2,019 research outputs found
A Serendipitous XMM-Newton Observation of the Intermediate Polar WX Pyx
We briefly describe a serendipitous observation of the little-studied
intermediate polar WX Pyx using XMM-Newton. The X-ray spin period is 1557.3
sec, confirming the optical period published in 1996. An orbital period of
approximately 5.54 hr is inferred from the separation of the spin-orbit
sidelobe components. The soft and hard band spin-folded light curves are nearly
sinusoidal in shape. The best-fit spectrum is consistent with a bremsstrahlung
temperature of about 18 keV. An upper limit of approximately 300 eV is assigned
to the presence of Fe line emission. WX Pyx lies near TX and TV Col in the
P_spin-P_orb plane.Comment: 5 pages, 5 figs; accepted A&A 2004 Dec
Magellan Spectroscopy of Low-Redshift Active Galactic Nuclei
We present an atlas of moderate-resolution (R ~ 1200-1600) optical spectra of
94 low-redshift (z < 0.5) active galactic nuclei taken with the Magellan 6.5 m
Clay Telescope. The spectra mostly cover the rest-frame region 3600-6000 Ang.
All the objects have preexisting Hubble Space Telescope imaging, and they were
chosen as part of an ongoing program to investigate the relationship between
black hole mass and their host galaxy properties. A significant fraction of the
sample has no previous quantitative spectroscopic measurements in the
literature. We perform spectral decomposition of the spectra and present
detailed fits and basic measurements of several commonly used broad and narrow
emission lines, including [O II] 3727, He II 4686, Hbeta, and [O III] 4959,
5007. Eight of the objects are narrow-line sources that were previously
misclassified as broad-line (type 1) Seyfert galaxies; of these, five appear
not to be accretion-powered.Comment: To appear in ApJS, 25 page
Topology of large scale structure as test of modified gravity
The genus of the iso-density contours is a robust measure of the topology of
large scale structure, and it is relatively insensitive to nonlinear
gravitational evolution, galaxy bias and redshift-space distortion. We show
that the growth of density fluctuations is scale-dependent even in the linear
regime in some modified gravity theories, which opens a new possibility of
testing the theories observationally. We propose to use the genus of the
iso-density contours, an intrinsic measure of the topology of large scale
structure, as a statistic to be used in such tests. In Einstein's general
theory of relativity, density fluctuations are growing at the same rate on all
scales in the linear regime, and the genus per comoving volume is almost
conserved as structures are growing homologously, so we expect that the
genus-smoothing scale relation is basically time-independent. However, in some
modified gravity models where structures grow with different rates on different
scales, the genus-smoothing scale relation should change over time. This can be
used to test the gravity models with large scale structure observations. We
studied the case of the f(R) theory, DGP braneworld theory as well as the
parameterized post-Friedmann (PPF) models. We also forecast how the modified
gravity models can be constrained with optical/IR or redshifted 21cm radio
surveys in the near future.Comment: Introduction and discussion expanded and refined, conclusion
unchanged, 10 pages, 8 figures. ApJ accepte
Theoretical model for the formation of caveolae and similar membrane invaginations
We study a physical model for the formation of bud-like invaginations on fluid lipid membranes under tension, and apply this model to caveolae formation. We demonstrate that budding can be driven by membrane-bound proteins, provided that they exert asymmetric forces on the membrane that give rise to bending moments. In particular, caveolae formation does not necessarily require forces to be applied by the cytoskeleton. Our theoretical model is able to explain several features observed experimentally in caveolae, where proteins in the caveolin family are known to play a crucial role in the formation of caveolae buds. These include 1), the formation of caveolae buds with sizes in the 100-nm range and 2), that certain N- and C-termini deletion mutants result in vesicles that are an order-of-magnitude larger. Finally, we discuss the possible origin of the morphological striations that are observed on the surfaces of the caveolae
Astrophysical parameters of ten poorly studied open star clusters
We present here the fundamental parameters of ten open star clusters,
nominated from Kronberger et al. (2006) who presented some new discovered
stellar groups on the basis of 2MASS photometry and DSS visual images. Star
counts and photometric parameters (radius, membership, distances, color excess,
age, luminosity function, mass function, total mass, and the dynamical
relaxation time) have been determined for these ten clusters for the first
time. In order to calibrate our procedures, the main parameters (distance, age,
and color excesses) have been re-estimated for another five clusters, which are
studied by Kronberger et al. (2006) as well.Comment: 10 pages, 5 figures, 2 tables; accepted in "Research in Astronomy and
Astrophysics Journal
Luminous X-ray AGN in Clusters of Galaxies
We present a study of X-ray AGN overdensities in 16 Abell clusters, within
the redshift range 0.073<z<0.279, in order to investigate the effect of the hot
inter-cluster environment on the triggering of the AGN phenomenon. The X-ray
AGN overdensities, with respect to the field expectations, were estimated for
sources with L_x>= 10^{42} erg s^{-1} (at the redshift of the clusters) and
within an area of 1 h^{-1}_{72} Mpc radius (excluding the core). To investigate
the presence or not of a true enhancement of luminous X-ray AGN in the cluster
area, we also derived the corresponding optical galaxy overdensities, using a
suitable range of -band magnitudes. We always find the latter to be
significantly higher (and only in two cases roughly equal) with respect to the
corresponding X-ray overdensities. Over the whole cluster sample, the mean
X-ray point-source overdensity is a factor of ~4 less than that corresponding
to bright optical galaxies, a difference which is significant at a >0.995
level, as indicated by an appropriate t-student test. We conclude that the
triggering of luminous X-ray AGN in rich clusters is strongly suppressed.
