1,463 research outputs found
The defect variance of random spherical harmonics
The defect of a function is defined as the
difference between the measure of the positive and negative regions. In this
paper, we begin the analysis of the distribution of defect of random Gaussian
spherical harmonics. By an easy argument, the defect is non-trivial only for
even degree and the expected value always vanishes. Our principal result is
obtaining the asymptotic shape of the defect variance, in the high frequency
limit. As other geometric functionals of random eigenfunctions, the defect may
be used as a tool to probe the statistical properties of spherical random
fields, a topic of great interest for modern Cosmological data analysis.Comment: 19 page
A Chandra view of the clumpy reflector at the heart of the Circinus galaxy
We present a spectral and imaging analysis of the X-ray reflecting structure
at the heart of the Circinus galaxy, investigating the innermost regions
surrounding the central black hole. By studying an archival 200 ks Chandra
ACIS-S observation, we are able to image the extended clumpy structure
responsible for both cold reflection of the primary radiation and neutral iron
Ka line emission. We measure an excess of the equivalent width of the iron Ka
line which follows an axisymmetric geometry around the nucleus on a hundred pc
scale. Spectra extracted from different regions confirm a scenario in which the
dominant mechanism is the reflection of the nuclear radiation from
Compton-thick gas. Significant differences in the equivalent width of the iron
Ka emission line (up to a factor of 2) are found. It is argued that these
differences are due to different scattering angles with respect to the line of
sight rather than to different iron abundances.Comment: 6 pages, 4 figures, accepted for publication on MNRA
Introducing Mexican needlets for CMB analysis: Issues for practical applications and comparison with standard needlets
Over the last few years, needlets have a emerged as a useful tool for the
analysis of Cosmic Microwave Background (CMB) data. Our aim in this paper is
first to introduce in the CMB literature a different form of needlets, known as
Mexican needlets, first discussed in the mathematical literature by Geller and
Mayeli (2009a,b). We then proceed with an extensive study of the properties of
both standard and Mexican needlets; these properties depend on some parameters
which can be tuned in order to optimize the performance for a given
application. Our second aim in this paper is then to give practical advice on
how to adjust these parameters in order to achieve the best properties for a
given problem in CMB data analysis. In particular we investigate localization
properties in real and harmonic spaces and propose a recipe on how to quantify
the influence of galactic and point source masks on the needlet coefficients.
We also show that for certain parameter values, the Mexican needlets provide a
close approximation to the Spherical Mexican Hat Wavelets (whence their name),
with some advantages concerning their numerical implementation and the
derivation of their statistical properties.Comment: 40 pages, 11 figures, published version, main modification: added
section on more realistic galactic and point source mask
X-ray observations of the Compton-thick Seyfert 2 galaxy, NGC 5643
We present results from a ~55 ks long XMM-Newton observation of the obscured
AGN, NGC 5643, performed in July 2009. A previous, shorter (about 10 ks)
XMM-Newton observation in February 2003 had left two major issues open, the
nature of the hard X-ray emission (Compton-thin vs Compton-thick) and of the
soft X-ray excess (photoionized vs collisionally ionized matter). The new
observation shows that the source is Compton-thick and that the dominant
contribution to the soft X-ray emission is by photoionized matter (even if it
is still unclear whether collisionally ionized matter may contribute as well).
We also studied three bright X-ray sources that are in the field of NGC 5643.
