68 research outputs found
Probing non-Gaussianities on Large Scales in WMAP5 and WMAP7 Data using Surrogates
Probing Gaussianity represents one of the key questions in modern cosmology,
because it allows to discriminate between different models of inflation. We
test for large-scale non-Gaussianities in the cosmic microwave background (CMB)
in a model-independent way. To this end, so-called first and second order
surrogates are generated by first shuffling the Fourier phases belonging to the
scales not of interest and then shuffling the remaining phases for the length
scales under study. Using scaling indices as test statistics we find highly
significant signatures for both non-Gaussianities and asymmetries on large
scales for the WMAP data of the CMB. We find remarkably similar results when
analyzing different ILC-maps based on the WMAP five and seven year data. Such
features being independent from the map-making procedure would disfavor the
fundamental principle of isotropy as well as canonical single-field slow-roll
inflation - unless there is some undiscovered systematic error in the
collection or reduction of the CMB data or yet unknown foreground
contributions.Comment: 4 pages, 3 figures, to appear in the Proceedings of Moriond Cosmology
201
Correlated spectral and temporal changes in 3C 390.3: a new link between AGN and Galactic Black Hole Binaries?
This work presents the results from a systematic search for evidence of
temporal changes (i.e., non-stationarity) associated with spectral variations
in 3C 390.3, using data from a two-year intensive RXTE monitoring campaign of
this broad-line radio galaxy. In order to exploit the potential information
contained in a time series more efficiently, we adopt a multi-technique
approach, making use of linear and non-linear techniques. All the methods show
suggestive evidences for non-stationarity in the temporal properties of 3C
390.3 between 1999 and 2000, in the sense that the characteristic time-scale of
variability decreases as the energy spectrum of the source softens. However,
only the non-linear, "scaling index method" is able to show conclusively that
the temporal characteristics of the source do vary, although the physical
interpretation of this result is not clear at the moment. Our results indicate
that the variability properties of 3C 390.3 may vary with time, in the same way
as they do in Galactic black holes in the hard state, strengthening the analogy
between the X-ray variability properties of the two types of object. This is
the first time that such a behavior is detected in an AGN X-ray light curve.
Further work is needed in order to investigate whether this is a common
behavior in AGN, just like in the Galactic binaries, or not.Comment: 15 pages, 12 figures, accepted for publication in A&
Spectral variability analysis of an XMM-Newton observation of Ark 564
We present a spectral variability analysis of the X-ray emission of the Narrow Line Seyfert 1 galaxy Ark 564 using the data from a ~100 ks XMM-Newton observation. Taking advantage of the high sensitivity of this long observation and the simple spectral shape of Ark 564, we determine accurately the spectral variability patterns in the source. We use standard cross-correlation methods to investigate the correlations between the soft and hard energy band light curves. We also generated 200 energy spectra from data stretches of 500 s duration each and fitted each one of them with a power law plus a bremsstrahlung component (for the soft excess) and we investigated the correlations between the various best fit model parameter values. The ``power law plus bremsstrahlung'' model describes the spectrum well at all times. The iron line and the absorption features, which are found in the time-averaged spectrum of the source are too weak to effect the results of the time resolved spectral fits. We find that the power law and the soft excess flux are variable, on all measured time scales. The power law slope is also variable, and leads the flux variations of both the power law and the bremsstrahlung components. Our results can be explained in the framework of time-dependent Comptonization models. They are consistent with a picture where instabilities propagate through an extended X-ray source, affecting first the soft and then the hard photons producing regions. The soft excess could correspond to ionized disc reflection emission, in which case it responds fast to the primary continuum variations. The time scales are such that light travel times might additionally influence the observed variability structure
Revisiting algorithms for generating surrogate time series
The method of surrogates is one of the key concepts of nonlinear data
analysis. Here, we demonstrate that commonly used algorithms for generating
surrogates often fail to generate truly linear time series. Rather, they create
surrogate realizations with Fourier phase correlations leading to
non-detections of nonlinearities. We argue that reliable surrogates can only be
generated, if one tests separately for static and dynamic nonlinearities.Comment: 5 pages, 4 figures, accepted for publication in PR
A model-independent test for scale-dependent non-Gaussianities in the CMB
We present a model-independent method to test for scale-dependent
non-Gaussianities in combination with scaling indices as test statistics.
Therefore, surrogate data sets are generated, in which the power spectrum of
the original data is preserved, while the higher order correlations are partly
randomised by applying a scale-dependent shuffling procedure to the Fourier
phases. We apply this method to the WMAP data of the cosmic microwave
background (CMB) and find signatures for non-Gaussianities on large scales.
