
Information Content in the Galaxy Angular Power Spectrum from the Sloan Digital Sky Survey and Its Implication on Weak Lensing Analysis. (arXiv:0807.1538v1 [astroph])
We analyze the photometric redshift catalog of the Sloan Digital Sky Survey
Data Release 5 (SDSS DR5) to estimate the Fisher information in the galaxy
angular power spectrum with the help of the RimesHamilton technique. It is
found that the amount of Fisher information contained in the galaxy angular
power spectrum is saturated at lensing multipole scale 300<= l <= 2000 in the
redshift range 0.1<= photoz <0.5. At l=2000, the observed information is two
orders of magnitude lower than the case of Gaussian fluctuations. This supports
observationally that the translinear regime of the density power spectrum
contains little independent information about the initial cosmological
conditions, which is consistent with the numerical trend shown by
RimesHamilton. Our results also suggest that the Gaussiannoise description
may not be valid in weak lensing measurements.
read more at astroph updates on arXiv.org 

The Host Galaxy and Central Engine of the Dwarf AGN POX 52. (arXiv:0807.1535v1 [astroph])
We present new multiwavelength observations of the dwarf Seyfert 1 galaxy
POX 52 in order to investigate the properties of the host galaxy and the active
nucleus, and to examine the mass of its black hole, previously estimated to be
~ 10^5 M_sun. Hubble Space Telescope ACS/HRC images show that the host galaxy
has a dwarf elliptical morphology (M_I = 18.4 mag, Sersic index n = 4.3) with
no detected disk component or spiral structure, confirming previous results
from groundbased imaging. Xray observations from both Chandra and XMM show
strong (factor of 2) variability over timescales as short as 500 s, as well as
a dramatic decrease in the absorbing column density over a 9 month period. We
attribute this change to a partial covering absorber, with a 94% covering
fraction and N_H = 58^{+8.4}_{9.2} * 10^21 cm^2, that moved out of the line
of sight in between the XMM and Chandra observations. Combining these data with
observations from the VLA, Spitzer, and archival data from 2MASS and GALEX, we
examine the spectral energy distribution (SED) of the active nucleus. Its shape
is broadly similar to typical radioquiet quasar SEDs, despite the very low
bolometric luminosity of L_bol = 1.3 * 10^43 ergs/s. Finally, we compare black
hole mass estimators including methods based on Xray variability, and optical
scaling relations using the broad Hbeta line width and AGN continuum
luminosity, finding a range of black hole mass from all methods to be M_bh =
(2.24.2) * 10^5 M_sun, with an Eddington ratio of L_bol/L_edd = 0.20.5.
read more at astroph updates on arXiv.org 

EAZY: A Fast, Public Photometric Redshift Code. (arXiv:0807.1533v1 [astroph])
We describe a new program for determining photometric redshifts, dubbed EAZY.
The program is optimized for cases where spectroscopic redshifts are not
available, or only available for a biased subset of the galaxies. The code
combines features from various existing codes: it can fit linear combinations
of templates, it includes optional flux and redshiftbased priors, and its
user interface is modeled on the popular HYPERZ code. A novel feature is that
the default template set, as well as the default functional forms of the
priors, are not based on (usually highly biased) spectroscopic samples, but on
semianalytical models. Furthermore, template mismatch is addressed by a novel
restframe template error function. This function gives different wavelength
regions different weights, and ensures that the formal redshift uncertainties
are realistic. We introduce a redshift quality parameter, Q_z, that provides a
robust estimate of the reliability of the photometric redshift estimate.
Despite the fact that EAZY is not "trained" on spectroscopic samples, the code
(with default parameters) performs very well on existing public datasets. For
Kselected samples in CDFSouth and other deep fields we find a 1sigma scatter
in dz/(1+z) of 0.034, and we provide updated photometric redshift catalogs for
the FIRES, MUSYC, and FIREWORKS surveys.
read more at astroph updates on arXiv.org 

