1,764 research outputs found
Spherical orbit closures in simple projective spaces and their normalizations
Let G be a simply connected semisimple algebraic group over an algebraically
closed field k of characteristic 0 and let V be a rational simple G-module of
finite dimension. If G/H \subset P(V) is a spherical orbit and if X is its
closure, then we describe the orbits of X and those of its normalization. If
moreover the wonderful completion of G/H is strict, then we give necessary and
sufficient combinatorial conditions so that the normalization morphism is a
homeomorphism. Such conditions are trivially fulfilled if G is simply laced or
if H is a symmetric subgroup.Comment: 24 pages, LaTeX. v4: Final version, to appear in Transformation
Groups. Simplified some proofs and corrected minor mistakes, added
references. v3: major changes due to a mistake in previous version
A combinatorial smoothness criterion for spherical varieties
We suggest a combinatorial criterion for the smoothness of an arbitrary
spherical variety using the classification of multiplicity-free spaces,
generalizing an earlier result of Camus for spherical varieties of type .Comment: 14 pages, 2 table
The X-ray flaring activity of the galactic nucleus observed with XMM-Newton
We report the results of XMM-Newton observations of Sgr A*, the radiative
counterpart of the massive black hole at the nucleus of our Galaxy, performed
in the frame of the guaranteed time survey program of the Galactic Center
region. The discovery of bright X-ray flares from Sgr A* with Chandra in
October 2000 have opened new perspectives to understand the processes at work
in this object and in general in black holes accreting at low accretion rates.
We report here the important results obtained with XMM-Newton on the Sgr A*
high-energy flaring activity and we discuss the implications on the models and
the future observational perspectives.Comment: 4 pages, 3 figures, proc. of the SF2A conf. held in Bordeaux, France,
June 2003, eds.: F. Combes, D. Barret and T. Contini, EdP-Sciences Conf.
Serie
The Impact of Bail Reform on Arrest Rates for Aggravated Assault In Two Cities
With the recent push to implement bail reform in various U.S. cities and states, the impact of such reform was studied using aggravated assault arrest statistics for Philadelphia, a city that implemented bail reform via prosecutorial discretion, and for Pittsburgh, which had not implemented bail reform. Using the time period of January 2017 through December 2019, a quantitative analysis was completed on aggravated assault arrest counts in both cities to ascertain whether the removal of bail as a deterrent caused aggravated assault arrests to increase. Using a t-test, linear regression, and ANOVA, it was determined that bail reform had minimal (if any) influence on aggravated assault arrests in Philadelphia. While this analysis fills a gap in the literature, it also suggests that much more needs to be done if the implications and, where appropriate, consequences of bail reform are to be understood by the politicians and government officials who will make decisions regarding its implementation
Effect of magnesium doping on the orbital and magnetic order in LiNiO2
In LiNiO2, the Ni3+ ions, with S=1/2 and twofold orbital degeneracy, are
arranged on a trian- gular lattice. Using muon spin relaxation (MuSR) and
electron spin resonance (ESR), we show that magnesium doping does not stabilize
any magnetic or orbital order, despite the absence of interplane Ni2+. A
disordered, slowly fluctuating state develops below 12 K. In addition, we find
that magnons are excited on the time scale of the ESR experiment. At the same
time, a g factor anisotropy is observed, in agreement with
orbital occupancy
Quantum optical non-linearities induced by Rydberg-Rydberg interactions: a perturbative approach
In this article, we theoretically study the quantum statistical properties of
the light transmitted through or reflected from an optical cavity, filled by an
atomic medium with strong optical non-linearity induced by Rydberg-Rydberg van
der Waals interactions. Atoms are driven on a two-photon transition from their
ground state to a Rydberg level via an intermediate state by the combination of
a weak signal field and a strong control beam. By using a perturbative
approach, we get analytic results which remain valid in the regime of weak
feeding fields, even when the intermediate state becomes resonant. Therefore
they allow us to investigate quantitatively new features associated with the
resonant behaviour of the system. We also propose an effective non-linear
three-boson model of the system which, in addition to leading to the same
analytic results as the original problem, sheds light on the physical processes
at work in the system
Classification of Reductive Monoid Spaces Over an Arbitrary Field
In this semi-expository paper we review the notion of a spherical space. In
particular we present some recent results of Wedhorn on the classification of
spherical spaces over arbitrary fields. As an application, we introduce and
classify reductive monoid spaces over an arbitrary field.Comment: This is the final versio
Effective constructions in plethysms and Weintraub's conjecture
We give a short proof of Weintraub's conjecture by constructing explicit
highest weight vectors in the symmetric power of an even exterior power
A New X-Ray Flare from the Galactic Nucleus Detected with the XMM-Newton Photon Imaging Cameras
Sgr A*, the compact radio source, believed to be the counterpart of the
massive black hole at the galactic nucleus, was observed to undergo rapid and
intense flaring activity in X-rays with Chandra in October 2000. We report here
the detection with XMM-Newton EPIC cameras of the early phase of a similar
X-ray flare from this source, which occurred on September 4, 2001. The source
2-10 keV luminosity increased by a factor about 20 to reach a level of 4
10^{34} erg s^{-1} in a time interval of about 900 s, just before the end of
the observation. The data indicate that the source spectrum was hard during the
flare. This XMM-Newton observation confirms the results obtained by Chandra and
suggests that, in Sgr A*, rapid and intense X-ray flaring is not a rare event.
This can constrain the emission mechanism models proposed for this source, and
also implies that the crucial multiwavelength observation programs planned to
explore the behaviour of the radio/sub-mm and hard X-ray/gamma-ray emissions
during the X-ray flares, have a good chance of success.Comment: 18 pages, 6 color figures, final version, accepted on October 24,
2002, to appear in ApJ, v584 n2 ApJ February 20, 2003 issu
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