Further tests are required to elucidate the origin of the detected anomalies.Comment: accepted for publication in PRL, minor revisions, results unchanged,
l(cut)-dependency adde
Search for non-Gaussianities in the WMAP data with the Scaling Index Method
In the recent years, non-Gaussianity and statistical isotropy of the Cosmic
Microwave Background (CMB) was investigated with various statistical measures,
first and foremost by means of the measurements of the WMAP satellite. In this
Review, we focus on the analyses that were accomplished with a measure of local
type, the so-called Scaling Index Method (SIM). The SIM is able to detect
structural characteristics of a given data set, and has proven to be highly
valuable in CMB analysis. It was used for comparing the data set with
simulations as well as surrogates, which are full sky maps generated by
randomisation of previously selected features of the original map. During these
investigations, strong evidence for non-Gaussianities as well as asymmetries
and local features could be detected. In combination with the surrogates
approach, the SIM detected the highest significances for non-Gaussianity to
date.Comment: 19 pages, 15 figures, Review Article, Accepted for publication in
Advances in Astronom
A panchromatic view of PKS 0558-504: an ideal laboratory to study the disk-jet link
PKS 0558-504 is the brightest radio-loud Narrow-Line Seyfert 1 galaxy at
X-ray energies. Here we present results from the radio, optical, UV, and X-ray
bands obtained with Swift, XMM, and ATCA during a 10-day monitoring campaign in
September 2008. The simultaneous coverage at several wavelengths makes it
possible to investigate in detail the broadband spectral energy distribution
(SED) and the energetic of this source. The main results can be summarized as
follows. The ATCA reveals the presence of an extended radio emission in PKS
0558-504 with two lobe-like structures at ~7" from the bright central source.
The extended radio structure and the low value of the radio-loudness similar to
radio-quiet Seyfert galaxies coupled with constraints from higher energy bands
argue against a jet-dominated emission. The study of the SED, which is
dominated by a nearly constant optical-UV emission, supports the conclusion
that PKS 0558-504 is accreting at super-Eddington rate. This conclusion was
reached assuming M_BH=2.5e8 M_sun, which was obtained with a new scaling method
based on X-ray spectral variability results. A comparison between the accretion
luminosity and the kinetic power associated with the jet suggests that in this
source the accretion power dominates in agreement with the results obtained
from Radiation-MHD simulations of Galactic black holes (GBHs) accreting at the
Eddington rate. The combined findings from this panchromatic investigation
strongly suggest that PKS 0558-504 is a large-scale analog of GBHs in their
highly accreting intermediate state. Importantly, PKS 0558-504 may also be the
prototype of the parent population of the very radio-loud NLS1s recently
detected at gamma-ray energies.Comment: 11 pages, 5 figures, 3 tables, accepted for publication in Ap
XMM-Newton timing mode observations of Mrk 421
We present the results of a detailed temporal analysis of the bright BL Lac
object Mrk 421 using the three available long timing mode observations by the
EPIC PN camera. This detector mode is characterized by its long life time and
is largely free of photon pile-up problems. The source was found in different
intensity and variability states differing by up to more than a factor of three
in count rate. A time resolved cross correlation analysis between the soft and
hard energy bands revealed that the characteristics of the correlated emission,
with lags of both signs, change on time scales of a few thousand seconds.
Individual spectra, resolved on time scales of a few hundread seconds, can be
quite well fitted by a broken power law. We find significant spectral
variations on time scales as short as 500-1000 sec. Both the hard and the soft
band spectral indices show a non-linear correlation with the source flux. A
comparison of the observed light curves with numerical results from
relativistic hydrodynamic computer simulations of the currently favored
shock-in-jet models indicates that any determination of the jet's physical
parameters from `simple' emission models must be regarded with caution: at any
time we are seeing the emission from several emission regions distinct in space
and time, which are connected by the complex hydrodynamic evolution of the
non-uniform jet.Comment: 16 pages, 16 figures. Accepted for publication in A&
Some doubts on the validity of the foreground Galactic contribution subtraction from microwave anisotropies
The Galactic foreground contamination in CMBR anisotropies, especially from
the dust component, is not easily separable from the cosmological or
extragalactic component. In this paper, some doubts will be raised concerning
the validity of the methods used to date to remove Galactic dust emission in
order to show that none of them achieves its goal.
First, I review the recent bibliography on the topic and discuss critically
the methods of foreground subtraction: the cross-correlation with templates,
analysis assuming the spectral shape of the Galactic components, the "maximum
entropy method", "internal linear combination", and "wavelet-based high
resolution fitting of internal templates". Second, I analyse the galactic
latitude dependence from WMAP data. The frequency dependence is discussed with
the data in the available literature. The result is that all methods of
subtracting the Galactic contamination are inaccurate. The galactic latitude
dependence analysis or the frequency dependence of the anisotropies in the
range 50-250 GHz put a constraint on the maximum Galactic contribution in the
power spectrum to be less than a ~10% (68% C. L.) for a ~1 degree scale, and
possibly higher for larger scales.
The origin of most of the signal in the CMBR anisotropies is not Galactic. In
any case, the subtraction of the Galaxy is not accurate enough to allow a
"precision Cosmology"; other sources of contamination (extragalactic, solar
system) are also present.Comment: 24 pages, 1 figure, accepted to be published in J. Astrophys. Ast
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