A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud. (arXiv:0807.1532v1 [astroph])
We present a study of the magnetic field of the Small Magellanic Cloud (SMC),
carried out using radio Faraday rotation and optical starlight polarization
data. Consistent negative rotation measures (RMs) across the SMC indicate that
the lineofsight magnetic field is directed uniformly away from us with a
strength 0.19 +/ 0.06 microGauss. Applying the ChandrasekharFermi method to
starlight polarization data yields an ordered magnetic field in the plane of
the sky of strength 1.6 +/ 0.4 microGauss oriented at a position angle 4 +/
12 degs, measured counterclockwise from the great circle on the sky joining
the SMC to the Large Magellanic Cloud (LMC). We construct a threedimensional
magnetic field model of the SMC, under the assumption that the RMs and
starlight polarization probe the same underlying largescale field. The vector
defining the overall orientation of the SMC magnetic field shows a potential
alignment with the vector joining the center of the SMC to the center of the
LMC, suggesting the possibility of a "panMagellanic'' magnetic field. A
cosmicray driven dynamo is the most viable explanation of the observed field
geometry, but has difficulties accounting for the observed unidirectional
field lines. A study of Faraday rotation through the Magellanic Bridge is
needed to further test the panMagellanic field hypothesis.
read more at astroph updates on arXiv.org 

Is the Gini coefficient a stable measure of galaxy structure?. (arXiv:0807.1531v1 [astroph])
The Gini coefficient, a nonparametric measure of galaxy morphology, has
recently taken up an important role in the automated identification of galaxy
mergers. I present a critical assessment of its stability, based on a
comparison of HST/ACS imaging data from the GOODS and UDF surveys. Below a
certain signaltonoise level, the Gini coefficient depends strongly on the
signaltonoise ratio, and thus becomes useless for distinguishing different
galaxy morphologies. Moreover, at all signaltonoise levels the Gini
coefficient shows a strong dependence on the choice of aperture within which it
is measured. Consequently, quantitative selection criteria involving the Gini
coefficient, such as a selection of merger candidates, cannot always be
straightforwardly applied to different datasets. I discuss whether these
effects could have affected previous studies that were based on the Gini
coefficient, and establish signaltonoise limits above which measured Gini
values can be considered reliable.
read more at astroph updates on arXiv.org 
» Determination of stellar, orbital and planetary parameters using complete MonteCarlo analysis  th 
Determination of stellar, orbital and planetary parameters using complete MonteCarlo analysis  the case of HATP7b. (arXiv:0807.1530v1 [astroph])
The recently discovered transiting very hot Jupiter, HATP7b, a planet
detected by the telescopes of HATNet, turned out to be among the ones subjected
to the highest irradiation from the parent star. As known, the combination of
photometric and spectroscopic data for such an object yields the stellar,
orbital and planetary parameters. In order to best characterize this particular
planet, we carried out a complex analysis based on a complete and simultaneous
MonteCarlo solution using all available data. We included the discovery light
curves, partial followup light curves, the radial velocity data, and we used
the stellar evolution models to infer the stellar properties. This
selfconsistent way of modeling provides the most precise estimate of the a
posteriori distributions of all of the system parameters of interest, and
avoids making assumptions on the values and uncertainties of any of the
internally derived variables describing the system. This analysis demonstrates
that even partial light curve information can be valuable. This may become very
important for future discoveries of planets with longer periods  and
therefore longer transit durations  where the chance of observing a full
event is small.
read more at astroph updates on arXiv.org
Jul. 12th, 2008 @ 11:41 pm

» Rapid optical and Xray timing observations of GX 3394: flux correlations at the onset of a low/har 
Rapid optical and Xray timing observations of GX 3394: flux correlations at the onset of a low/hard state. (arXiv:0807.1529v1 [astroph])
We present the discovery of optical/Xray flux correlations on rapid
timescales in the low/hard state of the Galactic black hole GX 3394. The
source had recently emerged from outburst and was associated with a
relativelyfaint counterpart with mag V~17. The optical (VLT/ULTRACAM) and
Xray (RXTE/PCA) data show a clear positive crosscorrelation function (CCF)
signal, with the optical peak lagging Xrays by ~ 150 ms, preceded by a shallow
rise and followed by a steep decline along with broad anticorrelation dips.
Examination of the light curves shows that the main CCF features are reproduced
in superpositions of flares and dips. The CCF peak is narrow and the Xray
autocorrelation function (ACF) is broader than the optical ACF, arguing
against reprocessing as the origin for the rapid optical emission. Xray
flaring is associated with spectral hardening, but no corresponding changes are
detected around optical peaks and dips. The variability may be explained in the
context of synchrotron emission with interaction between a jet and a corona.
The complex CCF structure in GX 3394 has similarities to that of another
remarkable Xray binary XTE J1118+480, in spite of showing a weaker maximum
strength. Such simultaneous multiwavelength, rapid timing studies provide key
constraints for modeling the inner regions of accreting stellar sources.
read more at astroph updates on arXiv.org
Jul. 12th, 2008 @ 11:41 pm

