2,263 research outputs found
Anti-Pluricanonical Systems On Q-Fano Threefolds
We investigate birationality of the anti-pluricanonical map , the
rational map defined by the anti-pluricanonical system , on
-Fano threefolds.Comment: 18 page
Symplectic symmetries of 4-manifolds
A study of symplectic actions of a finite group on smooth 4-manifolds is
initiated. The central new idea is the use of -equivariant
Seiberg-Witten-Taubes theory in studying the structure of the fixed-point set
of these symmetries. The main result in this paper is a complete description of
the fixed-point set structure (and the action around it) of a symplectic cyclic
action of prime order on a minimal symplectic 4-manifold with .
Comparison of this result with the case of locally linear topological actions
is made. As an application of these considerations, the triviality of many such
actions on a large class of 4-manifolds is established. In particular, we show
the triviality of homologically trivial symplectic symmetries of a surface
(in analogy with holomorphic automorphisms). Various examples and comments
illustrating our considerations are also included.Comment: 28 pages, 1 figure, 1 appendix, publishe
A new family of surfaces with and whose Albanese map has degree
We construct a new family of minimal surfaces of general type with
and , whose Albanese map is a quadruple cover of an abelian surface with
polarization of type . We also show that this family provides an
irreducible component of the moduli space of surfaces with and
. Finally, we prove that such a component is generically smooth of
dimension 4 and that it contains the 2-dimensional family of product-quotient
examples previously constructed by the first author. The main tools we use are
the Fourier-Mukai transform and the Schr\"odinger representation of the finite
Heisenberg group .Comment: 23 pages. To appear in the Journal of the London Mathematical
Society. This is a preprint version, slightly different from the published
versio
A Systematic Analysis of Fe II Emission in Quasars: Evidence for Inflow to the Central Black Hole
Broad Fe II emission is a prominent feature of the optical and ultraviolet
spectra of quasars. We report on a systematical investigation of optical Fe II
emission in a large sample of 4037 z < 0.8 quasars selected from the Sloan
Digital Sky Survey. We have developed and tested a detailed line-fitting
technique, taking into account the complex continuum and narrow and broad
emission-line spectrum. Our primary goal is to quantify the velocity broadening
and velocity shift of the Fe II spectrum in order to constrain the location of
the Fe II-emitting region and its relation to the broad-line region. We find
that the majority of quasars show Fe II emission that is redshifted, typically
by ~ 400 km/s but up to 2000 km/s, with respect to the systemic velocity of the
narrow-line region or of the conventional broad-line region as traced by the
Hbeta line. Moreover, the line width of Fe II is significantly narrower than
that of the broad component of Hbeta. We show that the magnitude of the Fe II
redshift correlates inversely with the Eddington ratio, and that there is a
tendency for sources with redshifted Fe II emission to show red asymmetry in
the Hbeta line. These characteristics strongly suggest that Fe II originates
from a location different from, and most likely exterior to, the region that
produces most of Hbeta. The Fe II-emitting zone traces a portion of the
broad-line region of intermediate velocities whose dynamics may be dominated by
infall.Comment: 20 pages, 14 figures, accepted for publication in Ap
Physical Structure and Nature of Supernova Remnants in M101
Supernova remnant (SNR) candidates in the giant spiral galaxy M101 have been
previously identified from ground-based H-alpha and [SII] images. We have used
archival Hubble Space Telescope (HST) H-alpha and broad-band images as well as
stellar photometry of 55 SNR candidates to examine their physical structure,
interstellar environment, and underlying stellar population. We have also
obtained high-dispersion echelle spectra to search for shocked high-velocity
gas in 18 SNR candidates, and identified X-ray counterparts to SNR candidates
using data from archival observations made by the Chandra X-ray Observatory.
Twenty-one of these 55 SNR candidates studied have X-ray counterparts, although
one of them is a known ultra-luminous X-ray source. The multi-wavelength
information has been used to assess the nature of each SNR candidate. We find
that within this limited sample, ~16% are likely remnants of Type Ia SNe and
~45% are remnants of core-collapse SNe. In addition, about ~36% are large
candidates which we suggest are either superbubbles or OB/HII complexes.
