451 research outputs found
On surfaces with p_g=2, q=1 and K^2=5
We consider minimal surfaces of general type with , and . We provide a stratification of the corresponding moduli space and we give
some bounds for the number and the dimensions of its irreducible components.Comment: 25 pages. To appear in Rendiconti del Circolo Matematico di Palerm
Limits to Quantum Gravity Effects from Observations of TeV Flares in Active Galaxies
We have used data from the TeV gamma-ray flare associated with the active
galaxy Markarian 421 observed on 15 May 1996 to place bounds on the possible
energy-dependence of the speed of light in the context of an effective quantum
gravitational energy scale. The possibility of an observable time dispersion in
high energy radiation has recently received attention in the literature, with
some suggestions that the relevant energy scale could be less than the Planck
mass and perhaps as low as 10^16 GeV. The limits derived here indicate this
energy scale to be in excess of 4x10^16 GeV at the 95% confidence level. To the
best of our knowledge, this constitutes the first convincing limit on such
phenomena in this energy regime.Comment: 6 pages, including 2 figure
Surfaces containing a family of plane curves not forming a fibration
We complete the classification of smooth surfaces swept out by a
1-dimensional family of plane curves that do not form a fibration. As a
consequence, we characterize manifolds swept out by a 1-dimensional family of
hypersurfaces that do not form a fibration.Comment: Author's post-print, final version published online in Collect. Mat
Deformation of canonical morphisms and the moduli of surfaces of general type
In this article we study the deformation of finite maps and show how to use
this deformation theory to construct varieties with given invariants in a
projective space. Among other things, we prove a criterion that determines when
a finite map can be deformed to a one--to--one map. We use this criterion to
construct new simple canonical surfaces with different and . Our
general results enable us to describe some new components of the moduli of
surfaces of general type. We also find infinitely many moduli spaces having one component whose general point corresponds to a
canonically embedded surface and another component whose general point
corresponds to a surface whose canonical map is a degree 2 morphism.Comment: 32 pages. Final version with some simplifications and clarifications
in the exposition. To appear in Invent. Math. (the final publication is
available at springerlink.com
Constraints on Gamma-ray Emission from the Galactic Plane at 300 TeV
We describe a new search for diffuse ultrahigh energy gamma-ray emission
associated with molecular clouds in the galactic disk. The Chicago Air Shower
Array (CASA), operating in coincidence with the Michigan muon array (MIA), has
recorded over 2.2 x 10^{9} air showers from April 4, 1990 to October 7, 1995.
We search for gamma rays based upon the muon content of air showers arriving
from the direction of the galactic plane. We find no significant evidence for
diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma
rays to normal hadronic cosmic rays at less than 2.4 x 10^{-5} at 310 TeV (90%
confidence limit) from the galactic plane region: (50 degrees < l < 200
degrees); -5 degrees < b < 5 degrees). This limit places a strong constraint on
models for emission from molecular clouds in the galaxy. We rule out
significant spectral hardening in the outer galaxy, and conclude that emission
from the plane at these energies is likely to be dominated by the decay of
neutral pions resulting from cosmic rays interactions with passive target gas
molecules.Comment: Astrophysical Journal, submitted, 11 pages, AASTeX Latex, 3
Postscript figure
BeppoSAX Observations of Markarian 501 in June 1999
We present the preliminary results of a long BeppoSAX observation of the BL
Lac object Mkn501 carried out in June 1999. The source was fainter than found
during the BeppoSAX pointings of 1997 and 1998, but is still detected with a
good signal-to-noise ratio up to ~40 keV. The X-ray spectrum in the energy
range 0.1-40 keV, produced through synchrotron radiation, is steeper than in
the previous years, it is clearly curved, and peaks (in nu*F_nu) at ~0.5 keV.
This energy is much lower than those at which the synchrotron component was
found to peak in 1997 and 1998. Some intraday variability suggests that
activity of the source on small time scales accompanies the large long time
scale changes of brightness and spectrum.Comment: 4 pages, Latex, 2 PostScript figures, to appear in the Proceedings of
the Conference "X-ray Astronomy '999: Stellar Endpoints, AGNs, and the
Diffuse X-ray Background" (Bologna, 6-10 September 1999
Likelihood Geometry
We study the critical points of monomial functions over an algebraic subset
of the probability simplex. The number of critical points on the Zariski
closure is a topological invariant of that embedded projective variety, known
as its maximum likelihood degree. We present an introduction to this theory and
its statistical motivations. Many favorite objects from combinatorial algebraic
geometry are featured: toric varieties, A-discriminants, hyperplane
arrangements, Grassmannians, and determinantal varieties. Several new results
are included, especially on the likelihood correspondence and its bidegree.
These notes were written for the second author's lectures at the CIME-CIRM
summer course on Combinatorial Algebraic Geometry at Levico Terme in June 2013.Comment: 45 pages; minor changes and addition
Correlated variability of Mkn 421 at X-ray and TeV wavelengths on timescales of hours
Mkn 421 was observed for about two days with BeppoSAX, prior to and partly
overlapping the start of a 1 week continuous exposure with ASCA in April 1998,
as part of a world-wide multiwavelength campaign. A pronounced, well defined,
flare observed in X-rays was also observed simultaneously at TeV energies by
the Whipple Observatory's 10 m gamma-ray telescope. These data provide the
first evidence that the X-ray and TeV intensities are well correlated on
time-scales of hours.Comment: 4 pages, 1 figure, presented at the VERITAS Workshop on the TeV
Astrophysics of Extragalactic Object
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