» NearIR Spectrograph for VSI (VLTI Spectro Imager): dispersing the light from an integrated optics b 
NearIR Spectrograph for VSI (VLTI Spectro Imager): dispersing the light from an integrated optics beamcombiner. (arXiv:0807.1617v1 [astroph])
We present the optical and cryomechanical solutions for the Spectrograph of
VSI (VLTI SpectroImager), the second generation nearinfrared (J, H and K
bands) interferometric instrument for the VLTI. The peculiarity of this
spectrograph is represented by the Integrated Optics (IO) beamcombiner, a
small and delicate component which is located inside the cryostat and makes VSI
capable to coherently combine 4, 6 or even 8 telescopes. The optics have been
specifically designed to match the IO combiner output with the IR detector
still preserving the needed spatial and spectral sampling, as well as the
required fringe spacing. A compact device that allows us to interchange
spectral resolutions (from R=200 to R=12000), is also presented.
read more at astroph updates on arXiv.org
Jul. 12th, 2008 @ 11:41 pm

» Magnetic field distribution in the quiet Sun: a reduced model approach. (arXiv:0807.1625v1 [astroph 
Magnetic field distribution in the quiet Sun: a reduced model approach. (arXiv:0807.1625v1 [astroph])
We simulate the dynamics and the evolution of quiet Sun magnetic elements to
produce a probability density function of the field strengths associated with
such elements. The dynamics of the magnetic field are simulated through a
numerical model in which magnetic elements are passively driven by an advection
field presenting spatiotemporal correlations which mimicks the granulation and
the mesogranulation scales observed on the solar surface. The field strength
can increase due to an amplification process which takes place where the
magnetic elements converge. Starting from a deltalike probability density
function centered on B=30 G, we obtain magnetic field strengths up to 2 kG (in
absolute value). To derive the statistical properties of the magnetic elements
several simulation runs are performed. The model is able to produce kG magnetic
fields in a time interval of the order of the granulation time scale. The mean
unsigned flux density and the mean magnetic energy density of the synthetic
quiet Sun reach respectively 100 G and 350 G in the stationary regime. The
derived probability density function of the magnetic field strength decreases
rapidly from B=30 G to B=100 G and presents a secondary maximum for B=2 kG.
From this result it follows that magnetic fields >700 G dominate the unsigned
flux density and magnetic energy density although the probability density
function of the field strength presents a maximum for B=30 G.
read more at astroph updates on arXiv.org
Jul. 12th, 2008 @ 11:41 pm

» Stability and mode analysis of solar coronal loops using thermodynamic irreversible energy principle 
Stability and mode analysis of solar coronal loops using thermodynamic irreversible energy principles II. Modes in twisted nonisothermal magnetic field configurations. (arXiv:0807.1722v1 [astroph])
We study the stability and the modes of non  isothermal coronal loop models
with different intensity values of the equilibrium twisted magnetic field.We
use an energy principle obtained via non  equilibrium thermodynamic
arguments. The principle is expressed in terms of Hermitian operators and
allows to consider together the coupled system of equations: the balance of
energy equation and the equation of motion, to obtain modes and eigenmodes in a
spectrum ranging from short to longwavelength disturbances without having to
use weak varying approximations of the equilibrium parameters. Longwavelength
perturbations introduce additional difficulties because the inhomogeneous
nature of the medium determines disturbances leading to continuous intervals of
eigenfrequencies which cannot be considered as purely sinusoidal.We analyze the
modification of periods, modes structure and stability when the helicity, the
magnetic field strength and the radius of the fluxtube are varied. The
efficiency of the damping due to the resonant absorption mechanism is analyzed
in a context of modes that can either impulsively release or storage magnetic
energy.We find that the onset of the instability is associated to a critical
value of the helicity and that the magnetic energy content has a determinant
role on the instability of the system with respect to the stabilizing effect of
the resonant absorption mechanism.
read more at astroph updates on arXiv.org
Jul. 12th, 2008 @ 11:41 pm