Furthermore, searching for optical SDSS counterparts of all the X-ray sources,
associated with our clusters, we found that about half appear to be background
QSOs, while others are background and foreground AGN or stars. The true
overdensity of X-ray point sources, associated to the clusters, is therefore
even smaller than what our statistical approach revealed.Comment: accepted for publication in ApJ Letter
On the agglomeration and breakage of particles in turbulent flows through pipe bends using CFD-PBE
Chandra Multi-wavelength Plane (ChaMPlane) Survey: Design and Initial Results
The Chandra Multiwavength Plane (ChaMPlane) Survey of the galactic plane
incorporates serendipitous sources from selected Chandra pointings in or near
the galactic plane (b 20 ksec; lack of bright diffuse or point
sources) to measure or constrain the luminosity function of low-luminosity
accretion sources in the Galaxy. The primary goal is to detect and identify
accreting white dwarfs (cataclysmic variables, with space density still
uncertain by a factor of >10-100), neutron stars and black holes (quiescent low
mass X-ray binaries) to constrain their space densities and thus origin and
evolution. Secondary objectives are to identify Be stars in high mass X-ray
binaries and constrain their space densities, and to survey the H-R diagram for
stellar coronal sources. A parallel optical imaging under the NOAO Long Term
Survey program provides deep optical images using the Mosaic imager on the CTIO
and KPNO 4-m telescopes. The 36arcmin X 36arcmin optical images (Halpha, R, V
and I) cover ~5X the area of each enclosed Chandra ACIS FOV, providing an
extended survey of emission line objects for comparison with Chandra.
Spectroscopic followup of optical counterparts is then conducted, thus far with
WIYN and Magellan. The X-ray preliminary results from both the Chandra and
optical surveys will be presented, including logN-logS vs. galactic position
(l,b) and optical idenifications.Comment: 4 pages, 7 figures (in 8 files), Astronomishe Nachrichten, in press
(Feb 2003). Proceedings of "X-ray Surveys, in the Light of New
Observatories", 4-6 September, Santander, Spain. Higher resolution figures
available at:
http://hea-www.harvard.edu/ChaMPlane/papers/champlane-santander.pd
Large-Scale Structure of the Universe as a Cosmic Standard Ruler
We propose to use the large-scale structure of the universe as a cosmic
standard ruler, based on the fact that the pattern of galaxy distribution
should be maintained in the course of time on large scales. By examining the
scale-dependence of the pattern in different redshift intervals it is possible
to reconstruct the expansion history of the universe, and thus to measure the
cosmological parameters governing the expansion of the universe. The features
in the galaxy distribution that can be used as standard rulers include the
topology of large-scale structure and the overall shapes of galaxy power
spectrum and correlation function. The genus, being an intrinsic topology
measure, is resistant against the non-linear gravitational evolution, galaxy
biasing, and redshift-space distortion effects, and thus is ideal for
quantifying the primordial topology of the large-scale structure. The expansion
history of the universe can be constrained by comparing among the genus
measured at different redshifts. In the case of initially Gaussian fluctuations
the genus accurately recovers the slope of the primordial power spectrum near
the smoothing scale, and the expansion history can be constrained by comparing
between the predicted and measured genus.Comment: 5 pages, 3 figures, submitted to ApJ
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