Existing radio observations are not sensitive enough to detect the non-thermal
emission from these SNR candidates. Several radio sources are coincident with
X-ray sources, but they are associated with either giant HII regions in M101 or
background galaxies. The archival HST H-alpha images do not cover the entire
galaxy and thus prevents a complete study of M101. Furthermore, the lack of HST
[SII] images precludes searches for small SNR candidates which could not be
identified by ground-based observations. Such high-resolution images are needed
in order to obtain a complete census of SNRs in M101 for a comprehensive
investigation of the distribution, population, and rates of SNe in this galaxy.Comment: 37 pages, 4 Tables, 7 Figures, accepted for publication in the
Astronomical Journa
Dynamic saturation in semiconductor optical amplifiers: accurate model, role of carrier density, and slow light
We developed an improved model in order to predict the RF behavior and the
slow light properties of the SOA valid for any experimental conditions. It
takes into account the dynamic saturation of the SOA, which can be fully
characterized by a simple measurement, and only relies on material fitting
parameters, independent of the optical intensity and the injected current. The
present model is validated by showing a good agreement with experiments for
small and large modulation indices.Comment: 9 pages, 5 figure
Simultaneous X-ray and radio observations of Young Stellar Objects in NGC 1333 and IC 348
Young Stellar Objects (YSOs) and in particular protostars are known to show a
variety of high-energy processes. Observations in the X-ray and centimetric
radio wavelength ranges are thought to constrain some of these processes, e.g.,
coronal-type magnetic activity. There is a well-known empirical correlation of
radio and X-ray luminosities in active stars, the so-called Guedel-Benz
relation. Previous evidence whether YSOs are compatible with this relation
remains inconclusive for the earliest evolutionary stages. The main difficulty
is that due to the extreme variability of these sources, simultaneous
observations are essential. Until now, only few YSOs and only a handful of
protostars have been observed simultaneously in the X-ray and radio range. To
expand the sample, we have obtained such observations of two young clusters
rich in protostars, NGC 1333 and IC 348. While the absolute sensitivity is
lower for these regions than for more nearby clusters like CrA, we find that
even in deep continuum observations carried out with the NRAO Very Large Array,
the radio detection fraction for protostars in these clusters is much lower
than the X-ray detection fraction. Very few YSOs are detected in both bands,
and we find the radio and X-ray populations among YSOs to be largely distinct.
We combine these new results with previous simultaneous Chandra and VLA
observations of star-forming regions and find that YSOs with detections in both
bands appear to be offset toward higher radio luminosities for given X-ray
luminosities when compared to the Guedel-Benz relation, although even in this
sensitive dataset most sources are too weak for the radio detections to provide
information on the emission processes. The considerably improved sensitivity of
the Expanded Very Large Array will provide a better census of the YSO radio
population as well as better constraints on the emission mechanisms.Comment: Accepted for publication in Ap
Discovery of fast, large-amplitude optical variability of V648 Car (=SS73-17)
We report on the discovery of large-amplitude flickering from V648 Car (=
SS73-17), a poorly studied object listed amongst the very few hard X-ray
emitting symbiotic stars. We performed milli-magnitude precision optical
photometry with the Swope Telescope at the Las Campanas Observatory, Chile, and
found that V648 Car shows large U-band variability over time scales of minutes.
To our knowledge, it is amongst the largest flickering of a symbiotic star ever
reported. Our finding supports the hypothesis that symbiotic WDs producing hard
X-rays are predominantly powered by accretion, rather than quasi-steady nuclear
burning, and have masses close to the Chandrasekhar limit. No significant
periodicity is evident from the flickering light curve. The ASAS long-term V
light curve suggests the presence of a tidally distorted giant accreting via
Roche Lobe overflow, and a binary period of about 520 days. On the basis of the
outstanding physical properties of V648 Car as hinted by its fast and long-term
optical variability, as well as by its nature as hard X-ray emitter, we
therefore call for simultaneous follow-up observations in different bands,
ideally combined with time-resolved optical spectroscopy.Comment: Accepted for publication in ApJ
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