The results show
that early dark-energy models are expected to produce significantly lower
values of effective bias and both spatial and angular correlation amplitudes
with respect to the standard CDM model. Among the cluster catalogues
studied in this work, it turns out that those based on \emph{eRosita},
\emph{Planck}, and South Pole Telescope observations are the most promising for
distinguishing between various dark-energy models.Comment: 16 pages, 10 figures. A&A in pres
Avalanche Size Scaling in Sheared Three-Dimensional Amorphous Solid
We have studied the statistics of plastic rearrangement events in a simulated
amorphous solid at T=0. Events are characterized by the energy release and the
``slip volume'', the product of plastic strain and system volume. Their
distributions for a given system size appear to be exponential, but a
characteristic event size cannot be inferred, because the mean values of these
quantities increase as with . In contrast to
results obtained in 2D models, we do not see simply connected avalanches. The
exponent suggests a fractal shape of the avalanches, which is also evidenced by
the mean fractal dimension and participation ratio.Comment: Accepted for publication in Physical Review Letter
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