The ULX NGC 5643 X-1 was confirmed to be very luminous, even if more than a
factor 2 fainter than in 2003. We then provided the first high quality spectrum
of the cluster of galaxies Abell 3602. The last source, CXOJ143244.5-442020, is
likely an unobscured AGN, possibly belonging to Abell 3602.Comment: 6 pages. Accepted for publication in A&
Exposing implicit biases and stereotypes in human and artificial intelligence: state of the art and challenges with a focus on gender
Biases in cognition are ubiquitous. Social psychologists suggested biases and stereotypes serve a multifarious set of cognitive goals, while at the same time stressing their potential harmfulness. Recently, biases and stereotypes became the purview of heated debates in the machine learning community too. Researchers and developers are becoming increasingly aware of the fact that some biases, like gender and race biases, are entrenched in the algorithms some AI applications rely upon. Here, taking into account several existing approaches that address the problem of implicit biases and stereotypes, we propose that a strategy to cope with this phenomenon is to unmask those found in AI systems by understanding their cognitive dimension, rather than simply trying to correct algorithms. To this extent, we present a discussion bridging together findings from cognitive science and insights from machine learning that can be integrated in a state-of-the-art semantic network. Remarkably, this resource can be of assistance to scholars (e.g., cognitive and computer scientists) while at the same time contributing to refine AI regulations affecting social life. We show how only through a thorough understanding of the cognitive processes leading to biases, and through an interdisciplinary effort, we can make the best of AI technology
NGC 1068: No change in the mid-IR torus structure despite X-ray variability
Context. Recent NuSTAR observations revealed a somewhat unexpected increase
in the X-ray flux of the nucleus of NGC 1068. We expect the infrared emission
of the dusty torus to react on the intrinsic changes of the accretion disk.
Aims. We aim to investigate the origin of the X-ray variation by
investigating the response of the mid-infrared environment.
Methods. We obtained single-aperture and interferometric mid-infrared
measurements and directly compared the measurements observed before and
immediately after the X-ray variations. The average correlated and
single-aperture fluxes as well as the differential phases were directly
compared to detect a possible change in the structure of the nuclear emission
on scales of 2 pc.
Results. The flux densities and differential phases of the observations
before and during the X-ray variation show no significant change over a period
of ten years. Possible minor variations in the infrared emission are
8 %.
Conclusions. Our results suggest that the mid-infrared environment of NGC
1068 has remained unchanged for a decade. The recent transient change in the
X-rays did not cause a significant variation in the infrared emission. This
independent study supports previous conclusions that stated that the X-ray
variation detected by NuSTAR observations is due to X-ray emission piercing
through a patchy section of the dusty region.Comment: 6 pages, 5 figures, 3 tables. Accepted for publication on A&
The changing X-ray time lag in MCG-6-30-15
MCG-6-30-15 is one of the most observed Narrow Line Seyfert 1 galaxies in the
X-ray band. In this paper we examine the X-ray time lags in this source using a
total of 600 ks in observations (440 ks exposure) taken with the XMM-Newton
telescope (300 ks in 2001 and 300 ks in 2013). Both the old and new
observations show the usual hard lag that increases with energy, however, the
hard lag turns over to a soft lag at frequencies below ~1e-4 Hz. The highest
frequencies (~1e-3 Hz) in this source show a clear soft lag, as previously
presented for the first 300 ks observation, but no clear iron K lag is detected
in either the old or new observation. The soft lag is more significant in the
old observation than the new. The observations are consistent with a
reverberation interpretation, where the soft, reflected emission is delayed
with respect to the hard powerlaw component. These spectral timing results
suggest that two distinct variability mechanisms are important in this source:
intrinsic coronal variations (which lead to correlated variability in the
reprocessed emission), and geometrical changes in the corona. Variability due
to geometrical changes does not result in correlated variability in the
reflection, and therefore inhibits the clear detection of an iron K lag.Comment: Resubmitted to MNRAS after minor corrections. 11 pages, 10 figure
On Nonlinear Functionals of Random Spherical Eigenfunctions
We prove Central Limit Theorems and Stein-like bounds for the asymptotic
behaviour of nonlinear functionals of spherical Gaussian eigenfunctions. Our
investigation combine asymptotic analysis of higher order moments for Legendre
polynomials and, in addition, recent results on Malliavin calculus and Total
Variation bounds for Gaussian subordinated fields. We discuss application to
geometric functionals like the Defect and invariant statistics, e.g.
polyspectra of isotropic spherical random fields. Both of these have relevance
for applications, especially in an astrophysical environment.Comment: 24 page
- …